View LT3845A_453231.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

LT3724 High Voltage, Current Mode Switching Regulator Controller FEATURES
n n n n n n n n n n n n n
DESCRIPTION
The LT(R)3724 is a DC/DC controller used for medium power, low part count, low cost, high efficiency supplies. It offers a wide 4V-60V input range (7.5V minimum startup voltage) and can implement step-down, step-up, inverting and SEPIC topologies. The LT3724 includes Burst Mode operation, which reduces quiescent current below 100A and maintains high efficiency at light loads. An internal high voltage bias regulator allows for simple biasing and can be back driven to increase efficiency. Additional features include fixed frequency current mode control for fast line and load transient response; a gate driver capable of driving large N-channel MOSFETs; a precision undervoltage lockout function; 10A shutdown current; short-circuit protection; and a programmable soft-start function that directly controls output voltage slew rates at startup which limits inrush current, minimizes overshoot and facilitates supply sequencing.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and Burst Mode is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 5731694, 6498466, 6611131.
Wide Input Range: 4V to 60V Output Voltages up to 36V (Step-Down) Burst Mode(R) Operation: <100A Supply Current 10A Shutdown Supply Current 1.3% Reference Accuracy 200kHz Fixed Frequency Drives N-Channel MOSFET Programmable Soft-Start Programmable Undervoltage Lockout Internal High Voltage Regulator for Gate Drive Thermal Shutdown Current Limit Unaffected by Duty Cycle 16-Pin Thermally Enhanced TSSOP Package
APPLICATIONS
n n n n n n
Industrial Power Distribution 12V and 42V Automotive and Heavy Equipment High Voltage Single Board Systems Distributed Power Systems Avionics Telecom Power
TYPICAL APPLICATION
High Voltage Step-Down Regulator
VIN 30V TO 60V CIN 68F 1M
95
Efficiency and Power Loss vs Load Current
12 10 EFFICIENCY EFFICIENCY (%) 8 6 4 LOSS VIN = 48V 65 0.1 1 LOAD CURRENT (A) 0 10
3724 TA01b
VIN
BOOST LT3724 TG SW 0.22F 10 47H SS3H9 VCC PGND SENSE+ SENSE-
3724 TA01a
90
Si7852 0.025 VOUT 24V 75W COUT 330F
85 80 75 70
POWER LOSS (W)
SHDN 68.1k 200k CSS Burst_EN VFB 40.2k 680pF 120pF 4.99k 93.1k 1000pF VC SGND
+
1F
2
3724fd
1
LT3724 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW VIN NC SHDN CSS BURST_EN VFB VC SGND 1 2 3 4 5 6 7 8 17 16 BOOST 15 TG 14 SW 13 NC 12 VCC 11 PGND 10 SENSE+ 9 SENSE-
Input Supply Voltage (VIN) ......................... 65V to -0.3V Boosted Supply Voltage (BOOST) .............. 80V to -0.3V Switch Voltage (SW)(Note 8) ........................ 65V to -1V Differential Boost Voltage (BOOST to SW)...................................... 24V to -0.3V Bias Supply Voltage (VCC) .......................... 24V to -0.3V SENSE+ and SENSE- Voltages ................... 40V to -0.3V BURST_EN Voltage .................................... 24V to -0.3V VC, VFB, CSS, and SHDN Voltages................. 5V to -0.3V CSS and SHDN Pin Currents .....................................1mA Operating Junction Temperature Range (Notes 2, 3) LT3724E ............................................. -40C to 125C LT3724I .............................................. -40C to 125C LT3724MP.......................................... -55C to 125C Storage Temperature.............................. -65C to 150C Lead Temperature (Soldering, 10 sec) .................. 300C (SENSE+ to SENSE-)................................... 1V to -1V
FE PACKAGE 16-LEAD PLASTIC TSSOP TJMAX = 125C, JA = 40C/W, JC = 10C/W EXPOSED PAD IS SGND (PIN 17), MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LT3724EFE#PBF LT3724IFE#PBF LEAD BASED FINISH LT3724MPFE TAPE AND REEL LT3724EFE#TRPBF LT3724IFE#TRPBF TAPE AND REEL LT3724MPFE#TR PART MARKING 3724EFE 3724IFE PART MARKING* 3724MPFE PACKAGE DESCRIPTION 16-Lead Plastic TSSOP 16-Lead Plastic TSSOP PACKAGE DESCRIPTION 16-Lead Plastic TSSOP TEMPERATURE RANGE -40C to 125C -40C to 125C TEMPERATURE RANGE -55C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, SENSE - = SENSE+ = 10V, SGND = PGND = SW = 0V, unless otherwise noted.
SYMBOL VIN PARAMETER Operating Voltage Range (Note 4) Minimum Start Voltage UVLO Threshold (Falling) UVLO Threshold Hysteresis VIN Supply Current VIN Burst Mode Current VIN Shutdown Current VCC > 9V VBURST_EN = 0V, VFB = 1.35V VSHDN = 0V CONDITIONS
l l l
ELECTRICAL CHARACTERISTICS
MIN 4 7.5 3.65
TYP
MAX 60
UNITS V V V mV A A A
3724fd
3.8 670 20 20 10
3.95
IVIN
15
2
LT3724 ELECTRICAL CHARACTERISTICS
SYMBOL VBOOST PARAMETER Operating Voltage Range Operating Voltage Range (Note 5) UVLO Threshold (Rising) UVLO Threshold Hysteresis BOOST Supply Current (Note 6) BOOST Burst Mode Current BOOST Shutdown Current Operating Voltage Range (Note 5) Output Voltage UVLO Threshold (Rising) UVLO Threshold Hysteresis VCC Supply Current (Note 6) VCC Burst Mode Current VCC Shutdown Current Short-Circuit Current Error Amp Reference Voltage Feedback Input Current Enable Threshold (Rising) Threshold Hysteresis Common Mode Range Current Limit Sense Voltage Input Current (ISENSE+ + ISENSE-) Operating Frequency MP Grade VFB(SS) ISS gm AV VC IVC VTG tTG tTG(OFF) tTG(ON) ISW Soft-Start Disable Voltage Soft-Start Disable Hysteresis Soft-Start Capacitor Control Current Error Amp Transconductance Error Amp DC Voltage Gain Error Amp Output Range Error Amp Sink/Source Current Gate Drive Output On Voltage (Note 7) CLOAD = 3300pF Gate Drive Output Off Voltage CLOAD = 3300pF Gate Drive Rise/Fall Time Minimum Switch Off Time Minimum Switch On Time SW Pin Sink Current VSW = 2V
l l l l l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 20V, VCC = BOOST = BURST_EN = 10V, SHDN = 2V, SENSE - = SENSE+ = 10V, SGND = PGND = SW = 0V, unless otherwise noted.
