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19-0165; Rev 2; 1/96
5V, Step-Down, Current-Mode PWM DC-DC Converters
__________________General Description
The MAX730A/MAX738A/MAX744A are 5V-output CMOS, step-down switching regulators. The MAX738A/ MAX744A accept inputs from 6V to 16V and deliver 750mA. The MAX744A guarantees 500mA load capability for inputs above 6V and has tighter oscillator frequency limits for low-noise (radio) applications. The MAX730A accepts inputs between 5.2V and 11V and delivers 450mA for inputs above 6V. Typical efficiencies are 85% to 96%. Quiescent supply current is 1.7mA and only 6A in shutdown. Pulse-width modulation (PWM) current-mode control provides precise output regulation and excellent transient responses. Output voltage accuracy is guaranteed to be 5% over line, load, and temperature variations. Fixed-frequency switching allows easy filtering of output ripple and noise, as well as the use of small external components. These regulators require only a single inductor value to work in most applications, so no inductor design is necessary. The MAX730A/MAX738A/MAX744A also feature cycleby-cycle current limiting, overcurrent limiting, undervoltage lockout, and programmable soft-start protection.
________________________________Features
o 750mA Load Currents (MAX738A/MAX744A) o High-Frequency, Current-Mode PWM o 159kHz to 212.5kHz Guaranteed Oscillator Frequency Limits (MAX744A) o 85% to 96% Efficiencies o 1.7mA Quiescent Current o 6A Shutdown Supply Current o Single Preselected Inductor Value, No Component Design Required o Overcurrent, Soft-Start, and Undervoltage Lockout Protection o Cycle-by-Cycle Current Limiting o 8-Pin DIP/SO Packages (MAX730A)
MAX730A/MAX738A/MAX744A
_________________Ordering Information
PART MAX730ACPA MAX730ACSA MAX730AC/D MAX730AEPA MAX730AESA MAX730AMJA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C PIN-PACKAGE 8 Plastic DIP 8 SO Dice* 8 Plastic DIP 8 SO 8 CERDIP
___________________________Applications
Portable Instruments Cellular Phones and Radios Personal Communicators Distributed Power Systems Computer Peripherals
Ordering Information continued at end of data sheet. *Contact factory for dice specifications.
__________Typical Operating Circuit
INPUT 6V TO 16V V+ 33H LX OUTPUT 5V
_________________Pin Configurations
TOP VIEW
68F
SHDN 1
MAX738A MAX744A
ON/OFF SHDN OUT
8 7
V+ LX GND OUT
100F
REF
2
SS 3 CC 4
MAX730A MAX738A MAX744A
DIP
6 5
REF SS GND
CC
Pin Configurations continued on last page.
________________________________________________________________ Maxim Integrated Products
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5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
ABSOLUTE MAXIMUM RATINGS
Pin Voltages V+ (MAX730A) ......................................................+12V, -0.3V V+ (MAX738A/MAX744A) .....................................+18V, -0.3V LX (MAX730A) .................................(V+ - 12V) to (V+ + 0.3V) LX (MAX738A/MAX744A) ................(V+ - 21V) to (V+ + 0.3V) OUT .................................................................................25V SS, CC, SHDN .........................................