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LTC3803-5 Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT
FEATURES

DESCRIPTIO
VIN and VOUT Limited Only by External Components 4.8V Undervoltage Lockout Threshold Operating Junction Temperature from -40C to 150C Adjustable Slope Compensation Internal Soft-Start Constant Frequency 200kHz Operation 1.5% Reference Accuracy Current Mode Operation for Excellent Line and Load Transient Response No Minimum Load Requirement Low Quiescent Current: 240A Low Profile (1mm) SOT-23 Package
The LTC(R)3803-5 is a constant frequency current mode flyback controller optimized for driving 4.5V and 6V-rated N-channel MOSFETs in high input voltage applications. The LTC3803-5 operates from inputs as low as 5V. Constant frequency operation is maintained down to very light loads, resulting in less low frequency noise generation over a wide range of load currents. Slope compensation can be programmed with an external resistor. The LTC3803-5 provides 1.5% output voltage accuracy and consumes only 240A of quiescent current. Groundreferenced current sensing allows LTC3803-5-based converters to accept input supplies beyond the LTC3803-5's absolute maximum VCC. For simplicity, the LTC3803-5 can be powered from a high VIN through a resistor, due to its internal 8V shunt regulator. An internal undervoltage lockout shuts down the IC when the input voltage falls below 3.2V, guaranteeing at least 3.2V of gate drive to the external MOSFET. The LTC3803-5 is available in a low profile (1mm) 6-lead SOT-23 (ThinSOTTM) package.
APPLICATIO S

42V and 12V Automotive Power Supplies Telecom Power Supplies Auxiliary/Housekeeping Power Supplies Power Over Ethernet
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. ThinSOT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
VIN 6V TO 50V PDZ9.1B 22k
Dual Output Wide Input Range Converter
VPH5-0155 1F 100V 3x MMBTA42 10nF LTC3803-5 ITH/RUN NGATE GND 8.06k VFB VCC SENSE 0.1F 57.6k ALL CAPACITORS ARE X7R, TDK
38035 TA01
10MQ100N 13V/0.3A 20mA MIN LOAD
90 85 VIN = 12V
EFFICIENCY (%)
22F 10V 1F 100V 1F 100V
80 VIN = 24V 75 70 VIN = 48V 65 60 VIN = 12V 0 2 6 8 4 OUTPUT POWER (W) 10
7.5k
PHM25NQ10T
4.7k B3100 0.012
6.5V/1.2A 47F 10V
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Efficiency and Power Loss vs Output Power
VIN = 8V 3.0 2.5
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POWER LOSS (W)
38035f
2.0 1.5 1.0 0.5 0 12
38035 TA01b
1
LTC3803-5
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW ITH/RUN 1 GND 2 VFB 3 6 NGATE 5 VCC 4 SENSE
VCC to GND (Current Fed) .................... 25mA into VCC* NGATE Voltage ......................................... - 0.3V to VCC VFB, ITH/RUN Voltages ..............................- 0.3V to 3.5V SENSE Voltage ........................................... - 0.3V to 1V NGATE Peak Output Current (<10s) ........................ 1A Operating Junction Temperature Range (Note 2) LTC3803E-5 ....................................... - 40C to 85C LTC3803H-5 (Note 3) ....................... - 40C to 150C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
*LTC3803-5 internal clamp circuit self regulates VCC voltage to 8V.
