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19-2527; Rev 0; 7/02
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters
General Description
The MAX1927/MAX1928 800mA step-down converters power low-voltage microprocessors in compact equipment requiring the highest possible efficiency. The MAX1927/MAX1928 are optimized for generating low output voltages (down to 750mV) at high efficiency using small external components. The supply voltage range is from 2.6V to 5.5V and the guaranteed minimum output current is 800mA. 1MHz pulse-width modulation (PWM) switching allows for small external components. A unique control scheme minimizes ripple at light loads, while maintaining a low 140A quiescent current. The MAX1927/MAX1928 include a low on-resistance internal MOSFET switch and synchronous rectifier to maximize efficiency and minimize external component count. No external diode is needed. 100% duty-cycle operation allows for a dropout voltage of only 340mV at 800mA. Other features include internal soft-start, power-OK (POK) output, and selectable forced PWM operation for lower noise at all load currents. The MAX1928 is available with several preset output voltages: 1.5V (MAX1928-15), 1.8V (MAX1928-18), and 2.5V (MAX1928-25). The MAX1927R has adjustable output range down to 0.75V. The MAX1927/MAX1928 are available in a tiny 10-pin MAX package. o 800mA Output Current o Output Voltages from 0.75V to 5V o 2.6V to 5.5V Input Voltage Range o Power-OK Output o No Schottky Diode Required o Selectable Forced PWM Operation o 1MHz Fixed-Frequency PWM Operation o 140A Quiescent Current o Soft-Start o 10-Pin MAX Package
Features
MAX1927/MAX1928
Ordering Information
PART MAX1927REUB PRESET TEMP PINOUTPUT PACKAGE RANGE VOLTAGE Adj. to 0.75V -40C to +85C 10 MAX 1.5V 1.8V 2.5V -40C to +85C 10 MAX -40C to +85C 10 MAX -40C to +85C 10 MAX
Applications
WCDMA Handsets PDAs and Palmtops DSP Core Power Battery-Powered Equipment
MAX1928EUB15 MAX1928EUB18 MAX1928EUB25
Pin Configuration
TOP VIEW
PWM 1 GND REF FB COMP 2 3 4 5 10 POK 9 BATT
VIN 2.6V TO 5.5V
Typical Operating Circuit
PWM L1 LX SHDN RC COMP CC Cf FB C2 VOUT 0.75V AT 800mA
BATT C1
MAX1927R MAX1928
8 LX 7 PGND 6 SHDN
MAX1927R
POK
MAX
REF
PGND GND
________________________________________________________________ Maxim Integrated Products
1
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.
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters MAX1927/MAX1928
ABSOLUTE MAXIMUM RATINGS
BATT, PWM, POK, COMP, SHDN to GND ...............-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V LX, REF, FB to GND ................................-0.3V to (VBATT + 0.3V) Continuous Power Dissipation (TA = +70C) 10-Pin MAX (derate 5.6mW/C above +70C) ...........444mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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
(VBATT = 3.6V, SHDN = BATT, CREF = 0.1F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER BATT Input Voltage Undervoltage Lockout Threshold Quiescent Current Quiescent Current in Dropout Shutdown Supply Current REFERENCE AND ERROR AMP MAX1927R FB Voltage Accuracy MAX1928-15 MAX1928-18 MAX1928-25 FB Input Current MAX1928 MAX1927R MAX1927R MAX1928-15 MAX1928-18 MAX1928-25 1.231 2.6V < VBATT < 5.5V VBATT = 3.6V VBATT = 2.6V VBATT = 3.6V VBATT = 2.6V 1.1 0.11 0.738 1.477 1.773 2.462 5 0.75 1.5 1.8 2.5 10 10 250 210 175 125 1.25 0.5 0.25 0.3 0.17 0.2 0.48 1.3 0.13 -0.55 1.6 0.15 0.762 1.523 1.827 2.538 15 150 A nA V SHDN = GND VBATT rising or falling (35mV hysteresis) No load, pulse skipping, PWM = GND 1MHz switching CONDITIONS MIN 2.6 2.15 2.35 140 2 190 0.1 340 10 TYP MAX 5.5 2.55 240 UNITS V V A mA A A