-0.3V to (V+ + 0.3V) Peak Switch Current (ILX) ........................................................2A Reference Current (IREF) ...................................................2.5mA Continuous Power Dissipation (TA = +70C) 8-Pin Plastic DIP (derate 9.09mW/C above +70C)...727mW 8-Pin SO (derate 5.88mW/C above +70C) ...............471mW 16-Pin Wide SO (derate 9.52mW/C above +70C) .....762mW 8-Pin CERDIP (derate 8.00mW/C above +70C) .......640mW Operating Temperature Ranges: MAX7_ _AC_ _....................................................0C to +70C MAX7_ _AE_ _ .................................................-40C to +85C MAX7_ _AMJA ..............................................-55C to +125C Junction Temperatures: MAX7_ _AC_ _/AE_ _...................................................+150C MAX7_ _AMJA.............................................................+175C Storage Temperature Range ............................-65C to +160C Lead Temperature (soldering, 10sec).............................+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
(Circuit of Figure 3, V+ = 9V for the MAX730A, V+ = 12V for the MAX738A/MAX744A, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER CONDITIONS 0mA < ILOAD < 450mA, MAX730AC V+ = 6.0V to 11.0V 0mA < ILOAD < 450mA, MAX730AE 0mA < ILOAD < 300mA, MAX730AM 0mA < ILOAD < 450mA, MAX738AC/AE V+ = 6.0V to 16.0V 0mA < ILOAD < 350mA, MAX738AM 0mA < ILOAD < 500mA, MAX744AC/AE 0mA < ILOAD < 375mA, MAX744AM V+ = 10.2V to 16.0V, 0mA < ILOAD < 750mA 0mA < ILOAD < 750mA, MAX744AC/AE 4.75 5.00 0mA < ILOAD < 600mA, MAX744AM 5.2 V+ = 5.2V to 11.0V V+ = 6.0V to 16.0V 0.15 0.15 0.15 11.0 6.0 16.0 6.0 16.0 V %/V 5.25 4.75 5.00 5.25 4.75 5.00 5.25 MAX730A MIN TYP MAX MAX738A MIN TYP MAX MAX744A MIN TYP MAX UNITS
Output Voltage
4.75 5.00
5.25
4.75 5.00
5.25
V
V+ = 9.0V to 16.0V
Input Voltage Range Line Regulation
2
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5V, Step-Down, Current-Mode PWM DC-DC Converters
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 3, V+ = 9V for the MAX730A, V+ = 12V for the MAX738A/MAX744A, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Load Regulation Efficiency CONDITIONS ILOAD = 0mA to 300mA ILOAD = 0mA to 750mA V+ = 9.0V, ILOAD = 300mA V+ = 12V, ILOAD = 750mA 1.7 V+ = 6.0V to 9.0V Supply Current (includes switch current) V+ = 9.0V to 12.0V V+ = 12.0V to 16.0V Standby Current Shutdown Input Threshold Shutdown Input Leakage Current Short-Circuit Current Undervoltage Lockout LX On Resistance LX Leakage Current Reference Voltage Reference Drift 130 Oscillator Frequency Compensation Pin Impedance V+ = 6.0V to 16.0V MAX744AC/AE MAX744AM 7500 7500 V+ rising V+ falling ILX = 500mA V+ = 12V, LX = 0V V+ = 12V, TA = +25C 0.5 1.0 1.15 1.23 50 170 210 130 1.30 0.5 1.0 1.15 1.23 50 160 190 159.0 185.0 212.5 159.0 7500 216.5 kHz 1.30 1.5 4.7 5.2 MAX744AC/AE MAX744AM MAX744AC/AE MAX744AM MAX744AC MAX744AE MAX744AM SHDN = 0V (Note 1) VIH VIL 2.0 0.25 1.0 1.5 5.7 6.0 5.0 0.5 1.0 1.15 1.23 50 1.30 6.0 100.0 2.0 0.25 1.0 1.5 5.7 6.0 5.7 6.0 100.0 2.0 0.25 1.0 6.0 3.0 92 MAX730A MIN TYP MAX 0.0005 0.0005 90 87 1.7 3.0 0.0005 90 87 1.7 1.2 3.0 2.5 3.0 3.0 3.5 4.0 4.3 4.5 100.0 A V A A V A V ppm/C mA MAX738A MIN TYP MAX MAX744A MIN TYP MAX UNITS %/mA %
MAX730A/MAX738A/MAX744A
Note 1: The standby current typically settles to 25A (over temperature) within 2 seconds; however, to decrease test time, the part is guaranteed at a 100A maximum value.
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5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
__________________________________________Typical Operating Characteristics
(Circuit of Figure 3, TA = +25C, unless otherwise noted.)