CONDITIONS VBOOST - VSW VBOOST - VSW VBOOST - VSW VBURST_EN = 0V VSHDN = 0V Over Full Line and Load Range
l l l l
MIN
TYP
MAX 75 20
UNITS V V V mV mA A A
5 400 1.4 0.1 0.1 8 6.25 500 1.7 80 20 -55 1.231 25 1.3 0 140 1.35 120 150 400 2 -150 190 175 165 200 200 1.185 300 2 275 340 62 1.2 30 9.8 0.1 60 350 300 300
IBOOST VCC
20 8.3
V V V mV mA A A mA V V nA V mV V mV A A A
IVCC
l
VBURST_EN = 0V VSHDN = 0V
l
2.1
-30 1.224 1.215
VFB IFB VSHDN VSENSE ISENSE fSW
Measured at VFB Pin
l
1.238 1.245 1.4 36 175
VSENSE+ - VSENSE- VSENSE(CM) = 0V VSENSE(CM) = 2.5V VSENSE(CM) > 4V
210 220 225
kHz kHz kHz V mV A
VFB Rising
400
mhos dB V A V V ns ns
Zero Current to Current Limit
10% to 90% or 90% to 10%, CLOAD = 3300pF
500
ns mA
3724fd
3
LT3724 ELECTRICAL CHARACTERISTICS
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3724 includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 3: The LT3724E is guaranteed to meet performance specifications from 0C to 125C junction temperature. Specifications over the - 40C to 125C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3724I is guaranteed over the full -40C to 125C operating junction temperature range. The LT3724MP is 100% tested and guaranteed over the -55C to 125C operating junction temperature range. Note 4: VIN voltages below the start-up threshold (7.5V) are only supported when the VCC is externally driven above 6.5V. Note 5: Operating range is dictated by MOSFET absolute maximum VGS. Note 6: Supply current specification does not include switch drive currents. Actual supply currents will be higher. Note 7: DC measurement of gate drive output "ON" voltage is typically 8.6V. Internal dynamic bootstrap operation yields typical gate "ON" voltages of 9.8V during standard switching operation. Standard operation gate "ON" voltage is not tested but guaranteed by design. Note 8: The -1V absolute maximum on the SW pin is a transient condition. It is guaranteed by design and not subject to test.
TYPICAL PERFORMANCE CHARACTERISTICS
Shutdown Threshold (Rising) vs Temperature
1.38 SHUTDOWN THRESHOLD, FALLING (V) SHUTDOWN THRESHOLD, RISING (V) 1.37 1.36 1.35 1.34 1.33 1.32 -50 -25 1.26 1.25 1.24 1.23 1.22 1.21 1.20 -50 -25 VCC (V)
Shutdown Threshold (Falling) vs Temperature
8.2 8.1 8.0 7.9 7.8 7.7 7.6 0 25 50 75 TEMPERATURE (C) 100 125
VCC vs Temperature
ICC = 20mA
0 25 50 75 TEMPERATURE (C)
100
125
7.5 -50 -25
0 25 50 75 TEMPERATURE (C)
100
125
3724 G01
3724 G02
3724 G03
VCC vs ICC(LOAD)
8.2 8.1 8.0 VCC (V) VCC (V) 7.9 7.8 7.7 7.6 7.5 0 15 20 25 30 35 TA = 25C 8 7 6 5 4 3 9
VCC vs VIN
ICC = 20mA TA = 25C ICC CURRENT LIMIT (mA)
70
ICC Current Limit vs Temperature
60
50
40
30
5
10
4
5
6
7
8 VIN (V)
9
10
11
12
20 -50 -25
ICC (LOAD) (mA)
3724 G04
0 25 50 75 TEMPERATURE (C)
100
125
3724 G05
3724 G06
3724fd
4
LT3724 TYPICAL PERFORMANCE CHARACTERISTICS
VCC UVLO Threshold (Rising) vs Temperature
6.5 VCC UVLO THRESHOLD, RISING (V) 25
ICC vs VCC (SHDN = 0V)
TA = 25C ERROR AMP TRANSCONDUCTANCE (Mhos)
Error Amp Transconductance vs Temperature
350 345 340 335 330 325 320 -50 -25
6.4
20 15
6.2
ICC (A)
6.3
10 5
6.1
6.0 -50 -25
0 25 50 75 TEMPERATURE (C)
100
125
0
0
2
4
6
8
10 12 14 16 18 20 VCC (V)
3724 G08
50 25 75 0 TEMPERATURE (C)
100
125
3724 G07
3724 G09
400 300 I(SENSE+ + SENSE-) (A) 200 100 0
I(SENSE+ + SENSE-) vs VSENSE (CM)
TA = 25C OPERATING FREQUENCY (kHz)
Operating Frequency vs Temperature
230 220 210 200 190 180 170 -50 -25 1.234 1.233 ERROR AMP REFERENCE (V) 1.232 1.231 1.230 1.229 1.228 50 25 75 0 TEMPERATURE (C) 100 125
Error Amp Reference vs Temperature
-100 -200
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VSENSE (CM) (V)
3724 G10
1.227 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3724 G11
3724 G12
Maximum Current Sense Threshold vs Temperature
160 CURRENT SENSE THRESHOLD (mV) VIN UVLO THRESHOLD, RISING (V) 158 156 154 152 150 148 146 144 142 140 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125 4.54 4.52 4.50 4.48 4.46 4.44 4.42
VIN UVLO Threshold (Rising) vs Temperature
3.86 VIN UVLO THRESHOLD, FALLING (V) 50 25 75 0 TEMPERATURE (C) 100 125
VIN UVLO Threshold (Falling) vs Temperature
3.84
3.82
3.80
3.78
4.40 -50 -25
3.76 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3724 G13
3724 G14
3724 G15
3724fd
5
LT3724 PIN FUNCTIONS
VIN (Pin 1): The VIN pin is the main supply pin and should be decoupled to SGND with a low ESR capacitor located close to the pin. NC (Pin 2): No Connection. SHDN (Pin 3): The SHDN pin has a precision IC enable threshold of 1.35V (rising) with 120mV of hysteresis. It is used to implement an undervoltage lockout (UVLO) circuit. See Application Information section for implementing a UVLO function. When the SHDN pin is pulled below a transistor VBE (0.7V), a low current shutdown mode is entered, all internal circuitry is disabled and the VIN supply current is reduced to approximately 10A. Typical pin input bias current is <10A and the pin is internally clamped to 6V. CSS (Pin 4): The soft-start pin is used to program the supply soft-start function. The pin is connected to VOUT via a ceramic capacitor (CSS) and 200k series resistor. During start-up, the supply output voltage slew rate is controlled to produce a 2A average current through the soft-start coupling capacitor. Use the following formula to calculate CSS for a given output voltage slew rate: CSS = 2A(tSS/VOUT) See the application section for more information on setting the rise time of the output voltage during start-up. Shorting this pin to SGND disables the soft-start function. BURST_EN (Pin 5): The BURST_EN pin is used to enable or disable Burst Mode operation. Connect the BURST_EN pin to ground to enable the burst mode function. Connect the pin to VCC to disable the burst mode function. VFB (Pin 6): The output voltage feedback pin, VFB, is externally connected to the supply output voltage via a resistive divider. The VFB pin is internally connected to the inverting input of the error amplifier. In regulation, VFB is 1.231V. VC (Pin 7): The VC pin is the output of the error amplifier whose voltage corresponds to the maximum (peak) switch current per oscillator cycle. The error amplifier is typically configured as an integrator circuit by connecting an RC network from the VC pin to SGND. This circuit creates the dominant pole for the converter regulation control loop. Specific integrator characteristics can be configured to optimize transient response. Connecting a 100pF or greater high frequency bypass capacitor from this pin to ground is recommended. When Burst Mode operation is enabled (see Pin 5 description), an internal low impedance clamp on the VC pin is set at 100mV below the burst threshold, which limits the negative excursion of the pin voltage. Therefore, this pin cannot be pulled low with a low impedance source. If the VC pin must be externally manipulated, do so through a 1k series resistance. SGND (Pin 8, 17): The SGND pin is the low noise ground reference. It should be connected to the -VOUT side of the output capacitors. Careful layout of the PCB is necessary to keep high currents away from this SGND connection. See the Application Information section for helpful hints on PCB layout of grounds. SENSE- (Pin 9): The SENSE- pin is the negative input for the current sense amplifier and is connected to the VOUT side of the sense resistor for step-down applications. The sensed inductor current limit is set to 150mV across the SENSE inputs. SENSE+ (Pin 10): The SENSE+ pin is the positive input for the current sense amplifier and is connected to the inductor side of the sense resistor for step-down applications. The sensed inductor current limit is set to 150mV across the SENSE inputs. PGND (Pin 11): The PGND pin is the high-current ground reference for internal low side switch and the VCC regulator circuit. Connect the pin directly to the negative terminal of the VCC decoupling capacitor. See the Application Information section for helpful hints on PCB layout of grounds.
