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19-2198; Rev 0; 10/01
KIT EVALUATION AVAILABLE
3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches
General Description
The MAX1830/MAX1831 constant-off-time, pulse-widthmodulated (PWM) step-down DC-DC converters are ideal for use in 5V and 3.3V to low-voltage conversion necessary in notebook and subnotebook computers. These devices feature internal synchronous rectification for high efficiency and reduced component count. They require no external Schottky diode. The internal 45m PMOS power switch and 55m NMOS synchronous-rectifier switch easily deliver continuous load currents up to 3A. The MAX1830 produces preset +2.5V, +1.8V, or +1.5V output voltage or an adjustable output from +1.1V to VIN. The MAX1831 produces preset +3.3V, +2.5V, and +1.5V output voltages and an adjustable output from +1.1V to VIN. They achieve efficiencies as high as 94%. The MAX1830/MAX1831 use a unique current-mode, constant-off-time, PWM control scheme, which includes Idle ModeTM to maintain high efficiency during lightload operation. The programmable constant-off-time architecture sets switching frequencies up to 1MHz, allowing the user to optimize performance tradeoffs between efficiency, output switching noise, component size, and cost. Both devices are designed for continuous output currents up to 3A, an internal digital soft-start to limit surge currents during startup, a 100% duty cycle mode for low-dropout operation, and a low-power shutdown mode that disconnects the input from the output and reduces supply current below 1A.The MAX1830/MAX1831 are available in 16-pin QSOP packages. For similar devices that provide continuous output currents of 1A to 3A, refer to the MAX1644, MAX1623, and MAX1742/MAX1842/MAX1843 data sheets. o 1.5% Output Accuracy o 94% Efficiency o Internal PMOS/NMOS Switches 45m/55m On-Resistance at VIN = +4.5V 50m/55m On-Resistance at VIN = +3V o Output Voltages +3.3V, +2.5V, or +1.5V Pin Selectable (MAX1831) +2.5V, +1.8V, or +1.5V Pin Selectable (MAX1830) +1.1V to VIN Adjustable (Both Devices) o +3V to +5.5V Input Voltage Range o 750A (max) Operating Supply Current o <1A Shutdown Supply Current o Programmable Constant-Off-Time Operation o Up to 1MHz Switching Frequency o Idle Mode Operation at Light Loads o Thermal Shutdown o Internal Digital Soft-Start Inrush Current Limiting o 100% Duty Cycle During Low-Dropout Operation o Output Short-Circuit Protection o 16-Pin QSOP Package
Features
MAX1830/MAX1831
Ordering Information
PART MAX1830EEE MAX1831EEE TEMP. RANGE -40C to +85C -40C to +85C PIN-PACKAGE 16 QSOP 16 QSOP
Applications
5V or 3.3V to Low-Voltage Conversion CPU I/O Supplies Chipset Supplies Notebook and Subnotebook Computers
Typical Configuration
INPUT +3V TO +5.5V IN LX OUTPUT +1.1V TO VIN
MAX1830 FB MAX1831 VCC PGND GND SHDN COMP TOFF FBSEL REF
Pin Configuration appears at end of data sheet.
Idle Mode is a trademark of Maxim Integrated Products, Inc.
________________________________________________________________ Maxim Integrated Products
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For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim's website at www.maxim-ic.com.
3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831
ABSOLUTE MAXIMUM RATINGS
VCC, IN, SHDN to GND ............................................-0.3V to +6V IN to VCC .............................................................................0.3V GND to PGND.....................................................................0.3V COMP, FB, TOFF, FBSEL, REF to GND .....-0.3V to (VCC + 0.3V) LX Current (Note 1) ...............................................................5.1A REF Short Circuit to GND Duration ............................Continuous ESD Protection .....................................................................2kV Continuous Power Dissipation (TA = +70C) 16-Pin QSOP (derate 14mW/C above +70C; part mounted on 1in2 of 1oz copper) .............................1.12W Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C Junction Temperature ......................................................+150C Lead Temperature (soldering, 10s) ................................ +300C
Note 1: LX has internal clamp diodes to PGND and IN. Applications that forward bias the diode should take care not to exceed the IC's package power dissipation.