MAX730A EFFICIENCY vs. OUTPUT CURRENT
100 (NOTES 3, 6) 90 EFFICIENCY (%) EFFICIENCY (%) V+ = 5.5V 80 V+ = 7V V+ = 9V 70 V+ = 11V 90 100
MAX738A EFFICIENCY vs. OUTPUT CURRENT
100 (NOTES 3, 6) 90 EFFICIENCY (%)
MAX744A EFFICIENCY vs. OUTPUT CURRENT
(NOTE 3)
V+ = 6V 80 V+ = 8V V+ = 12V 70 V+ = 16V
V+ = 6.0V 80 V+ = 9.0V V+ = 12.0V 70 V+ = 16.0V
60 0 200 600 800 OUTPUT CURRENT (mA) 400 1000
60 0 200 600 800 OUTPUT CURRENT (mA) 400 1000
60 0 200 400 600 800 OUTPUT CURRENT (mA) 1000
MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE
1400 (NOTES 3, 6) MAXIMUM OUTPUT CURRENT (mA) MAXIMUM OUTPUT CURRENT (mA) 1200 1000 800 600 400 200 4 1400
MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE, NO R1
3.0 (NOTES 3, 6) QUIESCENT SUPPLY CURRENT (mA) 2.5 2.0 1.5 1.0 0.5
QUIESCENT SUPPLY CURRENT vs. TEMPERATURE
MAX730A, V+ = 9.0V MAX738A, V+ = 12.0V MAX744A, V+ = 12.0V
1200 MAX744A MAX730A 1000
MAX738A
MAX744A
MAX738A
MAX730A
(NOTES 4, 5) 800 4 6 8 10 12 14 16 SUPPLY VOLTAGE (V) 6 8 10 12 14 16 0 -60 -40 -20 0 20 40 60 80 100 120 140 160 TEMPERATURE (C)
SUPPLY VOLTAGE (V)
MAX738A/MAX744A QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE
2.5 QUIESCENT SUPPLY CURRENT (mA) 1000
MAX738A/MAX744A PEAK INDUCTOR CURRENT vs. OUTPUT CURRENT
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STANDBY SUPPLY CURRENT vs. TEMPERATURE
(NOTES 4, 5)
PEAK INDUCTOR CURRENT (mA)
2.0
800
STANDBY SUPPLY CURRENT (A)
18 16 14 12 10 8 6 4 2 0
V+ = 16V MAX738A/MAX744A ONLY V+ = 12V MAX738A/MAX744A ONLY
1.5 UNDERVOLTAGE LOCKOUT HYSTERESIS UNDERVOLTAGE LOCKOUT ENABLED
600
1.0
400
V+ = 6.6V
V+ = 8.0V TO 16.0V
0.5
200 L1 = 100H C4 = 150F
V+ = 9V V+ = 6V -60 -40 -20 0 20 40 60 80 100 120 140 160
0 0 2 4 6 8 10 12 14 16
0 0 100 200 300 400 500 600 700 800
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5V, Step-Down, Current-Mode PWM DC-DC Converters
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 3, TA = +25C, unless otherwise noted.)
MAX730A/MAX738A/MAX744A
OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE
220
(NOTE 3)
MAX730A OSCILLATOR FREQUENCY vs. TEMPERATURE
240 220 200 180 160 140 V+ = 11.0V (NOTE 4)
210 OSCILLATOR FREQUENCY (kHz) 200 190 MAX744A 180 170 160 150 4 6 8 10 12 SUPPLY VOLTAGE (V) 14 16 OSCILLATOR FREQUENCY (kHz) MAX730A
V+ = 5.5V
V+ = 7.0V V+ = 9.0V
MAX738A 0 -60 -40 -20 0 20 40 60 80 100 120 140 160 TEMPERATURE (C)
MAX738A OSCILLATOR FREQUENCY vs. TEMPERATURE
200 (NOTE 4) 190 SUPPLY CURRENT (mA) 180 170 160 150 140 130 120 -60 -40 -20 0 20 40 60 80 100 120 140 160 TEMPERATURE (C) V+ = 12.0V V+ = 9.0V V+ = 16.0V OSCILLATOR FREQUENCY (kHz) V+ = 6.0V 200 210
MAX744A OSCILLATOR FREQUENCY vs. TEMPERATURE
(NOTE 4) V+ = 6.0V
V+ = 16.0V 190 V+ = 9.0V 180 V+ = 12.0V 170 -60 -40 -20 0
20 40 60 80 100 120 140 160 TEMPERATURE (C)
Note 3: Commercial temperature range external component values in Table 3. Note 4: Wide temperature range external component values in Table 3. Note 5: Standby and shutdown current includes all external component leakage currents. Capacitor leakage currents dominate at T > +85C, A Sanyo OS-CON capacitors were used. Note 6: Operation beyond the specifications listed in the electrical characteristics may exceed the power dissipation ratings of the device.