3724fd
6
LT3724 PIN FUNCTIONS
VCC (Pin 12): The VCC pin is the internal bias supply decoupling node. Use a low ESR 1F ceramic capacitor to decouple this node to PGND. Most internal IC functions are powered from this bias supply. An external diode connected from VCC to the BOOST pin charges the bootstrapped capacitor during the off-time of the main power switch. Back driving the VCC pin from an external DC voltage source, such as the VOUT output of the buck regulator supply, increases overall efficiency and reduces power dissipation in the IC. In shutdown mode this pin sinks 20A until the pin voltage is discharged to 0V. NC (Pin 13): No Connection. SW (Pin 14): In step-down applications the SW pin is connected to the cathode of an external clamping Schottky diode, the source of the power MOSFET and the inductor. The SW node voltage swing is from VIN during the on-time of the power MOSFET, to a Schottky voltage drop below ground during the off-time of the power MOSFET. In start-up and in operating modes where there is insufficient inductor current to freewheel the Schottky diode, an internal switch is turned on to pull the SW pin to ground so that the BOOST pin capacitor can be charged. Give careful consideration in choosing the Schottky diode to limit the negative voltage swing on the SW pin. TG (Pin 15): The TG pin is the bootstrapped gate drive for the top N-Channel MOSFET. Since very fast high currents are driven from this pin, connect it to the gate of the power MOSFET with a short and wide, typically 0.02" width, PCB trace to minimize inductance. BOOST (Pin 16): The BOOST pin is the supply for the bootstrapped gate drive and is externally connected to a low ESR ceramic boost capacitor referenced to SW pin. The recommended value of the BOOST capacitor, CBOOST, is 50 times greater than the total input capacitance of the topside MOSFET. In most applications 0.1F is adequate. The maximum voltage that this pin sees is VIN + VCC, ground referred. Exposed Pad (SGND) (Pin 17): The exposed leadframe is internally connected to the SGND pin. Solder the exposed pad to the PCB ground for electrical contact and optimal thermal performance.
3724fd
7
LT3724 FUNCTIONAL DIAGRAM
VIN UVLO (<4V) VIN VIN CIN 3.8V REGULATOR 1
8V VCC REGULATOR
VCC UVLO (<6V) INTERNAL SUPPLY RAIL
BST UVLO
BOOST 16 CBOOST BOOSTED SWITCH DRIVER
RA
-
SHDN
+
FEEDBACK REFERENCE 1.231V
DRIVE CONTROL
TG 15 SW 14 VCC 12 CVCC D2
M1
L1 D1
RSENSE
VFB
5 6
R1
R2
gm ERROR AMP
0.5V Q S R CURRENT SENSE COMPARATOR
VC 7 CC2 CC1 RC SOFT-START DISABLE/BURST ENABLE
~1V
1.185V CSS 4 CSS SGND 8 2A
8
-
+
-
BURST_EN
-
RB
+ + + - + -
BURST MODE OPERATION
3
NOL SWITCH LOGIC
VOUT COUT
D3 (OPTIONAL)
DRIVE CONTROL PGND 11
OSCILLATOR
SLOPE COMP GENERATOR
-
+
-
+
SENSE+ 10 SENSE- 9
3724 FD
3724fd
LT3724 OPERATIONS
(Refer to Functional Diagram)
The LT3724 is a PWM controller with a constant frequency, current mode control architecture. It is designed for low to medium power, switching regulator applications. Its high operating voltage capability allows it to step-up or down input voltages up to 60V without the need for a transformer. The LT3724 is used in nonsynchronous applications, meaning that a freewheeling rectifier diode (D1 of Function Diagram) is used instead of a bottom side MOSFET. For circuit operation, please refer to the Functional Diagram of the IC and Typical Application on the front page of the data sheet. The LT3800 is a similar part that uses synchronous rectification, replacing the diode with a MOSFET in a step-down application. Main Control Loop During normal operation, the external N-channel MOSFET switch is turned on at the beginning of each cycle. The switch stays on until the current in the inductor exceeds a current threshold set by the DC control voltage, VC, the output of the voltage control loop. The voltage control loop monitors the output voltage, via the VFB pin voltage, and compares it to an internal 1.231V reference. It increases the current threshold when the VFB voltage is below the reference voltage and decreases the current threshold when the VFB voltage is above the reference voltage. For instance, when an increase in the load current occurs, the output voltage drops causing the VFB voltage to drop relative to the 1.231V reference. The voltage control loop senses the drop and increases the current threshold. The peak inductor current is increased until the average inductor current equals the new load current and the output voltage returns to regulation. Current Limit/Short-Circuit The inductor current is measured with a series sense resistor (see the Typical Application on the front page). When the voltage across the sense resistor reaches the maximum current sense threshold, typically 150mV, the TG MOSFET driver is disabled for the remainder of that cycle. If the maximum current sense threshold is still exceeded at the beginning of the next cycle, the entire cycle is skipped. Cycle skipping keeps the inductor currents to a controlled value during a short-circuit, particularly when VIN is high. Setting the sense resistor value is discussed in the "Application Information" section.
VCC/Boosted Supply An internal VCC regulator provides VIN derived gate-drive power for start-up under all operating conditions with MOSFET gate charge loads up to 90nC. The regulator can operate continuously in applications with VIN voltages up to 60V, provided the VIN voltage and/or MOSFET gate charge currents do not create excessive power dissipation in the IC. Safe operating conditions for continuous regulator use are shown in Figure 1. In applications where these conditions are exceeded, VCC must be derived from an external source after start-up. The LT3724 regulator can, however, be used for "full time" use in applications where short-duration VIN transients exceed allowable continuous voltages.