Transconductance (gm)
S
Reference Voltage Accuracy Reference Supply Rejection PWM CONTROLLER P-Channel On-Resistance N-Channel On-Resistance Current-Sense Transresistance (RCS ) P-Channel Current-Limit Threshold P-Channel Pulse-Skipping Current Threshold N-Channel Negative Current-Limit Threshold
1.269 2 0.4 0.5 0.3 0.35
V mV
V/A A A A
2
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Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(VBATT = 3.6V, SHDN = BATT, CREF = 0.1F, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER N-Channel Synchronous Rectifier Turn-Off Threshold LX Leakage Current Maximum Duty Cycle Minimum Duty Cycle Internal Oscillator Frequency Thermal Shutdown Threshold POK COMPARATOR BATT Operating Voltage Range Output Low Voltage Output High Leakage Current IPOK = 0.1 mA VFB = 0.5V, IPOK = 1mA VPOK = 5.5V MAX1927R POK Threshold MAX1928-15 MAX1928-18 MAX1928-25 Output Valid to POK Release Delay LOGIC INPUTS (SHDN, PWM) Logic Input High Logic Input Low Logic Input Current 2.6V < VBATT < 5.5 V 2.6V < VBATT < 5.5 V VBATT = 5.5V 0.1 1.6 0.6 1 V V A POK transitions to high impedance 20ms after VFB > VPOK 0.650 1.305 1.566 2.175 15 0.675 1.350 1.620 2.250 20 1 0.01 5.5 0.1 1 0.700 1.395 1.674 2.325 25 ms V V V A 15C hysteresis PWM = GND PWM = BATT 0.85 15 1 160 1.15 VBATT = 5.5V, LX = GND or BATT -20 100 0 CONDITIONS MIN TYP 20 0.1 +20 MAX UNITS mA A % % MHz Degrees
MAX1927/MAX1928
ELECTRICAL CHARACTERISTICS
(VBATT = 3.6V, SHDN = BATT, CREF = 0.1F, TA = -40C to +85C, unless otherwise noted.)
PARAMETER BATT Input Voltage Undervoltage Lockout Threshold Quiescent Current Quiescent Current in Dropout Shutdown Supply Current REFERENCE AND ERROR AMP MAX1927R FB Voltage Accuracy MAX1928-15 MAX1928-18 MAX1928-25 FB Input Current MAX1928 0.732 1.47 1.764 2.45 5 0.768 1.53 1.836 2.55 15 A V SHDN = GND VBATT rising or falling (35mV hysteresis) No load, pulse skipping, PWM = GND CONDITIONS MIN 2.6 2.15 MAX 5.5 2.55 240 340 10 UNITS V V A A A
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3
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters MAX1927/MAX1928
ELECTRICAL CHARACTERISTICS (continued)
(VBATT = 3.6V, SHDN = BATT, CREF = 0.1F, TA = -40C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER FB Input Current Reference Voltage Accuracy Reference-Supply Rejection PWM CONTROLLER P-Channel On-Resistance N-Channel On-Resistance P-Channel Current-Limit Threshold P-Channel Pulse-Skipping Current Threshold LX Leakage Current Maximum Duty Cycle Minimum Duty Cycle Internal Oscillator Frequency POK COMPARATOR BATT Operating Voltage Range Output Low Voltage Output High Leakage Current IPOK = 0.1 mA VFB = 0.5V, IPOK = 1mA VPOK = 5.5V MAX1927R MAX1928-15 POK Threshold MAX1928-18 MAX1928-25 Output Valid to POK Release Delay LOGIC INPUTS (SHDN, PWM) Logic Input High Logic Input Low Logic Input Current 2.6V < VBATT < 5.5 V 2.6V < VBATT < 5.5 V VBATT = 5.5V 1.6 0.6 1 V V A POK transitions to high impedance 20ms after VFB > VPOK 0.650 1.305 1.566 2.175 15 PWM = GND 0.8 1 VBATT = 5.5V, LX = GND or BATT 2.6V < VBATT < 5.5V VBATT = 3.6V VBATT = 2.6V VBATT = 3.6V VBATT = 2.6V 1.1 0.10 -20 100 0 1.2 5.5 0.1 1 0.700 1.395 1.674 2.325 25 ms V 0.10 MAX1927R 1.22 CONDITIONS MIN MAX 150 1.269 2 0.4 0.5 0.30 0.35 1.6 0.16 +20 UNITS nA V mV
A A A % % MHz V V A
4
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Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters MAX1927/MAX1928
Typical Operating Characteristics
(Circuits of Figure 3 and 4, TA = +25C, unless otherwise noted.)