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5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 3, TA = +25C, unless otherwise noted.) MAX738A/MAX744A SWITCHING WAVEFORMS, CONTINUOUS CONDITION MAX738A/MAX744A SWITCHING WAVEFORMS, DISCONTINUOUS CONDITION
12V A 0V 200mA 0mA C 2s/div A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +12V B: INDUCTOR CURRENT, 200mA/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div, AC-COUPLED COUT = 390F, V+ = 12V, IOUT = 150A, 2s/div A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +12V B: INDUCTOR CURRENT, 200mA/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div, AC-COUPLED COUT = 390F, V+ = 12V, IOUT = 150A B
12V A 0V 200mA 0mA B
C
MAX730A LINE-TRANSIENT RESPONSE
MAX738A/MAX744A LINE-TRANSIENT RESPONSE
A A 16V B 11V B 6V 0V 100ms/div A: VOUT, 50mV/div, DC-COUPLED B: V+, 5V/div, 10.2V TO 16.0V IOUT = 750mA 0V 10.2V
100ms/div A: VOUT, 50mV/div, DC-COUPLED B: V+, 5V/div, 6.0V TO 11.0V IOUT = 300mA
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5V, Step-Down, Current-Mode PWM DC-DC Converters
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 3, TA = +25C, unless otherwise noted.) MAX730A LOAD-TRANSIENT RESPONSE MAX738A/MAX744A LOAD-TRANSIENT RESPONSE
MAX730A/MAX738A/MAX744A
A
A
300mA B 20mA
750mA B 50mA
50ms/div A: VOUT, 50mV/div, DC-COUPLED B: IOUT, 200mA/div, 20mA TO 300mA V+ = 9V
50ms/div A: VOUT, 50mV/div, DC-COUPLED B: IOUT, 500mA/div, 50mA TO 750mA V+ = 12V
______________________________________________________________Pin Description
PIN 8-PIN DIP/SO 16-PIN WIDE SO NAME FUNCTION
1
2
2
3
SHDN
Shutdown--active low. Ground to power-down chip, tie to V+ for normal operation. Output voltage falls to 0V when SHDN is low. Reference-Voltage Output (+1.23V) supplies up to 100A for extended loads. Bypass to GND with a capacitor that does not exceed 0.047F. Soft-Start. Capacitor between SS and GND provides soft-start and short-circuit protection. 510k resistor from SS to SHDN provides current boost. Compensation Capacitor Input externally compensates the outer feedback loop. Connect to OUT with a 330pF capacitor. Output Voltage Sense Input provides regulation feedback sensing. Connect to +5V output. Ground pins are internally connected. Connect both pins to ground. Drain of internal P-channel power MOSFET. Supply-Voltage Input. Bypass to GND with 1F ceramic and large-value electrolytic capacitors in parallel. The 1F capacitor must be as close to V+ and GND pins as possible. No Connect--no internal connections to these pins.
REF
3
7
SS
4 5 6 7 8
8 9 10, 11 12, 13, 14 1, 15, 16 4, 5, 6
CC OUT GND LX V+ N.C.
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5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
_________________Detailed Description
The MAX730A/MAX738A/MAX744A switch-mode regulators use a current-mode pulse-width-modulation (PWM) control system coupled with a simple step-down (buck) regulator topography. They convert an unregulated DC voltage from 5.2V to 11V for the MAX730A, and from 6V to 16V for the MAX738A/MAX744A. The current-mode PWM architecture provides cycle-bycycle current limiting, improved load-transient response characteristics, and simpler outer-loop design. The controller consists of two feedback loops: an inner (current) loop that monitors the switch current via the current-sense resistor and amplifier, and an outer (voltage) loop that monitors the output voltage through the error amplifier (Figure 1). The inner loop performs cycle-bycycle current limiting, truncating the power transistor ontime when the switch current reaches a predetermined threshold. This threshold is determined by the outer loop. For example, a sagging output voltage produces an error signal that raises the threshold, allowing the circuit to store and transfer more energy during each cycle. for overcurrent protection to function properly. A typical value is 0.1F.
Undervoltage Lockout
The MAX738A/MAX744A's undervoltage-lockout feature monitors the supply voltage at V+, and allows operation to start when V+ rises above 5.7V (6V guaranteed). When V+ falls, operation continues until the supply voltage falls below 5.45V (see the MAX738A/MAX744A Quiescent Supply Current vs. Supply Voltage graph in the Typical Operating Characteristics). The MAX730A is similar, starting operation at V+ > 4.7V and continuing to operate down to 4.45V. When an undervoltage condition is detected, control logic turns off the output power FET and discharges the SS capacitor to ground. This prevents partial turn-on of the power MOSFET and avoids excessive power dissipation. The control logic holds the output power FET off until the supply voltage rises above approximately 4.7V (MAX730A) or 5.7V (MAX738A/ MAX744A), at which time an SS cycle begins.