70 60 50 VIN (V) 40 30 20 10 SAFE OPERATING AREA 0 20 40 60 80 MOSFET TOTAL GATE CHARGE (nC) 100
3724 F01
Figure 1. VCC Regulator Continuous Operating Conditions
For higher converter efficiency and less power dissipation in the IC, VCC can also be supplied from an external supply such as the converter output. When an external supply back drives the internal VCC regulator through an external diode and the VCC voltage is pulled to a diode above its regulation voltage, the internal regulator is disabled and goes into a low current mode. VCC is the bias supply for most of the internal IC functions and is also used to charge the bootstrapped capacitor (CBOOST) via an external diode. The external MOSFET switch is biased from the bootstrapped capacitor. While the external MOSFET switch is off, an internal BJT switch, whose collector is connected to the SW pin and emitter is connected to the PGND pin, is turned on to pull the SW node to PGND and recharge the bootstrap capacitor. The switch stays on until
3724fd
9
LT3724 OPERATIONS
(Refer to Functional Diagram)
either the start of the next cycle or until the bootstrapped capacitor is fully charged. MOSFET Driver The LT3724 contains a high speed boosted driver to turn on and off an external N-channel MOSFET switch. The MOSFET driver derives its power from the boost capacitor which is referenced to the SW pin and the source of the MOSFET. The driver provides a large pulse of current to turn on the MOSFET fast and minimize transition times. Multiple MOSFETs can be paralleled for higher current operation. To eliminate the possibility of shoot through between the MOSFET and the internal SW pull-down switch, an adaptive nonoverlap circuit ensures that the internal pull-down switch does not turn on until the gate of the MOSFET is below its turn on threshold. Low Current Operation (Burst Mode Operation) To increase low current load efficiency, the LT3724 is capable of operating in Linear Technology's proprietary Burst Mode operation where the external MOSFET operates intermittently based on load current demand. The Burst Mode function is disabled by connecting the BURST_EN pin to VCC and enabled by connecting the pin to SGND. When the required switch current, sensed via the VC pin voltage, is below 15% of maximum, Burst Mode operation is employed and that level of sense current is latched onto the IC control path. If the output load requires less than this latched current level, the converter will overdrive the output slightly during each switch cycle. This overdrive condition is sensed internally and forces the voltage on the VC pin to continue to drop. When the voltage on VC drops 150mV below the 15% load level, switching is disabled, and the LT3724 shuts down most of its internal circuitry, reducing total quiescent current to 100A. When the converter output begins to fall, the VC pin voltage begins to climb. When the voltage on the VC pin climbs back to the 15% load level, the IC returns to normal operation and
switching resumes. An internal clamp on the VC pin is set at 100mV below the output disable threshold, which limits the negative excursion of the pin voltage, minimizing the converter output ripple during Burst Mode operation. During Burst Mode operation, the VIN pin current is 20A and the VCC current is reduced to 80A. If no external drive is provided for VCC, all VCC bias currents originate from the VIN pin, giving a total VIN current of 100A. Burst current can be reduced further when VCC is driven using an output derived source, as the VCC component of VIN current is then reduced by the converter duty cycle ratio. Start-Up The following section describes the start-up of the supply and operation down to 4V once the step-down supply is up and running. For the protection of the LT3724 and the switching supply, there are internal undervoltage lockout (UVLO) circuits with hysteresis on VIN, VCC and VBOOST, as shown in the Electrical Characteristics table. Start-up and continuous operation require that all three of these undervoltage lockout conditions be satisfied because the TG MOSFET driver is disabled during any UVLO fault condition. In startup, for most applications, VCC is powered from VIN through the high voltage linear regulator of the LT3724. This requires VIN to be high enough to drive the VCC voltage above its undervoltage lockout threshold. VCC, in turn, has to be high enough to charge the BOOST capacitor through an external diode so that the BOOST voltage is above its undervoltage lockout threshold. There is an NPN switch that pulls the SW node to ground each cycle during the TG power MOSFET off-time, ensuring the BOOST capacitor is kept fully charged. Once the supply is up and running, the output voltage of the supply can backdrive VCC through an external diode. Internal circuitry disables the high voltage regulator to conserve VIN supply current. Output voltages that are too low or too high to backdrive VCC require additional circuitry such as a voltage doubler or linear regulator. Once VCC is backdriven from a supply other than VIN, VIN can be reduced to 4V with normal operation maintained.
3724fd
10
LT3724 OPERATIONS
Soft-Start The soft-start function controls the slew rate of the power supply output voltage during start-up. A controlled output voltage ramp minimizes output voltage overshoot, reduces inrush current from the VIN supply, and facilitates supply sequencing. A capacitor, CSS, connected between VOUT of the supply and the CSS pin of the IC, programs the slew rate. The capacitor provides a current to the CSS pin which is proportional to the dV/dt of the output voltage. The soft-start circuit overrides the control loop and adjusts the inductor current until the output voltage slew rate yields a 2A current through the soft-start capacitor. If the current is greater than 2A, then the current threshold set by the DC control voltage, VC, is decreased and the inductor current is lowered. This in turn lowers the output current and the output voltage slew rate is decreased. If the current is less than 2A, then the current threshold set by the DC control voltage, VC, is increased and the inductor current is raised. This in turn increases the output current and the output voltage slew rate is increased. Once the output voltage is within 5% of its regulation voltage, the soft-start circuit is disabled and the main control regulates the output. The soft-start circuit is reactivated when the output voltage drops below 70% of its regulation voltage. Slope/Antislope Compensation The IC incorporates slope compensation to eliminate potential subharmonic oscillations in the current control loop. The IC's slope compensation circuit imposes an
(Refer to Functional Diagram)
artificial ramp on the sensed current to increase the rising slope as duty cycle increases. Unfortunately, this additional ramp typically affects the sensed current value, thereby reducing the achievable current limit value by the same amount as the added ramp represents. As such, the current limit is typically reduced as the duty cycle increases. The LT3724, however, contains antislope compensation circuitry to eliminate the current limit reduction associated with slope compensation. As the slope compensation ramp is added to the sensed current, a similar ramp is added to the current limit threshold. The end result is that the current limit is not compromised so the LT3724 can provide full power regardless of required duty cycle. Shutdown The LT3724 includes a shutdown mode where all the internal IC functions are disabled and the VIN current is reduced to less than 10A. The shutdown pin can be used for undervoltage lockout with hysteresis, micropower shutdown or as a general purpose on/off control of the converter output. The shutdown function has two thresholds. The first threshold, a precision 1.23V threshold with 120mV of hysteresis, disables the converter from switching. The second threshold, approximately a 0.7V referenced to SGND, completely disables all internal circuitry and reduces the VIN current to less than 10A. See the Application Information section for more information.
3724fd
11
LT3724 APPLICATIONS INFORMATION
The basic LT3724 step-down (buck) application, shown in the Typical Application on the front page, converts a larger positive input voltage to a lower positive or negative output voltage. This Application Information section assists selection of external components for the requirements of the power supply. RSENSE Selection The current sense resistor, RSENSE, monitors the inductor current of the supply (See Typical Application on front page). Its value is chosen based on the maximum required output load current. The LT3724 current sense amplifier has a maximum voltage threshold of, typically, 150mV. Therefore, the peak inductor current is 150mV/RSENSE. The maximum output load current, IOUT(MAX), is the peak inductor current minus half the peak-to-peak ripple current, IL. Allowing adequate margin for ripple current and external component tolerances, RSENSE can be calculated as follows: 100mV RSENSE = IOUT(MAX) Typical values for RSENSE are in the range of 0.005 to 0.05. Inductor Selection The critical parameters for selection of an inductor are minimum inductance value, volt-second product, saturation current and/or RMS current. The minimum inductance value is calculated as follows: L VOUT * VIN(MAX) - VOUT fSW * VIN(MAX) * IL mode instability may occur at duty cycles greater than 50%. Lower values of IL require larger and more costly magnetics. A value of IL = 0.3 * IOUT(MAX) produces a 15% of IOUT(MAX) ripple current around the DC output current of the supply. Some magnetics vendors specify a volt-second product in their datasheet. If they do not, consult the magnetics vendor to make sure the specification is not being exceeded by your design. The volt-second product is calculated as follows: Volt-second (sec) = (VIN(MAX) - VOUT ) * VOUT VIN(MAX) * fSW
The magnetics vendors specify either the saturation current, the RMS current or both. When selecting an inductor based on inductor saturation current, use the peak current through the inductor, IOUT(MAX) + IL/2. The inductor saturation current specification is the current at which the inductance, measured at zero current, decreases by a specified amount, typically 30%. When selecting an inductor based on RMS current rating, use the average current through the inductor, IOUT(MAX). The RMS current specification is the RMS current at which the part has a specific temperature rise, typically 40C, above 25C ambient. After calculating the minimum inductance value, the voltsecond product, the saturation current and the RMS current for your design, select an off-the-shelf inductor. A list of magnetics vendors can be found at www.linear.com, or contact the Linear Technology Application Department. For more detailed information on selecting an inductor, please see the "Inductor Selection" section of Linear Technology Application Note 44. Step-Down Converter: MOSFET Selection The selection criteria of the external N-channel standard level power MOSFET include on resistance(RDS(ON)), reverse transfer capacitance (CRSS), maximum drain source voltage (VDSS), total gate charge (QG), and maximum continuous drain current.