MAX1927R EFFICIENCY vs. LOAD CURRENT
MAX1927 toc01
MAX1928-25 EFFICIENCY vs. LOAD CURRENT
MAX1927 toc02
MAX1928-18 EFFICIENCY vs. LOAD CURRENT
VIN = 2.7V
MAX1927 toc03
100 90 EFFICIENCY (%) 80 70 60 50 40 1 10 100 VIN = 5V VIN = 3.6V
100 90 EFFICIENCY (%) 80 70 60 50 VIN = 5V VIN = 3.6V
100 90 80 EFFICIENCY (%) 70 60 50 40 VOUT = 1.8V 30 VIN = 5V
VIN = 3.6V
VOUT = 3.3V 40 1000 1 10 100 1000 LOAD CURRENT (mA) LOAD CURRENT (mA)
1
10
100
1000
LOAD CURRENT (mA)
MAX1928-15 EFFICIENCY vs. LOAD CURRENT
VIN = 2.7V 90 80 EFFICIENCY (%) 70 60 50 40 VOUT = 1.5V 30 1 10 100 1000 LOAD CURRENT (mA) 30 1 VIN = 3.6V VIN = 5V
MAX1927 toc04
MAX1927R EFFICIENCY vs. LOAD CURRENT
MAX1927 toc05
MAX1928-25 DROPOUT VOLTAGE vs. LOAD CURRENT
450 DROPOUT VOLTAGE (mV) 400 350 300 250 200 150 100
MAX1927 toc06
100
100 90 80 EFFICIENCY (%) 70 60 50 40 VOUT = 1V 10 100 VIN = 3.6V VIN = 5V VIN = 2.7V
500
50 0 0
VIN = 2.5V 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 LOAD CURRENT (A)
1000
LOAD CURRENT (mA)
MAX1928-18 OUTPUT VOLTAGE vs. LOAD CURRENT
MAX1927 toc07
NO-LOAD INPUT CURRENT vs. INPUT VOLTAGE
MAX1927 toc08
OSCILLATOR FREQUENCY vs. INPUT VOLTAGE
TA = +85C
MAX1927 toc09
1.90 1.88 1.86 OUTPUT VOLTAGE (V) 1.84 1.82 1.80 1.78 1.76 1.74 1.72 1.70 VIN = 3.6V
400 350 INPUT CURRENT (A) 300 250 200 150 100 50 0
1.06 OSCILLATOR FREQUENCY (MHz) 1.04 1.02 1.00 0.98 0.96 0.94 TA = -40C TA = +25C
0 100 200 300 400 500 600 700 800 900 1000 LOAD CURRENT (mA)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 INPUT VOLTAGE (V)
2.6
3.1
3.6
4.1
4.6
5.1
5.6
INPUT VOLTAGE (V)
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5
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters MAX1927/MAX1928
Typical Operating Characteristics (continued)
(Circuits of Figure 3 and 4, TA = +25C, unless otherwise noted.)
MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE
VOUT = 1V MAXIMUM LOAD CURRENT (A) 1.2 1.0 0.8 0.6 VOUT 0.4 0.2 0 2.6 3.1 3.6 4.1 4.6 5.1 5.6 IIN VOUT = 1.8V VOUT = 2.5V
MAX1927 toc10
STARTUP WAVEFORM
MAX1927 toc11
POK WAVEFORM
MAX1927 toc12
1.4
SHDN
5V/div
SHDN
5V/div
POK 1V/div 2V/div
200mA/div
VOUT 20ms/div
2V/div
1ms/div
INPUT VOLTAGE (V)
HEAVY-LOAD SWITCHING WAVEFORMS
MAX1927 toc13
LIGHT-LOAD SWITCHING WAVEFORMS
MAX1927 toc14
VOUT (AC-COUPLED)
10mV/div
VOUT (AC-COUPLED)
10mV/div
IL
200mA/div
LX
5V/div
LX 400ns/div
5V/div
IL 2ms/div
200mA/div
LOAD TRANSIENT
MAX1927 toc15
LINE TRANSIENT
MAX1927 toc16
VOUT (AC-COUPLED)
100mV/div
VOUT (AC-COUPLED)
10mV/div
900mA VIN ILOAD 500mA/div 250mA
4.2V 3V 2V/div
100s/div
1ms/div
6
_______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters
Pin Description
PIN 1 2 3 4 NAME PWM GND REF FB FUNCTION Forced-PWM Input. Drive to GND to use PWM at medium to heavy loads and pulse-skipping at light loads. Drive to BATT to force PWM operation at all loads. Ground Internal 1.25V Reference. Bypass to GND with a 0.1F capacitor. Output Feedback Sense Input. To set the output voltage to the preset voltage (MAX1928), connect FB directly to the output. To adjust the output voltage (MAX1927R), connect FB to the center of an external resistordivider between the output and GND. FB regulation voltage is 0.75V. Compensation Input. See the Compensation, Stability, and Output Capacitor section for compensation component selection. Shutdown Control Input. Drive low to shut down the converter. Drive high for normal operation. Power Ground Inductor Connection to the drains of the internal power MOSFETs. Supply Voltage Input. Connect to a 2.6V to 5.5V source. Bypass to GND with a low-ESR 10F capacitor. Power-OK Open-Drain Output. Once the soft-start routine has completed, POK goes high impedance 20ms after FB exceeds 90% of its expected final value.
BATT COMP SLOPE COMPENSATION PWM COMPARATOR BIAS P P
MAX1927/MAX1928
5 6 7 8 9 10
COMP SHDN PGND LX BATT POK
LX
MAX1927 MAX1928
1MHz OSC
PFM CURRENT COMPARATOR
PWM
ILIM COMPARATOR PWM CONTROL N
N
SHDN N-CHANNEL CURRENT COMPARATOR TO COMP REF 1.25V REFERENCE POWER-OK CONTROL POK
PGND
FB MAX1927R ONLY MAX1928 ONLY
GND
Figure 1. Simplified Functional Diagram _______________________________________________________________________________________ 7
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters MAX1927/MAX1928
Detailed Description
The MAX1927/MAX1928 PWM step-down DC-DC converters accept inputs as low as 2.6V, while delivering 800mA to output voltages as low as 0.75V. These devices operate in one of two modes to optimize noise and quiescent current. Under heavy loads, MAX1927/ MAX1928 operate in pulse-width modulation (PWM) mode and switch at a fixed 1MHz frequency. Under light loads, they operate in PFM mode to reduce power consumption. In addition, both devices provide selectable forced PWM operation for minimum noise at all load currents. the use of a small, low-valued, output filter capacitor. The resulting load regulation is 0.3% (typ) from 0 to 800mA.