Shutdown Mode
The MAX730A/MAX738A/MAX744A are shut down by keeping SHDN at ground. In shutdown mode, the output drops to 0V and the output power FET is held in an off state. The internal reference also turns off, which causes the SS capacitor to discharge. Typical standby current in shutdown mode is 6A. The actual design limit for standby current is much less than the 100A specified in the Electrical Characteristics (see Standby Current vs. Temperature in the Typical Operating Characteristics). However, testing to tighter limits is prohibitive because the current takes several seconds to settle to a final value. For normal operation, connect SHDN to V+. Note that coming out of shutdown mode initiates an SS cycle.
Programmable Soft-Start
Figures 1 and 2 show a capacitor and a resistor connected to the soft-start (SS) pin to ensure an orderly power-up. Typical values are 0.1F and 510k. SS controls both the SS timing and the maximum output current that can be delivered while maintaining regulation. The charging capacitor slowly raises the clamp on the error-amplifier output voltage, limiting surge currents at power-up by slowly increasing the cycle-by-cycle current-limit threshold. The 510k resistor sets the SS clamp at a value high enough to maintain regulation, even at currents exceeding 1A. This resistor is not necessary for lower-current loads. Refer to the Maximum Output Current vs. Supply Voltage graph in the Typical Operating Characteristics. Table 1 lists timing characteristics for selected capacitor values and circuit conditions. The overcurrent comparator trips when the load exceeds approximately 1.5A. An SS cycle begins when either an undervoltage or overcurrent fault condition triggers an internal transistor to momentarily discharge the SS capacitor to ground. An SS cycle also begins at power-up and when coming out of shutdown mode.
Continuous-/DiscontinuousConduction Modes
The input voltage, output voltage, load current, and inductor value determine whether the IC operates in continuous or discontinuous mode. As the inductor value or load current decreases, or the input voltage increases, the MAX730A/MAX738A/MAX744A tend to operate in discontinuous-conduction mode (DCM). In DCM, the inductor current slope is steep enough so it decays to zero before the end of the transistor off-time. In continuous-conduction mode (CCM), the inductor current never decays to zero, which is typically more efficient than DCM. CCM allows the MAX730A/ MAX738A/MAX744A to deliver maximum load current, and is also slightly less noisy than DCM, because the peak-to-average inductor current ratio is reduced.
Overcurrent Limiting
The overcurrent comparator triggers when the load current exceeds approximately 1.5A. On each clock cycle, the output FET turns on and attempts to deliver current until cycle-by-cycle or overcurrent limits are exceeded. Note that the SS capacitor must be greater than 0.01F
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5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
Table 1. Typical Soft-Start Times
MAX730A CIRCUIT CONDITIONS R1 (k) 510 510 510 510 510 510 510 None None None None None None V+ (V) 6 9 11 9 9 9 9 6 9 11 9 9 9 IOUT (mA) 0 0 0 150 300 150 150 0 0 0 150 150 150 C4 (F) 100 100 100 100 100 390 680 100 100 100 100 390 680 C1 = 0.01 2 1 1 1 1 3 4 16 10 8 34 39 40 SOFT-START TIME (ms) vs. C1 (F) C1 = 0.047 6 4 2 4 5 6 6 34 22 18 134 147 152 C1 = 0.1 11 6 4 8 9 9 9 51 34 28 270 280 285 C1 = 0.47 28 15 11 21 27 23 24 125 82 66 1263 1275 1280
MAX738A/MAX744A CIRCUIT CONDITIONS R1 (k) 510 510 510 510 510 None None None None V+ (V) 7 12 16 12 12 7 12 16 12 IOUT (mA) 0 0 0 300 750 0 0 0 300 C4 (F) 100 100 100 100 100 100 100 100 100 C1 = 0.01 1 1 1 1 1 12 7 6 27
SOFT-START TIME (ms) vs. C1 (F) C1 = 0.047 4 2 1 3 5 27 16 13 112 C1 = 0.1 6 3 2 5 8 40 25 20 215 C1 = 0.47 18 8 6 3 21 100 54 68 1114
Internal Reference
The +1.23V bandgap reference supplies up to 100A at REF. Connect a 0.01F bypass capacitor from REF to GND.