ORDER PART NUMBER LTC3803HS6-5 LTC3803ES6-5 S6 PART MARKING LTBMH LTBPF
S6 PACKAGE 6-LEAD PLASTIC TSOT-23
TJMAX = 150C, JA = 165C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL VTURNON VTURNOFF VHYST VCLAMP1mA VCLAMP25mA ICC PARAMETER VCC Turn On Voltage VCC Turn Off Voltage VCC Hysteresis VCC Shunt Regulator Voltage VCC Shunt Regulator Voltage Input DC Supply Current Normal Operation Undervoltage Shutdown Threshold (at ITH/RUN) Start-Up Current Source Regulated Feedback Voltage Error Amplifier Transconductance Output Voltage Line Regulation Output Voltage Load Regulation VFB Input Current Oscillator Frequency Minimum Switch On Duty Cycle Maximum Switch On Duty Cycle Gate Drive Rise Time Gate Drive Fall Time Peak Current Sense Voltage Peak Slope Compensation Output Current Soft-Start Time
LTC3803E-5: The indicates specifications which apply over the full -40C to 85C operating junction temperature range, otherwise specifications are at TJ = 25C. VCC = 5V, unless otherwise noted. (Note 2)
CONDITIONS

VITHSHDN IITHSTART VFB gm VO(LINE) VO(LOAD) IFB fOSC DCON(MIN) DCON(MAX) tRISE tFALL VIMAX ISLMAX tSFST
VTURNON - VTURNOFF ICC = 1mA, VITH/RUN = 0V ICC = 25mA, VITH/RUN = 0V (Note 4) VITH/RUN = 1.3V VCC = VTURNON - 100mV VCC = VTURNON + 100mV VITH/RUN = 0V 0C TJ 85C (Note 5) -40C TJ 85C (Note 5) ITH/RUN Pin Load = 5A (Note 5) VTURNOFF < VCC < VCLAMP (Note 5) ITH/RUN Sinking 5A (Note 5) ITH/RUN Sourcing 5A (Note 5) (Note 5) VITH/RUN = 1.3V VITH/RUN = 1.3V, VFB = 0.8V VITH/RUN = 1.3V, VFB = 0.8V CLOAD = 3000pF CLOAD = 3000pF RSL = 0 (Note 6) (Note 7)

MIN 4 3.3 0.05 6.2 6.3
TYP 4.8 4 0.8 8 8.1 240 40 0.28 0.34 0.800 0.800 333 0.1 3 3 10 200 6.5 80 40 40 100 5 0.7
MAX 5.7 4.9 9.9 10.3 350 90 0.45 0.8 0.812 0.816 500
UNITS V V V V V A A V A V V A/V mV/V mV/A mV/A nA kHz % % ns ns mV A ms

0.12 0.07 0.788 0.780 200
170 70
50 230 8.5 90
90
115
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LTC3803-5
ELECTRICAL CHARACTERISTICS
SYMBOL VTURNON VTURNOFF VHYST VCLAMP1mA VCLAMP25mA ICC PARAMETER VCC Turn On Voltage VCC Turn Off Voltage VCC Hysteresis VCC Shunt Regulator Voltage VCC Shunt Regulator Voltage Input DC Supply Current Normal Operation Undervoltage Shutdown Threshold (at ITH/RUN) Start-Up Current Source Regulated Feedback Voltage Error Amplifier Transconductance Output Voltage Line Regulation Output Voltage Load Regulation VFB Input Current Oscillator Frequency Minimum Switch On Duty Cycle Maximum Switch On Duty Cycle Gate Drive Rise Time Gate Drive Fall Time Peak Current Sense Voltage Peak Slope Compensation Output Current Soft-Start Time
LTC3803H-5: The indicates specifications which apply over the full -40C to 150C operating junction temperature range, otherwise specifications are at TA = 25C. VCC = 5V, unless otherwise noted. (Notes 2, 3)
CONDITIONS

VITHSHDN IITHSTART VFB gm VO(LINE) VO(LOAD) IFB fOSC DCON(MIN) DCON(MAX) tRISE tFALL VIMAX ISLMAX tSFST
VTURNON - VTURNOFF ICC = 1mA, VITH/RUN = 0V ICC = 25mA, VITH/RUN = 0V (Note 4) VITH/RUN = 1.3V VCC = VTURNON - 100mV VCC = VTURNON + 100mV VITH/RUN = 0V 0C TJ 85C (Note 5) -40C TJ 150C (Note 5) ITH/RUN Pin Load = 5A (Note 5) VTURNOFF < VCC < VCLAMP (Note 5) ITH/RUN Sinking 5A (Note 5) ITH/RUN Sourcing 5A (Note 5) (Note 5) VITH/RUN = 1.3V VITH/RUN = 1.3V, VFB = 0.8V VITH/RUN = 1.3V, VFB = 0.8V CLOAD = 3000pF CLOAD = 3000pF RSL = 0 (Note 6) (Note 7)

MIN 3.9 3.2 0.05 6.2 6.3
TYP 4.8 4 0.8 8 8.1 240 40 0.28 0.34 0.800 0.800 333 0.1 3 3 10 200 6.5 80 40 40 100 5 0.7