Forced PWM Operation
To force PWM-only operation, connect PWM to BATT. Forced PWM operation is desirable in sensitive RF and data-acquisition applications to ensure that switching noise does not interfere with sensitive IF and data sampling frequencies. A minimum load is not required during forced PWM operation because the synchronous rectifier passes reverse inductor current as needed to allow constant frequency operation with no load. Forced PWM operation has higher quiescent current than PFM (2mA typ compared to 140A) due to continuous switching.
PFM Operation and PWM Control Scheme
The PFM mode improves efficiency and reduces quiescent current to 140A at light loads. The MAX1927/ MAX1928 initiate pulse-skipping PFM operation when the peak inductor current drops below 130mA. During PFM operation, the MAX1927/MAX1928 switch only as necessary to service the load, reducing the switching frequency and associated losses in the internal switch, synchronous rectifier, and inductor. During PFM mode, a switching cycle initiates when the error amplifier senses that the output voltage has dropped below the regulation point. If the output voltage is low, the P-channel MOSFET switch turns on and conducts current to the output filter capacitor and load. The PMOS switch turns off when the PWM comparator is satisfied. The MAX1927/MAX1928 then wait until the error amplifier senses a low output voltage to start again. Some jitter is normal during the transition from PFM to PWM with loads around 100mA. This has no adverse impact on regulation. At loads greater than 130mA, the MAX1927/MAX1928 use a fixed-frequency, current-mode, PWM controller capable of achieving 100% duty cycle. Current-mode feedback provides cycle-by-cycle current limiting, superior load and line response, as well as overcurrent protection for the internal MOSFET and synchronous rectifier. A comparator at the P-channel MOSFET switch detects overcurrent conditions exceeding 1.1A. During PWM operation, the MAX1927/MAX1928 regulate output voltage by switching at a constant frequency and then modulating the power transferred to the load using the PWM comparator (Figure 1). The error-amp output, the main switch current-sense signal, and the slope compensation ramp are all summed at the PWM comparator. The comparator modulates the output power by adjusting the peak inductor current during the first half of each cycle based on the output-error voltage. The MAX1927/MAX1928 have relatively low ACloop gain coupled with a high-gain integrator to enable
8
100% Duty-Cycle Operation
The maximum on-time can exceed one internal oscillator cycle, which permits operation at 100% duty cycle. As the input voltage drops, the duty cycle increases until the internal P-channel MOSFET stays on continuously. Dropout voltage at 100% duty cycle is the output current multiplied by the sum of the internal PMOS onresistance (typically 0.25) and the inductor resistance. Near dropout, switching cycles can be skipped, reducing switching frequency. However, voltage ripple remains small because the current ripple is still low.
Synchronous Rectification
An N-channel synchronous rectifier eliminates the need for an external Schottky diode and improves efficiency. The synchronous rectifier turns on during the second half of each cycle (off-time). During this time, the voltage across the inductor is reversed, and the inductor current falls. In normal mode, the synchronous rectifier is turned off when either the output falls out of regulation (and another on-time begins) or when the inductor current approaches zero. In forced PWM mode, the synchronous rectifier remains active until the beginning of a new cycle.
Shutdown Mode
Driving SHDN to GND places the MAX1927/MAX1928 in shutdown mode. In shutdown, the reference, control circuitry, internal switching MOSFET, and synchronous rectifier turn off and the output becomes high impedance. Drive SHDN high for normal operation. Input current falls to 0.1A (typ) during shutdown mode.
POK Output
POK is an open-drain output that goes high impedance 20ms after the soft-start ramp has concluded and VFB is within 90% of the threshold. POK is low impedance when in shutdown.