____________Applications Information
Figure 3 shows the standard 5V step-down application circuits. Table 3 lists the components for the desired operating temperature range. These circuits are useful in systems that require high current at high efficiency and are powered by an unregulated supply, such as a battery or wall-plug AC-DC transformer. These circuits operate over the entire line, load, and temperature ranges using the single set of component values shown in Figure 3 and listed in Table 3.
Oscillator
The internal oscillator of the MAX730A typically operates at 170kHz (160kHz for the MAX738A and 185kHz for the MAX744A). The MAX744A is guaranteed to operate at a minimum of 159kHz and a maximum of 212.5kHz over the operating voltage and temperature range, making it ideal for use in portable communications systems. The Typical Operating Characteristics graphs indicate oscillator frequency stability over temperature and supply voltage.
Inductor Selection
The MAX730A/MAX738A/MAX744A require no inductor design because they are tested in-circuit, and are guaranteed to deliver the power specified in the Electrical Characteristics with high efficiency using a
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5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
VIN = +6.0V TO +16.0V SHDN OVERCURRENT COMPARATOR SLOPE COMPENSATION BIAS GEN OUT C5 330pF RAMP GEN RSENSE F/F R 1.23V BANDGAP REF C6 0.01F SS C1 0.1F OSC 1M 35% S PWM COMPARATOR Q LX L1 D1 1N5817 VOUT = +5V C4* CURRENT SENSE AMP V+ C2 C3* 1F
ERROR AMP
CC
R1 510k
SS CLAMP
MAX730A MAX738A MAX744A
UNDERVOLTAGE LOCKOUT
VUVLO
GND *SEE TABLE 2 FOR COMPONENT VALUES AND SUPPLIERS
Figure 1. Detailed Block Diagram with External Components
single 100H (MAX7__AC) or 33H (MAX7__AE/AM) inductor. The inductor's incremental saturation current rating should be greater than 1A, and its DC resistance should be less than 0.8. Table 2 lists inductor types and suppliers for various applications. The surfacemount inductors have nearly equivalent efficiencies to the larger through-hole inductors.
Capacitor ESR rises as the temperature falls, and excessive ESR is the most likely cause of trouble at temperatures below 0C. Sanyo OS-CON series through-hole and surface-mount tantalum capacitors exhibit low ESR at temperatures below 0C. Refer to Table 2 for recommended capacitor values and suggested capacitor suppliers.
Output Filter Capacitor Selection
The primary criterion for selecting the output filter capacitor is low equivalent series resistance (ESR). The product of the inductor current variation and the output capacitor's ESR determines the amplitude of the sawtooth ripple seen on the output voltage. Also, minimize the output filter capacitor's ESR to maintain AC stability. The capacitor's ESR should be less than 0.25 to keep the output ripple less than 50mVp-p over the entire current range (using a 100H inductor).
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Other Components
The catch diode should be a Schottky or high-speed silicon rectifier with a peak current rating of at least 1.5A for full-load (750mA) operation. The 1N5817 is a good choice. The 330pF outer-loop compensation capacitor provides the widest input voltage range and best transient characteristics. For low-current applications, the 510k resistor may be omitted (see the Maximum Output Current vs. Supply Voltage graph (R1 removed) in the Typical Operating Characteristics).