MAX 5.7 4.9 10.4 10.7 350 100 0.45 1 0.812 0.820 500
UNITS V V V V V A A V A V V A/V mV/V mV/A mV/A nA kHz % % ns ns mV A ms

0.08 0.07 0.788 0.780 200
170 70
50 230 8.5 90
85
115
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3803H-5 is guaranteed to meet specifications from -40C to 150C. The LTC3803E-5 is guaranteed to meet specifications from 0C to 85C with specifications over the -40C to 85C temperature range assured by design, characterization and correlation with statistical process controls. Junction temperature (TJ) is calculated from the ambient temperature TA and the power dissipation PD in the LTC3803-5 using the formula: TJ = TA + (PD * 230C/W) Note 3: High junction temperatures degrade operating lifetimes. Operating
lifetime at junction temperatures greater than 125C is derated to 1000 hours. Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 5: The LTC3803-5 is tested in a feedback loop that servos VFB to the output of the error amplifier while maintaining ITH/RUN at the midpoint of the current limit range. Note 6: Peak current sense voltage is reduced dependent on duty cycle and an optional external resistor in series with the SENSE pin (RSL). For details, refer to the programmable slope compensation feature in the Applications Information section. Note 7: Guaranteed by design.
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LTC3803-5 TYPICAL PERFOR A CE CHARACTERISTICS
Reference Voltage vs Temperature
812 808
VCC = 5V
VFB VOLTAGE (mV)
VFB VOLTAGE (mV)
804 800 796 792 788 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38035 G01
804 800 796 792 788 4.0 4.5 5.0 5.5 6.0 6.5 7.0 VCC SUPPLY VOLTAGE (V) 7.5
VFB VOLTAGE (mV)
Oscillator Frequency vs Temperature
220
OSCILLATOR FREQUENCY (kHz)
VCC = 5V
OSCILLATOR FREQUENCY (kHz)
215 210 205 200 195 190 185 180 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38035 G04
210 205 200 195 190 185 180 4.0 4.5 6.5 7.0 5.0 5.5 6.0 VCC SUPPLY VOLTAGE (V) 7.5
OSCILLATOR FREQUENCY (kHz)
VCC Undervoltage Lockout Thresholds vs Temperature
6.0 5.5 5.0
VOLTS
9.5 VTURNON
SUPPLY CURRENT (A)
4.5 4.0 VTURNOFF 3.5 3.0 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
3803 G07
VCC (V)
4
UW
Reference Voltage vs Supply Voltage
812 808 TA = 25C VCC VCLAMP1mA
812 808 804 800 796 792 788
Reference Voltage vs VCC Shunt Regulator Current
TA = 25C
0
5
10
15 ICC (mA)
20
25
38035 G03
38035 F02
Oscillator Frequency vs Supply Voltage
220 215 TA = 25C
220 215 210 205 200 195 190 185 180
Oscillator Frequency vs VCC Shunt Regulator Current
TA = 25C
0
5
15 10 ICC (mA)
20
25
38035 G06
38035 G05
VCC Shunt Regulator Voltage vs Temperature
10.5 10.0 280 300
ICC Supply Current vs Temperature
VCC = 5V VITH/RUN = 1.3V
9.0 8.5
ICC = 25mA ICC = 1mA
260
240
8.0 7.5 7.0 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38035 G08
220
200 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38035 G08
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LTC3803-5 TYPICAL PERFOR A CE CHARACTERISTICS
Start-Up ICC Supply Current vs Temperature
70
VCC = VTURNON - 0.1V
SHUTDOWN THRESHOLD (mV)
60 50 40 30 20 10 0 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38035 G10
400 350 300 250 200 150 100 50 0 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
3803 G11
ITH/RUN PIN CURRENT SOURCE (nA)
START-UP SUPPLY CURRENT (A)
Peak Current Sense Voltage vs Temperature
120 115 VCC = 5V 1.4 1.2