_______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters MAX1927/MAX1928
Table 1. FB Regulation Voltages
PART
LX
PRESET OUTPUT VOLTAGE 0.75V, Adjustable 1.5 V 1.8 V 2.5 V
MAX1927R MAX1928-15
R1
MAX1927R
FB
MAX1928-18 MAX1928-25
R2 50k
following equation to calculate the maximum RMS input current: I IRMS = OUT x VOUT x (VIN - VOUT ) VIN
Figure 2. Setting the Adjustable Output Voltage
Applications Information
Output Voltage Selection
The MAX1927/MAX1928 have preset output voltages. In addition, the MAX1927R has an adjustable output. To set the output voltage at the preset voltage, connect FB to the output. See Table 1 for a list of the preset voltages and their corresponding part numbers. The output voltage for the MAX1927R is adjustable from 0.75V to the input voltage by connecting FB to a resistor-divider between the output and GND (Figure 2). To determine the values of the resistor-divider, first select a value for feedback resistor R2 between 5k to 50k. R1 is then given by: V R1 = R2 x OUT - 1 VFB where VFB is 0.75V.
Compensation, Stability, and Output Capacitor
The MAX1927/MAX1928 are externally compensated with a resistor and a capacitor (see Figure 3, RC and CC) in series from COMP to GND. An additional capacitor (Cf) may be required from COMP to GND if highESR output capacitors are used. The capacitor integrates the current from the transimpedance amplifier, averaging output capacitor ripple. This sets the device speed for transient response and allows the use of small ceramic output capacitors because the phaseshifted capacitor ripple does not disturb the current regulation loop. The resistor sets the proportional gain of the output error voltage by a factor g m R C . Increasing this resistor also increases the sensitivity of the control loop to output ripple. The resistor and capacitor set a compensation zero that defines the system's transient response. The load creates a dynamic pole, shifting in frequency with changes in load. As the load decreases, the pole frequency decreases. System stability requires that the compensation zero must be placed to ensure adequate phase margin (at least 30 at unity gain). The following is a design procedure for the compensation network: 1) Select an appropriate converter bandwidth (fC) to stabilize the system while maximizing transient response. This bandwidth should not exceed 1/10 of the switching frequency. 2) Calculate the compensation capacitor, CC, based on this bandwidth: For the MAX1927: V 1 R2 1 OUT CC = x gm x x x IOUT(MAX) RCS R1+ R2 2fC
Input Capacitor Selection
Capacitor equivalent series resistance (ESR) is a major contributor to input ripple in high-frequency DC-DC converters. Ordinary aluminum-electrolytic capacitors have high ESR and should be avoided. Low-ESR aluminum electrolytic capacitors are acceptable and relatively inexpensive. Low-ESR tantalum capacitors or polymer capacitors are better and provide a compact solution for space-constrained surface-mount designs. Ceramic capacitors have the lowest ESR overall. The input filter capacitor reduces peak currents and noise at the input voltage source. Connect a low-ESR bulk capacitor (10F typ) to the input. Select this bulk capacitor to meet the input ripple requirements and voltage rating rather than capacitance value. Use the
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9
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters MAX1927/MAX1928
For the MAX1928: V 1 1 OUT CC = x x (gm ) x 2f C IOUT(MAX) RCS Resistors R1 and R2 are external to the MAX1927 (see the Setting the Output Voltage section). IOUT(MAX) is the maximum output current, R CS = 0.48V/A, and g m = 250S for the MAX1927. See the Electrical Characteristics table for MAX1928 gm values. Select the closest standard CC value that gives an acceptable bandwidth. 3) Calculate the equivalent load impedance, RL, by: RL = VOUT IOUT(MAX) cel out the dominant pole created by the output load and the output capacitance: 1 1 = 2 x RL x COUT 2 x RC x CC Solving for RC gives: R x COUT RC = L CC 5) Calculate the high-frequency compensation pole to cancel the zero created by the output capacitor's ESR: 1 1 = 2 x RESR x COUT 2 x RC x Cf