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = VCC = +3.3V, FBSEL = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Input Voltage SYMBOL VIN, VCC FBSEL = VCC VIN = +3V to +5.5V, ILOAD = 0 to 3A, VFB = VOUT Preset Output Voltage VOUT VIN = +3.7V to +5.5V, ILOAD = 0 to 3A, VFB = VOUT Adjustable Output Voltage Range AC Load Regulation Error DC Load Regulation Error Dropout Voltage Reference Voltage Reference Load Regulation Current-Limit Threshold Maximum Output RMS Current Idle Mode Current Threshold PMOS Switch On-Resistance NMOS Switch On-Resistance Switching Frequency No-Load Supply Current VDO VREF VREF ILIMIT IOUT(RMS) (Note 4) IIM RON, P RON, N f IIN + ICC ILX = 0.5A ILX = 0.5A (Note 2) VFB = 1.2V 325 VIN = 4.5V VIN = 3V VIN = 4.5V VIN = 3V 0.2 0.6 45 50 55 55 IREF = -1A to +10A 4.0 VCC = VIN = +3V, ILOAD = 3A 1.089 FBSEL = unconnected FBSEL = REF (MAX1830) FBSEL = GND FBSEL = REF (MAX1831) CONDITIONS MIN 3.0 2.487 1.492 1.791 1.084 3.283 2.525 1.515 1.818 1.100 3.333 TYP MAX 5.5 2.563 1.538 1.845 1.117 3.383 UNITS V
V
VCC = VIN = +3V to +5.5V, ILOAD = 0, FBSEL = GND
VREF 2 0.4 150 1.100 0.5 4.8
VIN
V % %
330 1.111 2 5.4 3.4 1.0 90 110 95 100 1 750
mV V mV A A A
m
MHz A
2
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCC = +3.3V, FBSEL = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER SYMBOL CONDITIONS SHDN = GND, into VCC and IN pins; LX = 0 or 3.3V Shutdown Supply Current SHDN = GND, into IN with LX = 0 SHDN = GND, into IN with LX = 3.3V Thermal Shutdown Threshold Undervoltage Lockout Threshold FB Input Bias Current Off-Time Startup Off-Time Minimum On-Time Soft-Start Time (Note 3) SHDN Input Current SHDN Logic Input Low Voltage SHDN Logic Input High Voltage FBSEL Input Current FBSEL = GND FBSEL = REF FBSEL Logic Thresholds FBSEL = unconnected FBSEL = VCC 0.9 0.7 VCC - 0.2 VCC - 0.2 ISHDN VIL VIH 2.0 -5 5 0.2 1.3 0.7 VCC + 0.2 VCC + 0.2 SHDN = 0 or VCC -0.5 tON (Note 2) 0.40 3 256 0.5 0.8 TSHDN VUVLO IFB tOFF Hysteresis = 15C VIN falling, hysteresis = 90mV VFB = 1.2V RTOFF = 110k RTOFF = 44k RTOFF = 440k 1.8 0 0.85 0.3 3.0 MIN TYP 0.2 0.2 0.1 165 2.6 70 1.00 0.4 3.9 4 tOFF 2.8 300 1.15 0.5 5.0 MAX 20 20 20 C V nA s s s s s cycles A V V A V V V V A UNITS
MAX1830/MAX1831
ELECTRICAL CHARACTERISTICS
(VIN = VCC = +3.3V, FBSEL = GND, TA = -40C to +85C, unless otherwise noted.) (Note 5)
PARAMETER Input Voltage SYMBOL VIN, VCC FBSEL = VCC VIN = +3V to +5.5V, ILOAD = 0 to 3A, VFB = VOUT Preset Output Voltage VOUT VIN = +3.7V to +5.5V, ILOAD = 0 to 3A, VFB = VOUT FBSEL = unconnected FBSEL = REF (MAX1830) FBSEL = GND FBSEL = REF (MAX1831) CONDITIONS MIN 3.0 2.475 1.485 1.782 1.078 3.267 TYP MAX 5.5 2.575 1.545 1.854 1.122 3.399 UNITS V
V
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCC = +3.3V, FBSEL = GND, TA = -40C to +85C, unless otherwise noted.) (Note 5)