SENSE PIN VOLTAGE (mV)
SOFT-START TIME (ms)
110 105 100 95 90 85 80 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38035 G13
UW
ITH/RUN Shutdown Threshold vs Temperature
500 450
1000
ITH/RUN Start-Up Current Source vs Temperature
VCC = VTURNON + 0.1V 900 VITH/RUN = 0V 800 700 600 500 400 300 200 100 0 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38035 G12
Soft-Start Time vs Temperature
VCC = 5V
1.0 0.8 0.6 0.4 0.2 0 -50 -30 -10 10 30 50 70 90 110 130 150 TEMPERATURE (C)
38035 G14
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LTC3803-5
PI FU CTIO S
ITH/RUN (Pin 1): This pin performs two functions. It serves as the error amplifier compensation point as well as the run/shutdown control input. Nominal voltage range is 0.7V to 1.9V. Forcing this pin below 0.28V causes the LTC3803-5 to shut down. In shutdown mode, the NGATE pin is held low. GND (Pin 2): Ground Pin. VFB (Pin 3): Receives the feedback voltage from an external resistive divider across the output. SENSE (Pin 4): This pin performs two functions. It monitors switch current by reading the voltage across an external current sense resistor to ground. It also injects a current ramp that develops slope compensation voltage across an optional external programming resistor. VCC (Pin 5): Supply Pin. Must be closely decoupled to GND (Pin 2). NGATE (Pin 6): Gate Drive for the External N-Channel MOSFET. This pin swings from 0V to VCC.
BLOCK DIAGRA
800mV REFERENCE
VCC SHUNT REGULATOR
SOFTSTART CLAMP CURRENT COMPARATOR R Q S
+
3 VFB
-
GND 2
20mV 1.2V
1
ITH/RUN
38035 BD
6
+
-
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5 VCC
0.3A 0.28V
+ -
SHUTDOWN COMPARATOR VCC < VTURNON UNDERVOLTAGE LOCKOUT
SHUTDOWN
ERROR AMPLIFIER
VCC SWITCHING LOGIC AND BLANKING CIRCUIT GATE DRIVER NGATE
6
200kHz OSCILLATOR
SLOPE COMP CURRENT RAMP
SENSE
4
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LTC3803-5
OPERATIO
The LTC3803-5 is a constant frequency current mode controller for flyback, SEPIC and DC/DC boost converter applications in a tiny ThinSOT package. The LTC3803-5 is designed so that none of its pins need to come in contact with the input or output voltages of the power supply circuit of which it is a part, allowing the conversion of voltages well beyond the LTC3803-5's absolute maximum ratings. Main Control Loop Due to space limitations, the basics of current mode DC/DC conversion will not be discussed here; instead, the reader is referred to the detailed treatment in Application Note 19, or in texts such as Abraham Pressman's Switching Power Supply Design. Please refer to the Block Diagram and the Typical Application on the front page of this data sheet. An external resistive voltage divider presents a fraction of the output voltage to the VFB pin. The divider must be designed so that when the output is at the desired voltage, the VFB pin voltage will equal the 800mV from the internal reference. If the load current increases, the output voltage will decrease slightly, causing the VFB pin voltage to fall below 800mV. The error amplifier responds by feeding current into the ITH/RUN pin. If the load current decreases, the VFB voltage will rise above 800mV and the error amplifier will sink current away from the ITH/RUN pin. The voltage at the ITH/RUN pin commands the pulse-width modulator formed by the oscillator, current comparator and RS latch. Specifically, the voltage at the ITH/RUN pin sets the current comparator's trip