4) Calculate the compensation resistance (RC) to can-
VIN 2.6V TO 5.5V BATT C1 10F
PWM L1 CDRH4D18 4.7H LX SHDN VOUT 1.8V AT 800mA C2 10F
MAX1928-18
CC 1200pF COMP RC 18k Cf 22pF
FB POK PGND
REF GND
C3 0.1F
Figure 3. Applications Circuit for the MAX1928
VIN 2.6V TO 5.5V BATT C1 10F SHDN LX FB POK PGND GND C3 0.1F R2 49.9k 1% R1 16.5k 1%
PWM
L1 CDRH4D18 4.7H
VOUT 1V AT 800mA C2 10F
MAX1927R
COMP CC 680pF RC 15k Cf 22pF
REF
Figure 4. Applications Circuit for the MAX1927 10 ______________________________________________________________________________________
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters
Solving for Cf gives: R x COUT Cf = ESR RC or 22pF, whichever is greater. ground pins at a single common node in a star ground configuration. The external voltage feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces, such as those from the LX pin, away from the voltage feedback network. Position the bypass capacitors as close as possible to their respective pins to minimize noise coupling. For optimum performance, place input and output capacitors as close to the device as possible. Connect GND and PGND to the highest quality system ground. The MAX1928 evaluation kit illustrates an example PC board layout and routing scheme.
MAX1927/MAX1928
Standard Application Circuits
Figures 3 and 4 are standard applications circuits for the MAX1927/MAX1928. Figure 3 illustrates the preset output voltages (MAX1928), while Figure 4 shows the adjustable configuration (MAX1927). Table 2 lists part numbers and suppliers for the components used in these circuits.
Chip Information
TRANSISTORS: 3282 PROCESS: BiCMOS
PC Board Layout and Routing
High switching frequencies and large peak currents make PC board layout a very important part of design. Good design minimizes EMI, noise on the feedback paths, and voltage gradients in the ground plane, all of which can result in instability or regulation errors. Connect the inductor, input filter capacitor, and output filter capacitor as close together as possible and keep their traces short, direct, and wide. Connect their
Table 2. Suggested Parts/Suppliers
PART Inductor Input/Output Capacitors COMP Capacitor REF Capacitor PART NUMBER CDRH3D16-4R7 JMK212BJ106MG GRM1881X1H561J EMK107BJ104KA MANUFACTURER Sumida Taiyo Yuden Murata Taiyo Yuden PHONE USA 847-956-0666 Japan 81-3-3607-5111 408-573-4150 770-436-1300 408-573-4150 WEBSITE www.sumida.com www.t-yuden.com www.murata.com www.t-yuden.com
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11
Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters MAX1927/MAX1928
Package Information
(The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)
e
10
4X S
10
INCHES MAX DIM MIN 0.043 A 0.006 A1 0.002 A2 0.030 0.037 0.120 D1 0.116 0.118 0.114 D2 0.116 0.120 E1 E2 0.114 0.118 H 0.187 0.199 L 0.0157 0.0275 L1 0.037 REF b 0.007 0.0106 e 0.0197 BSC c 0.0035 0.0078 0.0196 REF S 0 6
MILLIMETERS MAX MIN 1.10 0.15 0.05 0.75 0.95 3.05 2.95 3.00 2.89 3.05 2.95 2.89 3.00 4.75 5.05 0.40 0.70 0.940 REF 0.177 0.270 0.500 BSC 0.090 0.200 0.498 REF 0 6
H y 0.500.1 0.60.1
1
1
0.60.1
TOP VIEW
BOTTOM VIEW
D2 GAGE PLANE A2 A b D1 A1
E2 c E1 L1
L
FRONT VIEW
SIDE VIEW
PROPRIETARY INFORMATION TITLE:
PACKAGE OUTLINE, 10L uMAX/uSOP
APPROVAL DOCUMENT CONTROL NO. REV.
21-0061
I
1 1
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.
12 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
10LUMAX.EPS

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