3724fd
fSW is the switch frequency (200kHz). The typical range of values for IL is (0.2 * IOUT(MAX)) to (0.5 * IOUT(MAX)), where IOUT(MAX) is the maximum load current of the supply. Using IL = 0.3 * IOUT(MAX) yields a good design compromise between inductor performance versus inductor size and cost. Higher values of IL will increase the peak currents, requiring more filtering on the input and output of the supply. If IL is too high, the slope compensation circuit is ineffective and current
12
LT3724 APPLICATIONS INFORMATION
For maximum efficiency, minimize RDS(ON) and CRSS. Low RDS(ON) minimizes conduction losses while low CRSS minimizes transition losses. The problem is that RDS(ON) is inversely related to CRSS. Balancing the transition losses with the conduction losses is a good idea in sizing the MOSFET. Select the MOSFET to balance the two losses. Calculate the maximum conduction losses of the MOSFET: V PCOND = (IOUT(MAX) )2 OUT (RDS(ON) ) VIN Note that RDS(ON) has a large positive temperature dependence. The MOSFET manufacturer's data sheet contains a curve, RDS(ON) vs Temperature. Calculate the maximum transition losses: PTRAN = (k)(VIN)2 (IOUT(MAX))(CRSS)(fSW) where k is a constant inversely related to the gate driver current, approximated by k = 2 for LT3724 applications. The total maximum power dissipation of the MOSFET is the sum of these two loss terms: PFET(TOTAL) = PCOND + PTRAN To achieve high supply efficiency, keep the PFET(TOTAL) to less than 3% of the total output power. Also, complete a thermal analysis to ensure that the MOSFET junction temperature is not exceeded. TJ = TA + PFET(TOTAL) * JA where JA is the package thermal resistance and TA is the ambient temperature. Keep the calculated TJ below the maximum specified junction temperature, typically 150C. Note that when VIN is high, the transition losses may dominate. A MOSFET with higher RDS(ON) and lower CRSS may provide higher efficiency. MOSFETs with higher voltage VDSS specification usually have higher RDS(ON) and lower CRSS. Choose the MOSFET VDSS specification to exceed the maximum voltage across the drain to the source of the MOSFET, which is VIN(MAX) plus any additional ringing on the switch node. Ringing on the switch node can be greatly reduced with good PCB layout and, if necessary, an RC snubber. The internal VCC regulator operating range limits the maximum total MOSFET gate charge, QG, to 90nC. The QG vs VGS specification is typically provided in the MOSFET data sheet. Use QG at VGS of 8V. If VCC is back driven from an external supply, the MOSFET drive current is not sourced from the internal regulator of the LT3724 and the QG of the MOSFET is not limited by the IC. However, note that the MOSFET drive current is supplied by the internal regulator when the external supply back driving VCC is not available such as during startup or short-circuit. The manufacturer's maximum continuous drain current specification should exceed the peak switch current, IOUT(MAX) + IL/2. During the supply startup, the gate drive levels are set by the VCC voltage regulator, which is approximately 8V. Once the supply is up and running, the VCC can be back driven by an auxiliary supply such as VOUT. It is important not to exceed the manufacturer's maximum VGS specification. A standard level threshold MOSFET typically has a VGS maximum of 20V. Step-Down Converter: Rectifier Selection The rectifier diode (D1 on the Functional Diagram) in a buck converter generates a current path for the inductor current when the main power switch is turned off. The rectifier is selected based upon the forward voltage, reverse voltage and maximum current. A Schottky diode is recommended. Its low forward voltage yields the lowest power loss and highest efficiency. The maximum reverse voltage that the diode will see is VIN(MAX). In continuous mode operation, the average diode current is calculated at maximum output load current and maximum VIN: IDIODE(AVG) =IOUT(MAX) VIN(MAX) - VOUT VIN(MAX)
To improve efficiency and to provide adequate margin for short-circuit operation, a diode rated at 1.5 to 2 times the maximum average diode current, IDIODE(AVG), is recommended.
3724fd
13
LT3724 APPLICATIONS INFORMATION
Step-Down Converter: Input Capacitor Selection A local input bypass capacitor is required for buck converters because the input current is pulsed with fast rise and fall times. The input capacitor selection criteria are based on the bulk capacitance and RMS current capability. The bulk capacitance will determine the supply input ripple voltage. The RMS current capability is used to keep from overheating the capacitor. The bulk capacitance is calculated based on maximum input ripple, VIN: CIN(BULK) = IOUT(MAX) * VOUT VIN * fSW * VIN(MIN) Step-Down Converter: Output Capacitor Selection The output capacitance, COUT , selection is based on the design's output voltage ripple, VOUT , and transient load requirements. VOUT is a function of IL and the COUT ESR. It is calculated by: 1 VOUT = IL * ESR + (8 * fSW * COUT ) The maximum ESR required to meet a VOUT design requirement can be calculated by: ESR(MAX) = (VOUT )(L)(fSW ) V VOUT * 1- OUT VIN(MAX)
VIN is typically chosen at a level acceptable to the user. 100mV-200mV is a good starting point. Aluminum electrolytic capacitors are a good choice for high voltage, bulk capacitance due to their high capacitance per unit area. The capacitor's RMS current is:
ICIN(RMS) = IOUT VOUT (VIN - VOUT ) (VIN )2
Worst-case VOUT occurs at highest input voltage. Use paralleled multiple capacitors to meet the ESR requirements. Increasing the inductance is an option to lower the ESR requirements. For extremely low VOUT, an additional LC filter stage can be added to the output of the supply. Application Note 44 has some good tips on sizing an additional output filter. Output Voltage Programming A resistive divider sets the DC output voltage according to the following formula: V R2 = R1 OUT - 1 1.231V The external resistor divider is connected to the output of the converter as shown in Figure 2. Tolerance of the feedback resistors will add additional error to the output voltage. Example: VOUT = 12V; R1 = 10k 12V R2 = 10k - 1 = 87.48k - use 86.6k 1% 1.231V
If applicable, calculate it at the worst case condition, VIN = 2VOUT. The RMS current rating of the capacitor is specified by the manufacturer and should exceed the calculated ICIN(RMS). Due to their low ESR (Equivalent Series Resistance), ceramic capacitors are a good choice for high voltage, high RMS current handling. Note that the ripple current ratings from aluminum electrolytic capacitor manufacturers are based on 2000 hours of life. This makes it advisable to further derate the capacitor or to choose a capacitor rated at a higher temperature than required. The combination of aluminum electrolytic capacitors and ceramic capacitors is an economical approach to meeting the input capacitor requirements. The capacitor voltage rating must be rated greater than VIN(MAX). Multiple capacitors may also be paralleled to meet size or height requirements in the design. Locate the capacitor very close to the MOSFET switch and use short, wide PCB traces to minimize parasitic inductance.