PARAMETER Adjustable Output Voltage Range Reference Voltage Current-Limit Threshold Idle Mode Current Threshold PMOS Switch On-Resistance NMOS Switch On-Resistance No-Load Supply Current FB Input Bias Current Off-Time VREF ILIMIT IIM RON, P RON, N IIN + ICC IFB tOFF ILX = 0.5A ILX = 0.5A VFB = 1.2V VFB = 1.2V RTOFF = 110k 0 0.8 VIN = 4.5V VIN = +3V VIN = 4.5V VIN = +3V SYMBOL CONDITIONS VCC = VIN = +3V to +5.5V, ILOAD = 0, FBSEL = GND MIN VREF 1.078 3.9 0.14 TYP MAX VIN 1.122 5.4 1.0 90 110 95 100 750 360 1.2 A nA s m UNITS V V A A
Note 2: Note 3: Note 4: Note 5:
Not production tested. Soft-start time is measured with respect to the number of cycles on LX. This is a metal migration limit. Maximum output current may be limited by thermal capability to a lower value than this. Specifications from 0C to -40C are guaranteed by design, not production tested.
Typical Operating Characteristics
(Circuit of Figure 1, TA = +25C, unless otherwise noted.) EFFICIENCY vs. OUTPUT CURRENT (VIN = 5.0V, L = 1.5H)
MAX1830 toc01
EFFICIENCY vs. OUTPUT CURRENT (VIN = 3.3V, L = 1.5H)
MAX1830 toc02
EFFICIENCY vs. OUTPUT CURRENT (fPWM = 300kHz)
95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 VOUT = 1.8V VIN = 3.3V RTOFF = 160k L = 2.5H VOUT = 1.8V VIN = 5.0V RTOFF = 220k L = 5.2H
MAX1830 toc03
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 0.001 0.01 0.1 1 VOUT = 1.8V RTOFF = 75k VOUT = 1.5V RTOFF = 82k VOUT = 3.3V RTOFF = 39k
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 55 VOUT = 1.5V VOUT = 2.5V RTOFF = 62k RTOFF = 39k VOUT = 1.8V RTOFF = 51k
100
10
50 0.001
0.01
0.1
1
10
50 0.001
0.01
0.1
1
10
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches
Typical Operating Characteristics (continued)
(Circuit of Figure 1, TA = +25C, unless otherwise noted.) NORMALIZED OUTPUT ERROR vs. OUTPUT CURRENT
MAX1830 toc04
MAX1830/MAX1831
SWITCHING FREQUENCY vs. OUTPUT CURRENT
VIN = 5.0V, VOUT = 1.5V, L = 1.5H
MAX1830 toc05
STARTUP AND SHUTDOWN
MAX1830 toc06
0.10 NORMALIZED OUTPUT ERROR (%) 0.05 0 -0.05 -0.10 -0.15 -0.20 -0.25 0.001 VIN = 5.0V VOUT = 1.5V L = 1.5H VIN = 3.3V VOUT = 1.5V L = 1.5H
1200 1000 FREQUENCY (kHz) 800 600 400 200 0 VIN = 3.3V, VOUT = 1.5V, L = 1.5H
IIN 1A/div
VSHDN 5V/div
VOUT 1V/div 0 0.5 1.0 1.5 2.0 2.5 3.0 400s/div VIN = 3.3V, VOUT = 1.5V, ROUT = 0.5, RTOFF = 82k, L = 1.5H
0.01
0.1
1
10
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
LOAD-TRANSIENT RESPONSE
MAX1830 toc07
LINE-TRANSIENT RESPONSE
MAX1830 toc08
VIN 2V/div VOUT 50mV/div AC-COUPLE
0
IOUT 2A/div 10s/div VIN = 3.3V, VOUT = 1.5V, RTOFF = 82k, L = 1.5H, IOUT = 0.1A TO 3A 20s/div VOUT = 1.8V, IOUT = 1A, RTOFF = 75k, L = 1.5H
VOUT 50mV/div AC-COUPLED
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831
Pin Description