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5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
Table 2. Component Values and Suppliers
Production Method MAX730AC/MAX738AC/MAX744AC Commercial Temp. Range Inductors L1 = 33H to 100H Sumida (708) 956-0666 CD54-101KC (MAX730AC) CD105-101KC (MAX738AC/MAX744AC) Coiltronics (407) 241-7876 CTX100 series Capacitors C3 = 68F, 16V C4 = 100F, 6.3V Matsuo (714) 969-2491 267 series Sprague (603) 224-1961 595D/293D series MAX730AE/M, MAX738AE/M, MAX744AE/M Wide Temp. Range Inductors L1 = 33H Sumida (708) 956-0666 CD54-330N (MAX730AC) CD105-330N (MAX738AE/M, MAX744AE/M) Coiltronics (407) 241-7876 CTX50 series Capacitors C3 = 68F, 16V C4 = 100F, 6.3V Matsuo (714) 969-2491 267 series Sprague (603) 224-1961 595D/293D series C3 = 150F, 16V C4 = 220F, 10V C3 = 150F, 16V C4 = 150F, 16V or 390F, 6.3V Nichicon (708) 843-7500 PL series Low-ESR electrolytics Sanyo (619) 661-6322 OS-CON series Low-ESR organic semiconductor (Rated from -55C to +105C) Mallory (317) 273-0090 THF series C3 = 100F, 20V C4 = 220F, 10V (Rated from -55C to +125C)
Surface Mount
L1 = 33H to 100H Miniature ThroughHole Sumida (708) 956-0666 RCH654-101K (MAX730A) RCH895-101K (MAX738A/MAX744A)
L1 = 33H Sumida (708) 956-0666 RCH654-330M (MAX730A) RCH895-330M (MAX738A/MAX744A)
L1 = 100H Low-Cost ThroughHole Maxim MAXL001 100H iron-power toroid Renco (516) 586-5566 RL1284-100
C3 = 150F, 16V C4 = 390F, 6.3V Maxim MAXC001 150F, low-ESR electrolytic United Chemicon (708) 843-7500
Printed Circuit Layouts
A good layout is essential for clean, stable operation. The layouts and component placement diagrams given in Figures 4, 5, 6, and 7 have been successfully tested over a wide range of operating conditions. Note that the 1F bypass capacitor (C2) must be positioned as close to the V+ and GND pins as possible. Also, place the output capacitor as close to the OUT and GND pins as possible. The traces connecting the input and output filter capacitors and the catch diode must be short to minimize inductance and capacitance. For this reason, avoid using sockets, and solder the IC directly to the PC board. Use an uninterrupted ground plane if possible.
Output-Ripple Filtering
A simple lowpass pi-filter (Figure 3) can be added to the output to reduce output ripple to about 5mVp-p. The cutoff frequency shown is 21kHz. Since the filter inductor is in series with the circuit output, its resistance should be minimized so the voltage drop across it is not excessive.
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11
5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
FROM SHDN
R1 510k
SS CLAMP
SS
1M 35% C1 1.23V
MAX730A MAX738A MAX744A
Figure 2. Block Diagram of Soft-Start Circuitry
MAX730A +5.2V TO +11.0V MAX738A/MAX744A +6.0V TO +16.0V
C2 1F C3* 8 1 SHDN V+ LX 7 D1 1N5817 R1 510k L1* OUTPUT +5V C5 330pF C4* PART MAX730A MAX738A C6 0.01F MAX744A
OPTIONAL LOWPASS OUTPUT FILTER
L2 25H OUTPUT C7 2.2F
FILTER OUTPUT
MAX730A MAX738A OUT 5 MAX744A
3 SS GND 6 REF 2 CC 4
INPUT SUPPLY RANGE (V) 6.0 to 11.0 6.0 to 16.0 10.2 to 16.0 6.0 to 9.0 9.0 to 16.0
GUARANTEED OUTPUT CURRENT AT 5V (mA) 450 450 750 500 750
C1 0.1F
*SEE TABLE 2 FOR COMPONENT VALUES AND SUPPLIERS.
NOTE: PIN NUMBERS REFER TO 8-PIN PACKAGES.
Figure 3. Standard +5V Step-Down Application Circuit
12 ______________________________________________________________________________________
5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
MAX730 EVALUATION KIT
Figure 4. DIP PC Layout, Through-Hole Component Placement Diagram (1x scale)
Figure 5. DIP PC Layout, Component Side (1x scale)
Figure 6. DIP PC Layout, Solder Side (1x scale)
Figure 7. DIP PC Layout, Drill Guide (1x scale)
______________________________________________________________________________________
13
5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
____Pin Configurations (continued)
TOP VIEW
SHDN REF 1 2 8 7 V+ LX GND OUT
__Ordering Information (continued)
PART MAX738ACPA MAX738ACWE MAX738AC/D MAX738AEPA MAX738AEWE MAX738AMJA MAX744ACPA TEMP. RANGE 0C to +70C 0C to +70C 0C to +70C -40C to +85C -40C to +85C -55C to +125C 0C to +70C PIN-PACKAGE 8 Plastic DIP 16 Wide SO Dice* 8 Plastic DIP 16 Wide SO 8 CERDIP 8 Plastic DIP 16 Wide SO Dice* 8 Plastic DIP 16 Wide SO 8 CERDIP
SS 3 CC 4
MAX730A
6 5
SO
V+ 1 SHDN 2 REF 3 N.C. 4 N.C. 5 N.C. 6 SS 7 CC 8 16 V+ 15 V+ 14 LX
MAX738A MAX744A
13 LX 12 LX 11 GND 10 GND 9 OUT
MAX744ACWE 0C to +70C MAX744AC/D 0C to +70C MAX744AEPA -40C to +85C MAX744AEWE -40C to +85C MAX744AMJA -55C to +125C *Contact factory for dice specifications.