3724fd
14
LT3724 APPLICATIONS INFORMATION
L1 VOUT R2 VFB PIN R1
3724 F03 3724 F02
VSUPPLY RA SHDN PIN RB COUT
Figure 2. Output Voltage Feedback Divider
Figure 3. Undervoltage Lockout Circuit
The VFB pin input bias current is typically 25nA, so use of extremely high value feedback resistors could cause a converter output that is slightly higher than expected. Bias current error at the output can be estimated as: VOUT(BIAS) = 25nA * R2 Supply UVLO and Shutdown The SHDN pin has a precision voltage threshold with hysteresis which can be used as an undervoltage lockout threshold (UVLO) for the power supply. Undervoltage lockout keeps the LT3724 in shutdown until the supply input voltage is above a certain voltage programmed by the user. The hysteresis voltage prevents noise from falsely tripping UVLO. Resistors are chosen by first selecting RB. Then: VSUPPLY(ON) RA = RB * - 1 1.35V VSUPPLY(ON) is the input voltage at which the undervoltage lockout is disabled and the supply turns on. Example: Select RB = 49.9k, VSUPPLY(ON) = 14.5V (based on a 15V minimum input voltage) 14.5V RA = 49.9k * -1 1.35V = 486.1k (499k resistor is selected) If low supply current in standby mode is required, select a higher value of RB. The supply turn off voltage is 9% below turn on. In the example the VSUPPLY(OFF) would be 13.2V.
If additional hysteresis is desired for the enable function, an external positive feedback resistor can be used from the LT3724 regulator output. The shutdown function can be disabled by connecting the SHDN pin to the VIN through a large value pull-up resistor. This pin contains a low impedance clamp at 6V, so the SHDN pin will sink current from the pull-up resistor(RPU): I SHDN = VIN - 6V RPU
Because this arrangement will clamp the SHDN pin to the 6V, it will violate the 5V absolute maximum voltage rating of the pin. This is permitted, however, as long as the absolute maximum input current rating of 1mA is not exceeded. Input SHDN pin currents of <100A are recommended: a 1M or greater pull-up resistor is typically used for this configuration. Soft-Start The soft-start function forces the programmed slew rate while the converter output rises to 95% of regulation, which corresponds to 1.185V on the VFB pin. Once 95% regulation is achieved, the soft-start circuit is disabled. The soft-start circuit will re-enable when the VFB pin drops below 70% of regulation, which corresponds to 300mV of control hysteresis on the VFB pin. This allows for a controlled recovery from a "brown-out" condition.
CSS1 VOUT A LT3724 RSS CSS
3724 F04
Figure 4.Soft-Start Circuit
3724fd
15
LT3724 APPLICATIONS INFORMATION
The desired soft-start rise time (tSS) is programmed via a programming capacitor CSS1, using a value that corresponds to 2A average current during the soft-start interval. This capacitor value follows the relation: CSS1 = 2 * 10 -6 * t SS VOUT
TIME, 250s/DIV
3724 F05
VOUT
VOUT(SS) V(VC)
RSS is typically set to 200k for most applications. Considerations for Low-Voltage Output Applications The LT3724 CSS pin biases to 220mV during the soft-start cycle, and this voltage is increased at Figure 4 node "A" by the 2A signal current through RSS, so the output has to reach this value before the soft-start function is engaged. The value of this output soft-start startup voltage offset (VOUT(SS)) follows the relation: VOUT(SS) = 220mV + RSS * 2 * 10- 6 Which is typically 0.64V for RSS = 200k. In some low voltage output applications, it may be desirable to reduce the value of this soft-start startup voltage offset. This is possible by reducing the value of RSS. With reduced values of RSS, the signal component caused by voltage ripple on the output must be minimized for proper soft-start operation. Peak-to-peak output voltage ripple (VOUT) will be imposed on node "A" through the capacitor CSS1. The value of RSS can be set using the following equation: RSS = 1.3 * 10 - 6 VOUT
Figure 5. Soft-Start Characteristic Showing Excessive Ripple Component
VOUT
VOUT(SS) V(VC)
TIME, 250s/DIV
3724F06
Figure 6. Desirable Soft-Start Characteristic
This is typically accomplished by increasing output capacitance and/or reducing output capacitor ESR. External Current Limit Foldback Circuit An additional startup voltage offset can occur during the period before the LT3724 soft-start circuit becomes active. Before the soft-start circuit throttles back the VC pin in response to the rising output voltage, current as high as the peak programmed current limit (IMAX) can flow in the switched inductor. Switching will stop once the softstart circuit takes hold and reduces the voltage on the VC pin, but the output voltage will continue to increase as the stored energy in the inductor is transferred to the output capacitor. With IMAX in the inductor, the resulting leading-edge rise on VOUT due to energy stored in the inductor follows the relation: VOUT =IMAX L * COUT
1/2
It is important to use low ESR output capacitors for LT3724 voltage converter designs to minimize this ripple voltage component. A design with an excessive ripple component can be evidenced by observing the VC pin during the start cycle. The soft-start cycle should be evaluated to verify that the reduced RSS value allows operation without excessive modulation of the VC pin before finalizing the design. If VC pin has an excessive ripple component during the soft-start cycle, converter output ripple should be reduced.
3724fd
16
LT3724 APPLICATIONS INFORMATION
Inductor current typically does not reach IMAX in the few cycles that occur before soft-start becomes active, but can with high input voltages or small inductors, so the above relation is useful as a worst-case scenario. This energy transfer increase in output voltage is typically small, but for some low voltage applications with relatively small output capacitors, it can become significant. The voltage rise can be reduced by increasing output capacitance, which puts additional limitations on COUT for these low voltage supplies. Another approach is to add an external current limit foldback circuit which reduces the value of IMAX during start-up. An external current limit foldback circuit can be easily incorporated into an LT3724 DC/DC converter application by placing a 1N4148 diode and a 47k resistor from the converter output (VOUT) to the LT3724's VC pin. This limits the peak current to 0.25 * IMAX when VOUT = 0V. A current limit foldback circuit also has the added advantage of providing reduced output current in the DC/DC converter during short-circuit fault conditions, so a foldback circuit may be useful even if the soft-start function is disabled. If the soft-start circuit is disabled by shorting the CSS pin to ground, the external current limit foldback circuit must be modified by adding an additional diode and resistor. The 2-diode, 2-resistor network shown also provides 0.25 * IMAX when VOUT = 0V.
VC 1N4148 39k VC 1N4148 1N4148 27k
VOUT
3724 F07
Figure 8. Current Limit Foldback Circuit for Applications that have Soft-Start Disabled (CSS Pin Shorted to SGND)
Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. Express percent efficiency as: % Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are individual loss terms as a percentage of input power. Although all dissipative elements in the circuit produce losses, four main contributors usually account for most of the losses in LT3724 circuits: 1. LT3724 VIN and VCC current loss 2. I2R conduction losses 3. MOSFET transition loss 4. Schottky diode conduction loss 1. The VIN and VCC currents are the sum of the quiescent currents of the LT3724 and the MOSFET drive currents. The quiescent currents are in the LT3724 Electrical Characteristics table. The MOSFET drive current is a result of charging the gate capacitance of the power MOSFET each cycle with a packet of charge, QG. QG is found in the MOSFET data sheet. The average charging current is calculated as QG * fSW . The power loss term due to these currents can be reduced by backdriving VCC with a lower voltage than VIN such as VOUT.