threshold. The current comparator monitors the voltage across a current sense resistor in series with the source terminal of the external MOSFET. The LTC3803-5 turns on the external power MOSFET when the internal free-running 200kHz oscillator sets the RS latch. It turns off the MOSFET when the current comparator resets the latch or when 80% duty cycle is reached, whichever happens first. In this way, the peak current levels through the flyback transformer's primary and secondary are controlled by the ITH/RUN voltage. Since the ITH/RUN voltage is increased by the error amplifier whenever the output voltage is below nominal, and
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decreased whenever output voltage exceeds nominal, the voltage regulation loop is closed. For example, whenever the load current increases, output voltage will decrease slightly, and sensing this, the error amplifier raises the ITH/RUN voltage by sourcing current into the ITH/RUN pin, raising the current comparator threshold, thus increasing the peak currents through the transformer primary and secondary. This delivers more current to the load, bringing the output voltage back up. The ITH/RUN pin serves as the compensation point for the control loop. Typically, an external series RC network is connected from ITH/RUN to ground and is chosen for optimal response to load and line transients. The impedance of this RC network converts the output current of the error amplifier to the ITH/RUN voltage which sets the current comparator threshold and commands considerable influence over the dynamics of the voltage regulation loop. Start-Up/Shutdown The LTC3803-5 has two shutdown mechanisms to disable and enable operation: an undervoltage lockout on the VCC supply pin voltage, and a forced shutdown whenever external circuitry drives the ITH/RUN pin low. The LTC38035 transitions into and out of shutdown according to the state diagram (Figure 1).
LTC3803-5 SHUT DOWN VCC < VTURNOFF (NOMINALLY 4V) V > VITHSHDN VITH/RUN < VITHSHDN ITH/RUN AND VCC > VTURNON (NOMINALLY 0.28V) (NOMINALLY 4.8V) LTC3803-5 ENABLED
38035 F01
Figure 1. Start-Up/Shutdown State Diagram
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LTC3803-5
OPERATIO
The undervoltage lockout (UVLO) mechanism prevents the LTC3803-5 from trying to drive a MOSFET with insufficient VGS. The voltage at the VCC pin must exceed VTURNON (nominally 4.8V) at least momentarily to enable LTC3803-5 operation. The VCC voltage is then allowed to fall to VTURNOFF (nominally 4V) before undervoltage lockout disables the LTC3803-5. The ITH/RUN pin can be driven below VSHDN (nominally 0.28V) to force the LTC3803-5 into shutdown. An internal 0.3A current source always tries to pull this pin towards VCC. When the ITH/RUN pin voltage is allowed to exceed VSHDN, and VCC exceeds VTURNON, the LTC3803-5 begins to operate and an internal clamp immediately pulls the ITH/RUN pin up to about 0.7V. In operation, the ITH/RUN pin voltage will vary from roughly 0.7V to 1.9V to represent current comparator thresholds from zero to maximum. Internal Soft-Start An internal soft-start feature is enabled whenever the LTC3803-5 comes out of shutdown. Specifically, the ITH/RUN voltage is clamped and is prevented from reaching maximum until roughly 0.7ms has passed. This allows the input and output currents of LTC3803-5based power supplies to rise in a smooth and controlled manner on start-up.
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Powering the LTC3803-5 In the simplest case, the LTC3803-5 can be powered from a high voltage supply through a resistor. A built-in shunt regulator from the VCC pin to GND will draw as much current as needed through this resistor to regulate the VCC voltage to around 8V as long as the VCC pin is not forced to sink more than 25mA. This shunt regulator is always active, even when the LTC3803-5 is in shutdown, since it serves the vital function of protecting the VCC pin from seeing too much voltage. The VCC pin must be bypassed to ground immediately adjacent to the IC pins with a ceramic or tantalum capacitor. Proper supply bypassing is necessary to supply the high transient currents required by the MOSFET gate driver. 10F is a good starting point. Adjustable Slope Compensation The LTC3803-5 injects a 5A peak current ramp out through its SENSE pin which can be used for slope compensation in designs that require it. This current ramp is approximately linear and begins at zero current at 6.5% duty cycle, reaching peak current at 80% duty cycle. Additional details are provided in the Applications Information section.
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LTC3803-5
APPLICATIO S I FOR ATIO
Many LTC3803-5 application circuits can be derived from the topology shown in Figure 2. The LTC3803-5 itself imposes no limits on allowed power output, input voltage VIN or desired regulated output voltage VOUT; these are all determined by the ratings on the external power components. The key factors are: Q1's maximum drain-source voltage (BVDSS), on-resistance (RDS(ON)) and maximum drain current, T1's saturation flux level and winding insulation breakdown voltages, CIN and COUT's maximum working voltage, ESR, and maximum ripple current ratings, and D1 and RSENSE's power ratings.
VIN T1 RVCC 5 1 CC 2 VCC ITH/RUN NGATE LTC3803-5 GND VFB R1 3 R2
38035 F02
D1
*
CIN LPRI LSEC COUT
*
CVCC
6 4 RSL
Q1
SENSE
RSENSE
Figure 2. Typical LTC3803-5 Application Circuit
SELECTING FEEDBACK RESISTOR DIVIDER VALUES The regulated output voltage is determined by the resistor divider across VOUT (R1 and R2 in Figure 2). The ratio of R2 to R1 needed to produce a desired VOUT can be calculated: R2 = VOUT - 0.8 V * R1 0.8 V
Choose resistance values for R1 and R2 to be as large as possible in order to minimize any efficiency loss due to the static current drawn from VOUT, but just small enough so that when VOUT is in regulation, the error caused by the nonzero input current to the VFB pin is less than 1%. A good rule of thumb is to choose R1 to be 80k or less.
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TRANSFORMER DESIGN CONSIDERATIONS Transformer specification and design is perhaps the most critical part of applying the LTC3803-5 successfully. In addition to the usual list of caveats dealing with high frequency power transformer design, the following should prove useful. Turns Ratios Due to the use of the external feedback resistor divider ratio to set output voltage, the user has relative freedom in selecting transformer turns ratio to suit a given application. Simple ratios of small integers, e.g., 1:1, 2:1, 3:2, etc. can be employed which yield more freedom in setting total turns and mutual inductance. Simple integer turns ratios also facilitate the use of "off-the-shelf" configurable transformers such as the Coiltronics VERSA-PACTM series in applications with high input to output voltage ratios. For example, if a 6-winding VERSA-PAC is used with three windings in series on the primary and three windings in parallel on the secondary, a 3:1 turns ratio will be achieved. Turns ratio can be chosen on the basis of desired duty cycle. However, remember that the input supply voltage plus the secondary-to-primary referred version of the flyback pulse (including leakage spike) must not exceed the allowed external MOSFET breakdown rating. Leakage Inductance Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the output switch (Q1) turn-off. This is increasingly prominent at higher load currents, where more stored energy must be dissipated. In some cases a "snubber" circuit will be required to avoid overvoltage breakdown at the MOSFET's drain node. Application Note 19 is a good reference on snubber design. A bifilar or similar winding technique is a good way to minimize troublesome leakage inductances. However, remember that this will limit the primary-to-secondary breakdown voltage, so bifilar winding is not always practical.
VERSA-PAC is a trademark of Coiltronics, Inc.
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VOUT
9
LTC3803-5
APPLICATIO S I FOR ATIO
CURRENT SENSE RESISTOR CONSIDERATIONS The external current sense resistor (RSENSE in Figure 2) allows the user to optimize the current limit behavior for the particular application. As the current sense resistor is varied from several ohms down to tens of milliohms, peak switch current goes from a fraction of an ampere to several amperes. Care must be taken to ensure proper circuit operation, especially with small current sense resistor values. For example, a peak switch current of 5A requires a sense resistor of 0.020. Note that the instantaneous peak power in the sense resistor is 0.5W and it must be rated accordingly. The LTC3803-5 has only a single sense line to this resistor. Therefore, any parasitic resistance in the ground side connection of the sense resistor will increase its apparent value. In the case of a 0.020 sense resistor, one milliohm of parasitic resistance will cause a 5% reduction in peak switch current. So the resistance of printed circuit copper traces and vias cannot necessarily be ignored. PROGRAMMABLE SLOPE COMPENSATION The LTC3803-5 injects a ramping current through its SENSE pin into an external slope compensation resistor (RSL in Figure 2). This current ramp starts at zero right after the NGATE pin has been high for the LTC3803-5's minimum duty cycle of 6.5%. The current rises linearly towards a peak of 5A at the maximum duty cycle of 80%, shutting off once the NGATE pin goes low. A series resistor (RSL) connecting the SENSE pin to the current sense resistor (RSENSE) thus develops a ramping voltage drop. From the perspective of the SENSE pin, this ramping voltage adds to the voltage across the sense resistor, effectively reducing the current comparator threshold in proportion to duty cycle. This stabilizes the control loop
VIN RVCC LTC3803-5 VCC CVCC GND
38035 F03
Figure 3. Powering the LTC3803-5 Via the Internal Shunt Regulator
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against subharmonic oscillation. The amount of reduction in the current comparator threshold (VSENSE) can be calculated using the following equation: VSENSE = Duty Cycle - 6.5% * 5A * RSL 73.5% Note: LTC3803-5 enforces 6.5% < Duty Cycle < 80%. A good starting value for RSL is 5.9k, which gives a 30mV drop in current comparator threshold at 80% duty cycle. Designs not needing slope compensation may replace RSL with a short circuit. VCC SHUNT REGULATOR An internal shunt regulator allows the LTC3803-5 to be powered through a single dropping resistor from VIN to VCC, in conjunction with a bypass capacitor, CVCC, that closely decouples VCC to GND (see Figure 3). The shunt regulator can draw up to 25mA through the VCC pin to GND to drop enough voltage across RVCC to regulate VCC to around 8V. For applications where VIN is low enough such that the static power dissipation in RVCC is acceptable, using the VCC shunt regulator is the simplest way to power the LTC3803-5. EXTERNAL PREREGULATOR The circuit in Figure 4 shows another way to power the LTC3803-5. An external series preregulator consisting of series pass transistor Q1, Zener diode D1, and bias resistor RB brings VCC above the VCC turn-on threshold, enabling the LTC3803-5.
8V TO 75 VIN RB 100k D1 6.8V Q1 MMBTA42 CVCC 0.1F LTC3803-5 VCC GND
38035 F04
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Figure 4. Powering the LTC3803-5 with an External Preregulator
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LTC3803-5
TYPICAL APPLICATIO S
2W Isolated Housekeeping Telecom Converter
PRIMARY SIDE 10V, 100mA OUTPUT BAS516 T1
VIN 36V TO 75V 9.2k 1nF BAS516 1k 220k
806
PACKAGE DESCRIPTIO
0.62 MAX
0.95 REF
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 1.00 MAX DATUM `A'
0.30 - 0.50 REF
NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193
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.
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*
2.2F
1F
BAS516
*
2.2F
*
FDC2512
SECONDARY SIDE 10V, 100mA OUTPUT
22k
1 2 3
LTC3803-5 6 ITH/RUN NGATE GND VFB VCC SENSE 5 4 5.6k 1F PRIMARY GROUND
SECONDARY SIDE GROUND
T1: PULSE ENGINEERING PA0648 OR TYCO TTI8698 0.1
38035 TA03
S6 Package 6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
2.90 BSC (NOTE 4)
1.22 REF
1.4 MIN
2.80 BSC
1.50 - 1.75 (NOTE 4) PIN ONE ID
0.95 BSC
0.30 - 0.45 6 PLCS (NOTE 3)
0.80 - 0.90 0.01 - 0.10
0.09 - 0.20 (NOTE 3)
1.90 BSC
S6 TSOT-23 0302
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LTC3803-5
TYPICAL APPLICATIO S
90% Efficient Synchronous Flyback Converter
VIN 36V TO 72V 270k CIN T1 VOUT* 3.3V 1.5A CO
*
1n D1 ITH/RUN GATE Q1 560 5k
38035 TA04a
*
0.1F
EFFICIENCY (%)
EFFICIENCY (%)
33k 1 2
6
LTC3803-5 5 VCC GND VFB SENSE 4 1F 10V
8.06k
3
25.5k* RFB VOUT
RCS
T1: PULSE ENGINEERING PA1006 Q1: FAIRCHILD FDC2512 Q2: VISHAY Si9803
RCS: VISHAY OR IRC, 80m D1: PHILIPS BAS516 CIN: TDK 1F, 100V, X5R *FOR 5V OUTPUT CHANGE CO: TDK 100F, 6.3V, X5R RFB TO 42.2k
RELATED PARTS
PART NUMBER LT(R)1425 LT1725 LTC1772 LTC1871 LTC1872 LT1950 LT1952 LT3420 LT3468/LT3468-1 LTC3806 LTC4441 DESCRIPTION Isolated Flyback Switching Regulator with No External Power Devices General Purpose Isolated Flyback Controller SOT-23 Constant Frequency Current Mode Step-Down DC/DC Controller Wide Input Range, No RSENSETM Current Mode Flyback, Boost and SEPIC Controller SOT-23 Constant Frequency Current Mode Boost DC/DC Controller Current Mode PWM Controller Current Mode PWM Controller Photoflash Capacitor Charger with Automatic Refresh Photoflash Capacitor Charger in 5-Pin SOT-23 Synchronous Flyback Controller 6A N-Channel MOSFET Driver COMMENTS No Optoisolator or "Third Winding" Required, Up to 6W Output No Optoisolator Required, VIN and VOUT Limited Only by External Power Components 550kHz Switching Frequency, 2.4V to 9.8V VIN Range Adjustable Switching Frequency, Programmable Undervoltage Lockout, Optional Burst Mode(R) Operation at Light Load 550kHz Switching Frequency, 2.4V to 9.8V VIN Range Controller for Forward Converters from 30W to 300W Synchronous Controller for Forward Converters from 30W to 500W Specialized Flyback Charges High Voltage Photoflash Capacitors Quickly and Efficiently Minimal Component Count, Uses Small Transformers; VIN from 2.5V to 16V High Efficiency (89%); Multiple Output with Excellent Cross Regulation Gate Drive Adjustable from 5V to 8V, Adjustable Blanking Prevents Ringing, 10-Lead MSSOP Package
Burst Mode is a registered trademark of Linear Technology Corporation. No RSENSE is a trademark of Linear Technology Corporation.
12
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
U
Synchronous Flyback 3.3VOUT
91
92 91 90
Synchronous Flyback 5VOUT
*
Q2
90
89 88 87 86
89
88 0.5
1.5 1.0 OUTPUT CURRENT (A)
2.0
38035 TA04b
85 0.5
1.0
1.5 2.0 OUTPUT CURRENT (A)
2.5
38035 TA04c
38035f LT/TP 1104 1K * PRINTED IN THE USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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