Wide SO
__________________________________________________________Chip Topographies
MAX730A
SHDN V+
MAX738A/MAX744A
SHDN V+
REF
REF
LX 0.116" (2.946mm)
LX 0.131" (3.327mm)
SS GND
SS
CC 0.072" (1.828mm) OUT
GND
CC 0.116" (2.946mm)
OUT
TRANSISTOR COUNT: 274 (MAX730A) 286 (MAX738A/MAX744A); SUBSTRATE CONNECTED TO V+.
14
______________________________________________________________________________________
5V, Step-Down, Current-Mode PWM DC-DC Converters
________________________________________________________Package Information
D1
DIM A A1 A2 A3 B B1 C D D1 E E1 e eA eB L INCHES MAX MIN 0.200 - - 0.015 0.175 0.125 0.080 0.055 0.022 0.016 0.065 0.050 0.012 0.008 0.390 0.348 0.035 0.005 0.325 0.300 0.280 0.240 0.100 BSC 0.300 BSC 0.400 - 0.150 0.115 15 0 MILLIMETERS MIN MAX - 5.08 0.38 - 3.18 4.45 1.40 2.03 0.41 0.56 1.27 1.65 0.20 0.30 8.84 9.91 0.13 0.89 7.62 8.26 6.10 7.11 2.54 BSC 7.62 BSC - 10.16 2.92 3.81 0 15
21-324A
MAX730A/MAX738A/MAX744A
E D A3 A A2 E1
L
A1 e B
C B1 eA eB
8-PIN PLASTIC DUAL-IN-LINE PACKAGE
DIM A A1 B C D E e H h L
E
H
INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.197 0.189 0.157 0.150 0.050 BSC 0.244 0.228 0.020 0.010 0.050 0.016 8 0
MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 4.80 5.00 3.80 4.00 1.27 BSC 5.80 6.20 0.25 0.50 0.40 1.27 0 8
21-325A
D A e B
0.127mm 0.004in.
h x 45
A1
C
L
8-PIN PLASTIC SMALL-OUTLINE PACKAGE
______________________________________________________________________________________
15
5V, Step-Down, Current-Mode PWM DC-DC Converters MAX730A/MAX738A/MAX744A
___________________________________________Package Information (continued)
DIM INCHES MAX MIN 0.200 - 0.023 0.014 0.065 0.038 0.045 0.023 0.015 0.008 0.405 - 0.310 0.220 0.320 0.290 0.100 BSC 0.200 0.125 - 0.150 0.060 0.015 0.055 - - 0.005 15 0 MILLIMETERS MIN MAX - 5.08 0.36 0.58 0.97 1.65 0.58 1.14 0.20 0.38 - 10.29 5.59 7.87 7.37 8.13 2.54 BSC 3.18 5.08 3.81 - 0.38 1.52 - 1.40 0.13 - 0 15
21-326D
S1
S
D A B2
E1 E
A B B1 B2 C D E E1 e L L1 Q S S1
Q L e B L1 B1
C
8-PIN CERAMIC DUAL-IN-LINE PACKAGE
DIM A A1 B C D E e H h L
E
H
INCHES MAX MIN 0.104 0.093 0.012 0.004 0.019 0.014 0.013 0.009 0.413 0.398 0.299 0.291 0.050 BSC 0.419 0.394 0.030 0.010 0.050 0.016 8 0
MILLIMETERS MIN MAX 2.35 2.65 0.10 0.30 0.35 0.49 0.23 0.32 10.10 10.50 7.40 7.60 1.27 BSC 10.00 10.65 0.25 0.75 0.40 1.27 0 8
21-589B
D A e B
0.127mm 0.004in.
h x 45
A1
C
L
16-PIN PLASTIC SMALL-OUTLINE (WIDE) PACKAGE
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.
16 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 (c) 1996 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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