47k VOUT
3724 F03
Figure 7. Current Limit Foldback Circuit for Applications that use Soft-Start
3724fd
17
LT3724 APPLICATIONS INFORMATION
2. I2R losses are calculated from the DC resistances of the MOSFET, the inductor, the sense resistor, and the input and output capacitors. In continuous conduction mode the average output current flows through the inductor and RSENSE but is chopped between the MOSFET and the Schottky diode. The resistances of the MOSFET (RDS(ON)) and the RSENSE multiplied by the duty cycle can be summed with the resistances of the inductor and RSENSE to obtain the total series resistance of the circuit. The total conduction power loss is proportional to this resistance and usually accounts for between 2% to 5% loss in efficiency. 3. Transition losses of the MOSFET can be substantial with input voltages greater than 20V. See MOSFET Selection section. 4. The Schottky diode can be a major contributor of power loss especially at high input to output voltage ratios (low duty cycles) where the diode conducts for the majority of the switch period. Lower Vf reduces the losses. Note that oversizing the diode does not always help because as the diode heats up the Vf is reduced and the diode loss term is decreased. I2R losses and the Schottky diode loss dominate at high load currents. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss in efficiency. PCB Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation. These items are illustrated graphically in the layout diagram of Figure 9. 1. Keep the signal and power grounds separate. The signal ground consists of the LT3724 SGND pin, the exposed pad on the backside of the LT3724 IC and the (-) terminal of VOUT. The signal ground is the quiet ground and does not contain any high, fast currents. The power ground consists of the Schottky diode anode, the (-) terminal of the input capacitor, and the ground return of the VCC capacitor. This ground has very fast high currents and is considered the noisy ground. The two grounds are connected to each other only at the (-) terminal of VOUT. 2. Use short wide traces in the loop formed by the MOSFET, the Schottky diode and the input capacitor to minimize high frequency noise and voltage stress from parasitic inductance. Surface mount components are preferred. 3. Connect the VFB pin directly to the feedback resistors independent of any other nodes, such as the SENSE- pin. Connect the feedback resistors between the (+) and (-) terminals of COUT. Locate the feedback resistors in close proximity to the LT3724 to keep the high impedance node, VFB, as short as possible. 4. Route the SENSE- and SENSE+ traces together and keep as short as possible. 5. Locate the VCC and BOOST capacitors in close proximity to the IC. These capacitors carry the MOSFET driver's high peak currents. Place the small signal components away from high frequency switching nodes (BOOST, SW, and TG). In the layout shown in Figure 9, place all the small signal components on one side of the IC and all the power components on the other. This helps to keep the signal and power grounds separate. 6. A small decoupling capacitor (100pF) is sometimes useful for filtering high frequency noise on the feedback and sense nodes. If used, locate as close to the IC as possible. 7. The LT3724 packaging will efficiently remove heat from the IC through the exposed pad on the backside of the part. The exposed pad is soldered to a copper footprint on the PCB. Make this footprint as large as possible to improve the thermal resistance of the IC case to ambient air. This helps to keep the LT3724 at a lower temperature. 8. Make the trace connecting the gate of MOSFET M1 to the TG pin of the LT3724 short and wide.
3724fd
18
LT3724 APPLICATIONS INFORMATION
VIN+
RA
1
VIN
BOOST LT3724 TG SW 17 VCC PGND SENSE+ SENSE-
16 15 14
CBOOST
CIN M1 VIN- L1 RSENSE
RB
3 4
SHDN CSS
+
CSS
RCSS
D2 12 11 10 9 CVCC D3 D1 COUT VOUT
5 6 7
BURST_EN VFB VC SGND
R2 R1 CC1
RC CC2
8
-
3724 F06
Figure 9. LT3724 Layout Diagram (See PCB Layout Checklist).
Minimum On-Time Considerations (Step-Down Converters) Minimum on-time (tTG(ON)) is the least amount of time that the LT3724 is capable of turning the MOSFET on and then off again. It is determined by internal timing delays and the gate charge of the MOSFET. Applications with high input to output differential voltages operate at low duty cycles and may approach this minimum on-time, typically 300nS. The LT3724 switching frequency is internally set to
200kHz, therefore, the minimum duty cycle of the MOSFET switch is 6%. When the duty cycle needs to be less than 6% the output will stay regulated, but cycle skipping may occur. Cycle skipping results in an increase in inductor ripple current. If it is important that cycle skipping does not occur, follow this guideline which takes into account worst case fSW and tTG(ON): VIN(MAX) 9 * VOUT This is only an issue for supplies with VOUT < 7V.
3724fd
19
LT3724 TYPICAL APPLICATIONS
12V to 24V/50W Boost (Step-Up) Converter
D1 BAV99 VIN 8V TO16V 1 CIN 33F x2 25V C1 1500pF 0.1F 25V RCSS 200k R3 4.7M 3 4 5 6 7 R1 10k C2 120pF R6 40.2k C3 4700pF 8 VIN BOOST LT3724 SHDN CSS BURST_EN VFB VC SGND VCC PGND SENSE+ SENSE- 12 11 10 9 C4 1F 25V M1 COUT1 330F 35V COUT2 2.2F x3 50V TG SW 16 15 14 L1 10H D2 SBM540 RSENSE 0.015
VOUT 24V AT 50W
R2 187k
3724 TA02
CIN = SANYO, 25SVP33M L1 = VISHAY, IHLP-5050FD-011 M1 = SILICONIX, Si7370DP COUT1 = SANYO, 35CV330AXA COUT2 = TDK, C4532X7R1H225K D2 = DIODESINC., SBM540 RSENSE = IRC LRF2512-01-R0I5-F
Efficiency and Power Loss vs Load Current