PIN 1, 3, 14, 16 2, 4 5 6 7 8 9 10 11 12 13, 15 NAME LX IN SHDN COMP TOFF FB GND REF FBSEL VCC PGND FUNCTION Connection for the drains of the PMOS power switch and NMOS synchronous-rectifier switch. Connect the inductor from this node to the output filter capacitor and load. Supply Voltage Input for the internal PMOS power switch Shutdown Control Input. Drive SHDN low to disable the reference, control circuitry, and internal MOSFETs. Drive high or connect to VCC for normal operation. Integrator Compensation. Connect a capacitor from COMP to VCC for integrator compensation. See Integrator Amplifier section. Off-Time Select Input. Sets the PMOS power switch off-time during constant-off-time operation. Connect a resistor from TOFF to GND to adjust the PMOS switch off-time. Feedback Input for both preset-output and adjustable-output operating modes. Connect directly to output for fixed-voltage operation or to a resistive divider for adjustable operating modes. Analog Ground Reference Output. Bypass REF to GND with a 1F capacitor. Feedback Select Input. Selects output voltage. See Table 3 for programming instructions. Analog Supply Voltage Input. Supplies internal analog circuitry. Bypass VCC with a 10 and 2.2F lowpass filter (Figure 1). Power Ground. Internally connected to the internal NMOS synchronous-rectifier switch.
_______________Detailed Description
The MAX1830/MAX1831 synchronous, current-mode, constant-off-time, PWM DC-DC converters step down input voltages of +3V to +5.5V to preset output voltages, or to an adjustable output voltage from +1.1V to VIN. The MAX1830 has preset outputs +2.5V, +1.8V, and +1.5V. The MAX1831 has preset outputs of +3.3V, +2.5V or +1.5V. Both devices deliver up to 3A of continuous output current. Internal switches composed of a 45m PMOS power switch and a 55m NMOS synchronous rectifier switch improve efficiency, reduce component count, and eliminate the need for an external Schottky diode. The MAX1830/MAX1831 optimize efficiency by operating in constant-off-time mode under heavy loads and in Maxim's proprietary Idle Mode under light loads. A single resistor-programmable constant-off-time control sets switching frequencies up to 1MHz, allowing the user to optimize performance trade-offs in efficiency, switching noise, component size, and cost. Under lowdropout conditions, the device operates in a 100% duty-cycle mode, where the PMOS switch remains continuously on. Idle Mode enhances light-load efficiency by skipping cycles, thus reducing transition and gatecharge losses.
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When power is drawn from a regulated supply, constantoff-time PWM architecture essentially provides constantfrequency operation. This architecture has the inherent advantage of quick response to line and load transients. The MAX1830/MAX1831 current-mode, constant-offtime PWM architecture regulates the output voltage by changing the PMOS switch on-time relative to the constant off-time. Increasing the on-time increases the peak inductor current and the amount of energy transferred to the load per pulse.
Modes of Operation
The current through the PMOS switch determines the mode of operation: constant-off-time mode (for load currents greater than half the Idle Mode threshold), or Idle Mode (for load currents less than half the Idle Mode threshold). Current sense is achieved through a proprietary architecture that eliminates current-sensing I2R losses. Constant-Off-Time Mode Constant-off-time operation occurs when the current through the PMOS switch is greater than the Idle Mode threshold current (which corresponds to a load current of half the Idle Mode threshold). In this mode, the regu-
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831
INPUT 10 CIN 22F, 6.3V X5R IN LX L 1.5F SUMIDA CDRH-6D28 OUTPUT COUT 120F, 4V Panasonic SP
MAX1830 FB MAX1831 VCC PGND SHDN COMP GND FBSEL REF
2.2F
470pF
1F TOFF RTOFF
MAX1830 VOUT = 2.5V, FBSEL = VOUT = 1.8V, FBSEL = VOUT = 1.5V, FBSEL =
VCC REF FLOATING
MAX1831 VOUT = 2.5V, FBSEL = VOUT = 3.3V, FBSEL = VOUT = 1.5V, FBSEL =
VCC REF FLOATING
Figure 1. Typical Circuit
FBSEL FB DIGITAL SOFT-START VIN 3.0V TO 5.5V CIN CERAMIC
FEEDBACK SELECTION COMP 470pF 10 VCC REF Gm REF
MAX1830 MAX1831
IN
CURRENT SENSE SKIP PWM LOGIC AND DRIVERS
VIN
2.2F SHDN REF 1F GND
SUMMING COMPARATOR
LX COUT
VOUT
REF
TIMER
CURRENT SENSE PGND
TOFF RTOFF
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
Figure 2. Functional Diagram
lation comparator turns the PMOS switch on at the end of each off-time, keeping the device in continuous-conduction mode. The PMOS switch remains on until the output is in regulation or the current limit is reached. When the PMOS switch turns off, it remains off for the programmed off-time (t OFF ). To control the current under short-circuit conditions, the PMOS switch remains off for approximately 4 x tOFF when VOUT < VOUT(NOM) / 4.
Idle Mode Under light loads, the devices improve efficiency by switching to a pulse-skipping Idle Mode. Idle Mode operation occurs when the current through the PMOS switch is less than the Idle Mode threshold current. Idle Mode forces the PMOS to remain on until the current through the switch reaches the Idle Mode threshold, thus minimizing the unnecessary switching that degrades efficiency under light loads. In Idle Mode, the
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831
Table 1. Recommended Component Values (IOUT = 3.0A)
VIN (V) 5 5 5 5 5 3.3 3.3 3.3 3.3 VOUT (V) 3.3 2.5 1.8 1.5 1.1 2.5 1.8 1.5 1.1 fPWM (kHz) 800 865 850 860 625 570 850 860 680 L (H) 2.2 2.2 2.2 2.2 2.2 1.5 1.5 1.5 1.5 RTOFF (k) 39 56 75 82 130 39 51 62 100
OPERATING FREQUENCY (kHz)
device operates in discontinuous conduction. Currentsense circuitry monitors the current through the NMOS synchronous switch, turning it off before the current reverses. This prevents current from being pulled from the output filter through the inductor and NMOS switch to ground. As the device switches between operating modes, no major shift in circuit behavior occurs.
MAXIMUM RECOMMENDED OPERATING FREQUENCY vs. INPUT VOLTAGE
VOUT = 1.5V 1200 1000 800 600 400 200 0 2.6 3.1 3.6 4.1 VIN (V) 4.6 5.1 5.6 VOUT = 1.8V VOUT = 2.5V VOUT = 3.3V
MAX1830/MAX1831
1400
100% Duty-Cycle Operation
When the input voltage drops near the output voltage, the duty cycle increases until the PMOS MOSFET is on continuously. The dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal PMOS switch and parasitic resistance in the inductor. The PMOS switch remains on continuously as long as the current limit is not reached.
Internal Digital Soft-Start Circuit
Soft-start allows a gradual increase of the current-limit level at startup to reduce input-surge currents. The MAX1830/MAX1831 contain internal digital soft-start circuits, controlled by a counter, a digital-to-analog converter (DAC), and the current-limit comparator. At power-on or in shutdown mode, the soft-start counter is reset to zero. When the MAX1830/MAX1831 are enabled or powered up, its counter starts counting LX switching cycles, and the DAC begins incrementing the comparison voltage applied to the current-limit comparator. The DAC ramps up the internal current limit in four 25% steps, as the count increases to 256 cycles. As a result, the main output capacitor charges up relatively slowly. The exact time of the output rise depends on nominal switching frequency, output capacitance, and the load current, and is typically 1ms.
Figure 3. Maximum Recommended Operating Frequency vs. Input Voltage
reduce supply current to less than 1A. In shutdown, all circuitry and internal MOSFETs turn off, and the LX node becomes high impedance. Drive SHDN to a logic-level high or connect to VCC for normal operation.
Summing Comparator
Three signals are added together at the input of the summing comparator (Figure 2): an output-voltage error signal relative to the reference voltage, an integrated output-voltage error correction signal, and the sensed PMOS switch current. The integrated error signal is provided by a transconductance amplifier with an external capacitor at COMP. This integrator provides high DC accuracy without the need for a high-gain amplifier. Connecting a capacitor at COMP modifies the overall loop response (see Integrator Amplifier).
Shutdown
Drive SHDN to a logic-level low to place the MAX1830/MAX1831 in low-power shutdown mode and
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches
Table 2. Output Voltage Programming
PIN FBSEL VCC Unconnected REF GND FB Output voltage Output voltage Output voltage Resistive divider OUTPUT VOLTAGE (V) MAX1830 2.5 1.5 1.8 MAX1831 2.5 1.5 3.3
evenly distribute the copper area connected at the IC among the high-current pins.
MAX1830/MAX1831
Power Dissipation
Power dissipation in the MAX1830/MAX1831 is dominated by conduction losses in the two internal power switches. Power dissipation due to supply current in the control section and average current used to charge and discharge the gate capacitance of the internal switches (i.e., switching losses) is approximately: PDS = C x VIN2 x fPWM where C = 5nF and f PWM is the switching frequency in PWM mode. This number is reduced when the switching frequency decreases as the part enters Idle Mode. Combined conduction losses in the two power switches are approximated by: PD = IOUT2 x RPMOS where RPMOS is the on-resistance of the PMOS switch. The junction-to-ambient thermal resistance required to dissipate this amount of power is calculated by: JA = (TJ,MAX - TA,MAX) / PD(TOT) where: JA = junction-to-ambient thermal resistance TJ,MAX = maximum junction temperature TA,MAX = maximum ambient temperature PD(TOT) = total losses
Adjustable
LX
VOUT
MAX1830 MAX1831
R2
FB R1 = 30k R2 = R1(VOUT / VREF - 1) VREF = 1.1V R1
Figure 4. Adjustable Output Voltage
Synchronous Rectification
In a step-down regulator without synchronous rectification, an external Schottky diode provides a path for current to flow when the inductor is discharging. Replacing the Schottky diode with a low-resistance NMOS synchronous switch reduces conduction losses and improves efficiency. The NMOS synchronous-rectifier switch turns on following a short delay after the PMOS power switch turns off, thus preventing cross conduction or "shoot through." In constant-off-time mode, the synchronous-rectifier switch turns off just prior to the PMOS power switch turning on. While both switches are off, inductor current flows through the internal-body diode of the NMOS switch. The internal-body diode's forward voltage is relatively high. An external Schottky diode from PGND to LX can improve efficiency. Junction-to-ambient thermal resistance, JA, is highly dependent on the amount of copper area immediately surrounding the IC leads. The MAX1830/MAX1831 evaluation kit has 0.7in2 of copper area and a thermal resistance of +71C/W with no forced airflow. Airflow over the board significantly reduces the junction-toambient thermal resistance. For heatsinking purposes,
Design Procedure
For typical applications, use the recommended component values in Table 1. For other applications, take the following steps: 1) Select the desired PWM-mode switching frequency; 1MHz is a good starting point. See Figure 3 for maximum operating frequency. 2) Select the constant off-time as a function of input voltage, output voltage, and switching frequency. 3) Select RTOFF as a function of off-time. 4) Select the inductor as a function of output voltage, off-time, and peak-to-peak inductor current.
Thermal Resistance
Setting the Output Voltage
The output of the MAX1830/MAX1831 is selectable between one of three preset output voltages. For a preset output voltage, connect FB to the output voltage and connect FBSEL as indicated in Table 2. For an adjustable output voltage, connect FBSEL to GND and connect FB to a resistive divider between the output
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831
voltage and ground (Figure 4). Regulation is maintained for adjustable output voltages when VFB = VREF. Use 30k for R1. R2 is given by the equation: V R2 = R1 OUT - 1 VREF where VREF is typically 1.1V. L= VOUT x t OFF IOUT x LIR
where: IOUT = maximum DC load current LIR = ratio of peak-to-peak AC inductor current to DC load current, typically 0.25 The peak-inductor current at full load is 1.125 x IOUT if the above equation is used; otherwise, the peak current is calculated by: IPEAK = IOUT + VOUT x t OFF 2xL
Programming the Switching Frequency and Off-Time
The MAX1830/MAX1831 feature a programmable PWM mode-switching frequency, which is set by the input and output voltage and the value of RTOFF, connected from TOFF to GND. RTOFF sets the PMOS power switch off-time in PWM mode. Use the following equation to select the off-time according to your desired switching frequency in PWM mode: t OFF =
(VIN - VOUT - VPMOS ) fPWM ( VIN - VPMOS + VNMOS )
Choose an inductor with a saturation current at least as high as the peak-inductor current. The inductor you select should exhibit low losses at your chosen operating frequency.
Capacitor Selection
The input-filter capacitor reduces peak currents and noise at the voltage source. Use a low-ESR and lowESL capacitor located no further than 5mm from IN. Select the input capacitor according to the RMS input ripple-current requirements and voltage rating: IRIPPLE = ILOAD VOUT VIN - VOUT VIN
where: tOFF = the programmed off-time VIN = the input voltage VOUT = the output voltage VPMOS = the voltage drop across the internal PMOS power switch VNMOS = the voltage drop across the internal NMOS synchronous-rectifier switch fPWM = switching frequency in PWM mode Select RTOFF according to the formula: RTOFF = (tOFF - 0.07s) (110k / 1.00s) Recommended values for RTOFF range from 36k to 430k for off-times of 0.4s to 4s.
(
)
where IRIPPLE = input RMS current ripple. The output-filter capacitor affects the output-voltage ripple, output load-transient response, and feedback-loop stability. For stable operation, the MAX1830/MAX1831 require a minimum output ripple voltage of VRIPPLE 1% VOUT. The minimum ESR of the output capacitor should be: ESR > 1% x L t OFF
Inductor Selection
The key inductor parameters must be specified: inductor value (L) and peak current (IPEAK). The following equation includes a constant, denoted as LIR, which is the ratio of peak-to-peak inductor AC current (ripple current) to maximum DC load current. A higher value of LIR allows smaller inductance but results in higher losses and ripple. A good compromise between size and losses is found at approximately a 25% ripple-current to load-current ratio (LIR = 0.25), which corresponds to a peak-inductor current 1.125 times the DC load current:
Stable operation requires the correct output-filter capacitor. When choosing the output capacitor, ensure that: t COUT OFF 79FV / s VOUT
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831
MAX1830/MAX1831 MAXIMUM RECOMMENDED CONTINUOUS OUTPUT CURRENT vs. TEMPERATURE
3.50 3.40 3.30 OUTPUT CURRENT (A) BURST CURRENT (A) 3.20 3.10 3.00 2.90 2.80 2.70 2.60 2.50 2.40 25 35 45 55 65 75 85 TEMPERATURE (C) 0.7IN2 OF 1-OZ COPPER 2.40 0 20 40 60 80 100 DUTY CYCLE (%) 2.60 IOUT IS A 100Hz SQUARE WAVE FROM 1A TO THE BURST CURRENT 3.20 TA = +85C 3.00 2.80 3.40 TA = +55C
MAXIMUM RECOMMENDED BURST CURRENT vs. BURST CURRENT DUTY CYCLE
Figure 5. Maximum Recommended Continuous Output Current vs. Temperature
Figure 6. Maximum Recommended Burst Current vs. Burst Current Duty Cycle
Integrator Amplifier
An internal transconductance amplifier fine tunes the output DC accuracy. A capacitor, CCOMP, from COMP to VCC compensates the transconductance amplifier. For stability, choose CCOMP = 470pF. A large capacitor value maintains a constant average output voltage but slows the loop response to changes in output voltage. A small capacitor value speeds up the loop response to changes in output voltage but decreases stability.
Frequency Variation with Output Current
The operating frequency of the MAX1830/MAX1831 is determined primarily by tOFF (set by RTOFF), VIN, and VOUT as shown in the following formula: fPWM =
(VIN - VOUT - VPMOS ) [tOFF (VIN - VPMOS + VNMOS )]
High-Current Thermal Considerations
High ambient temperatures can limit the maximum current or duty factor of the output current, depending on the total copper, are connected to the MAX1830/ MAX1831 and available airflow. Figure 5 shows the maximum recommended continuous output current vs. ambient temperature. Figure 6 shows the maximum recommended output current vs. the output current duty cycle at high temperatures. These figures are based on 0.7in2 of 1oz copper in free air. Figure 6 assumes that the output current is a square wave with a 100Hz frequency. The duty cycle is defined as the duration of the burst current divided by the period of the square wave. This figure shows the limitations for continuous bursts of output current. Note that if the thermal limitations of the MAX1830/ MAX1831 are exceeded, it enters thermal shutdown to prevent destructive failure.
However, as the output current increases, the voltage drop across the NMOS and PMOS switches increases and the voltage across the inductor decreases. This causes the frequency to drop. The change in frequency can be approximated with the following formula: fPWM = -IOUT x RPMOS / (VIN x tOFF) where RPMOS is the resistance of the internal MOSFETs (50m typ).
Circuit Layout and Grounding
Good layout is necessary to achieve the MAX1830/ MAX1831s' intended output power level, high efficiency, and low noise. Good layout includes the use of a ground plane, careful component placement, and correct routing of traces using appropriate trace widths.
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11
3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches MAX1830/MAX1831
The following points are in order of decreasing importance: 1) Minimize switched-current and high-current ground loops. Connect the input capacitor's ground, the output capacitor's ground, and PGND. Connect the resulting island to GND at only one point. 2) Connect the input filter capacitor less than 5mm away from IN. The connecting copper trace carries large currents and must be at least 1mm wide, preferably 2.5mm. 3) Place the LX node components as close together and as near to the device as possible. This reduces resistive and switching losses as well as noise. 4) A ground plane is essential for optimum performance. In most applications, the circuit is located on a multilayer board, and full use of the four or more layers is recommended. Use the top and bottom layers for interconnections and the inner layers for an uninterrupted ground plane. Avoid large AC currents through the ground plane.
Pin Configuration
TOP VIEW
LX 1 IN 2 LX 3 IN 4 SHDN 5 COMP 6 TOFF 7 FB 8 16 LX 15 PGND 14 LX
___________________Chip Information
TRANSISTOR COUNT: 3662
MAX1830 MAX1831
13 PGND 12 VCC 11 FBSEL 10 REF 9 GND
QSOP
12
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3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches
Package Information
QSOP.EPS
MAX1830/MAX1831
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
13 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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