100 98 EFFICIENCY (%) 96 94 92 90 88 LOSS VIN = 12V 3.0 2.5 2.0 1.5 1.0 0.5 0 10
3724 F08
VIN = 16V VIN = 12V
POWER LOSS (W)
VIN = 8V
0.1
1 LOAD CURRENT (A)
3724fd
20
LT3724 TYPICAL APPLICATIONS
High Voltage LED Driver with Dimmer Control
LED VIN 8V TO 60V CIN 22F 1 R1 4.7M VIN BOOST LT3724 SHDN CSS BURST_EN VFB VC SGND VCC PGND SENSE+ SENSE- 12 11 10 9 TG SW 16 15 14 CVCC 1F 16V ADJUST ILED: 0.15V ILED = RSENSE RSENSE 0.5
3724 TA03
L1 300H C1 (OPTIONAL) D1 B170 M1 ZXMN10A07F
OPTIONAL DIMMER CONTROL 1kHz
3 4 5 6
M2 2N7002
C1 = OPTIONAL TO REDUCE LED RIPPLE CURRENT CIN = TDK, C4532X7R2A225K D1 = DIODESINC., B170 M1 = ZETEX, ZXMN10A07F RSENSE = VISHAY, WSL2010R0150FEA L1 = COILTRONICS, CTX300-4
7 C1 100pF 8
3724fd
21
LT3724 TYPICAL APPLICATIONS
4.5V to 20V Input to 12V at 25W Output SEPIC Converter with 60V Input Transient Capability
VIN 4.5V TO 20V TO 60V TRANSIENT CIN1 22F 2x 25V CIN2 25V 1F RA 100k
1 3
VIN SHDN
BOOST TG LT3724 SW VCC PGND SENSE+ SENSE-
16 15 14 12 11 10 9
D1B GSD2004 C7 0.1F
L1 20H M1
*
C5 22F 3x 25V
C1 390pF R2 130k
RB 49.9k
R3 200k
D2
VOUT 12V AT 25W
4 5 6 7
CSS
BURST_EN VFB VC SGND
R6 10 C4 1F 25V C6 56pF R7 10 D1A GSD2004
L1 20H RSENSE 0.010
*
COUT1 330F 16V COUT2 22F 25V
3724 TA07a
R1 14.7k
R4 47k
C2 120pF
R5 40.2k C3 680pF
8
D3 D1N4148
C5, CIN1, COUT2 = TDKC453X7R1E226M COUT1 = SANYO, OS-CON 16SVP330M D2 = ON SEMI, MBRD660 L1 = COILCRAFT VERSAPAC VP5-D83 M1 = VISHAY, Si7852DP
Efficiency and Power Loss vs Load Current
92 91 90 EFFICIENCY (%) 89 88 87 86 85 1 LOAD CURRENT (A) LOSS VIN = 15V VIN = 10V VIN = 20V VIN = 15V 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 10
3724 TA07b
POWER LOSS (W)
0.1
3724fd
22
LT3724 TYPICAL APPLICATIONS
12V Step-Down with VCC Back Driven from VOUT and Ceramic Capacitor in Output Filter
VIN 15V TO 60V CIN 100F 100V
+
2.2F x2 100V R3 49.9k C1 3300pF
R2 499k
1 3
VIN SHDN
BOOST TG LT3724 SW
16 15 14 D2A BAV99 C4 1F 16V R7 20
C6 0.1F 16V
M1 Si7852DP
RCSS 200k
4 5 6 7
CSS BURST_EN VFB VC SGND VCC PGND 12 11
R4 130k
L1 47H D1
VOUT 12V AT 50W RSENSE 0.020 COUT 33F x3 16V
10 SENSE+ SENSE- 9
D2B BAV99
R5 14.7k
C2 120pF
R6 15k C3 680pF
8
CIN: TDK, C4532X7R2A225MT COUT: TDK, C4532X7R1C336MT D1: DIODESINC., PDS5100H L1: COEV DU1971-470M M1: VISHAY Si7852DP
3724 TA04
3724fd
23
LT3724 PACKAGE DESCRIPTION
(Reference LTC DWG # 05-08-1663 Rev I) Exposed Pad Variation BC
FE Package 16-Lead Plastic TSSOP (4.4mm)
3.58 (.141)
4.90 - 5.10* (.193 - .201) 3.58 (.141) 16 1514 13 12 11 10 9
0.48 (.019) REF
6.60 0.10 4.50 0.10
SEE NOTE 4
2.94 (.116) 0.45 0.05 1.05 0.10 0.65 BSC
DETAIL B
6.40 2.94 (.252) (.116) BSC
0.51 (.020) REF DETAIL B IS THE PART OF THE LEAD FRAME FEATURE FOR REFERENCE ONLY NO MEASUREMENT PURPOSE
RECOMMENDED SOLDER PAD LAYOUT
12345678 1.10 (.0433) MAX
0 - 8
4.30 - 4.50* (.169 - .177)
0.25 REF
0.09 - 0.20 (.0035 - .0079)
0.50 - 0.75 (.020 - .030)
0.65 (.0256) BSC
NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE
0.195 - 0.30 (.0077 - .0118) TYP
0.05 - 0.15 (.002 - .006)
FE16 (BC) TSSOP REV I 1210
4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE
3724fd
24
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
LT3724 REVISION HISTORY
REV D DATE 3/11 DESCRIPTION Deleted last paragraph of Description Minor text edits made to SW and BOOST pin descriptions in Pin Functions section Minor text edits made to Main Control Loop and Current Limit/Short Circuit sections in Operations Revised High Voltage LED Driver with Dimmer Control in Typical Applications Revised Typical Application drawing and Related Parts list
(Revision history begins at Rev D)
PAGE NUMBER 1 7 9 21 24
3724fd
25
LT3724 TYPICAL APPLICATION
Inverting -12V 1.5A Converter
VIN 18V TO 36V R3 2M 0.1F 3 4 R1 88.7k R2 10.2k CSS 1000pF RCSS 200k 6 R6, 40.2k CC2 680pF CC1 120pF 7 8 SHDN CSS VCC VFB VC GND PGND SENSE+ 1 VIN BOOST LT3724 TG SW 16 15 14 12 11 10 D1A 1F 16V D1B 0.1F 16V M1 L1 47H D2 VOUT -12V 1.5A RSENSE 0.040 COUT1 330F 16V
+
CIN1 220F 50V
D1 = BAV99 D2 = ON SEMI, MBRD350 L1 = COEV, DU1311-470M M1 = VISHAY, Si7370DP CIN1 = SANYO, 50CV220KX COUT1 = SANYO, 16SVP330M
3724 TA05
RELATED PARTS
PART NUMBER LT3845A LTC3891 LT3844 LT3741 LTC3824 LTC3834/LTC3834-1 LTC3835/LTC3835-1 LTC3859 DESCRIPTION 60V, Low IQ, High Voltage Synchronous Step-Down DC/DC Controller 60V, Low IQ, High Voltage Synchronous Step-Down DC/DC Controller 60V, Low IQ, Single Output Step-Down DC/DC Controller High Power, Constant Current, Constant Voltage, Step-Down Controller 60V, Low IQ, Step-Down DC/DC Controller with 100% Duty Cycle Low IQ, Single Output Synchronous Step-Down DC/DC Controller with 99% Duty Cycle Low IQ, Triple Output Buck/Buck/Boost Synchronous DC/DC Controller COMMENTS Adjustable Fixed Frequency 100kHz to 500kHz, 4V VIN 60V, 1.23V VOUT 36V, IQ = 120A, TSSOP-16 Phase-Lockable Fixed Frequency 50kHz to 900kHz, 4V VIN 60V, 0.8V VOUT 24V, IQ = 50A Synchronizable Fixed Frequency 50kHz to 600kHz, 4V VIN 60V, 1.23V VOUT 36V, IQ = 120A, TSSOP-16 Fixed 200kHz to 1MHz Operating Frequency, 6% Current Regulation, 6V VIN 36V, VOUT Up to (VIN - 2V) Selectable Fixed Frequency 200kHz to 600kHz, 4V VIN 60V, 0.8V VOUT VIN, IQ = 40A, MSOP-10E Phase-Lockable Fixed Frequency 140kHz to 650kHz, 4V VIN 36V, 0.8V VOUT 10V, IQ = 30A/80A All Outputs Remain in Regulation Through Cold Crank 2.5V VIN 38V, VOUT(BUCKS) Up to 24V, VOUT(BOOST) Up to 60V, IQ = 55A
26 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2005
+
9 SENSE-
3724fd LT 0311 REV D * PRINTED IN USA

▲Up To Search▲

Price & Availability of LT3845A

	To Download LT3845A Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .