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19-0190; Rev 0; 9/93
3.3V, Step-Down, Current-Mode PWM DC-DC Converters
__________________General Description
The MAX748A/MAX763A are 3.3V-output CMOS, stepdown switching regulators. The MAX748A accepts inputs from 3.3V to 16V and delivers up to 500mA. The MAX763A accepts inputs between 3.3V and 11V and delivers up to 500mA. Typical efficiencies are 85% to 90%. Quiescent supply current is 1.4mA (MAX763A), and only 0.2A 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. A 22H inductor works in most applications, so no magnetics design is necessary. The MAX748A/MAX763A also feature cycle-by-cycle current limiting, overcurrent limiting, undervoltage lockout, and programmable soft-start protection. The MAX748A is available in 8-pin DIP and 16-pin wide SO packages; the MAX763A comes in 8-pin DIP and SO packages.
________________________________Features
o Up to 500mA Load Currents o Guaranteed 159kHz to 219.5kHz Current-Mode PWM o 85% to 90% Efficiencies o 1.7mA Quiescent Current (MAX748A) 1.4mA Quiescent Current (MAX763A) o 0.2A Shutdown Supply Current o 22H 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 (MAX763A)
MAX748A/MAX763A
_________________Ordering Information
PART MAX748ACPA MAX748ACWE MAX748AC/D MAX748AEPA MAX748AEWE MAX748AMJA 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 16 Wide SO Dice* 8 Plastic DIP 16 Wide SO 8 CERDIP
___________________________Applications
5V-to-3.3V Converters Cellular Phones Portable Instruments Hand-Held Computers Computer Peripherals
Ordering Information continued on last page. * Contact factory for dice specifications.
____________Typical Operating Circuit
INPUT 3.3V TO 16V V+ 22H LX OUTPUT 3.3V
____________________Pin Configurations
TOP VIEW
MAX748A
ON/OFF SHDN OUT 100F REF SS GND CC
SHDN 1 REF 2
8 7
V+ LX GND OUT
SS 3 CC 4
MAX748A MAX763A
6 5
DIP
Pin Configurations continued on last page.
_______________________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
ABSOLUTE MAXIMUM RATINGS
Pin Voltages: V+ (MAX748A) ......................................................+17V, -0.3V V+ (MAX763A) ......................................................+12V, -0.3V LX (MAX748A) .................................(V+ - 21V) to (V+ + 0.3V) LX (MAX763A) .................................(V+ - 12V) to (V+ + 0.3V) OUT .................................................................................25V SS, CC, SHDN ..........................................-0.3V to (V+ + 0.3V) Peak Switch Current (ILX) .....................................................2.0A Reference Current (IREF) ...................................................2.5mA Continuous Power Dissipation (TA = +70C) 8-Pin Plastic DIP (derate 6.90mW/C above +70C)...552mW 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/E ..............................................................+150C MAX7_ _ AM .................................................................+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+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Input Voltage Range C/E temp. ranges, V+ = 4.0V to 16V, 0mA < ILOAD < 300mA M temp. range, V+ = 4.0V to 16V, 0mA < ILOAD < 250mA C/E temp. ranges, V+ = 4.75V to 16V, 0mA < ILOAD < 500mA M temp. range, V+ = 4.75V to 16V, 0mA < ILOAD < 400mA C/E temp. ranges, V+ = 4.0V to 11V, 0mA < ILOAD < 300mA M temp. range, V+ = 4.0V to 11V, 0mA < I LOAD < 250mA C/E temp. ranges, V+ = 4.75V to 11V, 0mA < ILOAD < 500mA M temp. range, V+ = 4.75V to 11V, 0mA < ILOAD < 400mA Line Regulation Load Regulation ILOAD = 0mA to 500mA 0.13 0.001 CONDITIONS MAX748A TYP MAX 16.0 3.3 3.465 MAX763A TYP MAX 11.0 UNITS V
MIN 3.3 3.135
MIN 3.3
3.135
3.3
3.465
3.135
3.3
3.465
3.135
3.3
3.465 V 3.135 3.3 3.465
Output Voltage
3.135
3.3
3.465
3.135
3.3
3.465
3.135
3.3 0.13 0.001
3.465 %/V %/mA
2
________________________________________________________________________________________________
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
ELECTRICAL CHARACTERISTICS (continued)
(Circuit of Figure 3, V+ = 5V, ILOAD = 0mA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER Efficiency Supply Current Shutdown Current Shutdown Input Threshold Shutdown Input Leakage Current Short-Circuit Current Undervoltage Lockout LX On Resistance LX Leakage Current Reference Voltage Reference Drift Oscillator Frequency Compensation Pin Impedance V+ falling ILX = 500mA V+ = 12V, LX = 0 TA = +25C TA = TMIN to TMAX 159 1.15 1.2 2.7 1.0 10 1.22 50 180 7500 212.5 159 1.30 1.15 3.0 V+ = 5V CONDITIONS ILOAD = 300mA ILOAD = 100mA Includes switch current SHDN = 0V (Note 1) VIH VIL 2.0 0.25 1.0 1.2 2.7 1.0 10 1.22 50 200 7500 212.5 1.30 3.0 MAX748A TYP MAX 88 90 1.7 0.2 3.0 100.0 2.0 0.25 1.0 A A V nA V ppm/C kHz MAX763A TYP MAX 88 90 1.4 0.2 2.5 100.0 mA A V UNITS %
MIN
MIN
Note 1: The standby current typically settles to 10A (over temperature) within 2 seconds; however, to decrease test time, the part is guaranteed at a 100A maximum value.
________________________________________________Typical Operating Characteristics
(Circuit of Figure 3, TA = +25C, VOUT = 3.3V, unless otherwise noted.) MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE
1000 MAX748A MAXIMUM OUTPUT CURRENT (mA) 800 MAX763A EFFICIENCY (%) 600 90 85 80 75 70 NOTE 2 NOTE 2 65 60 0 2 4 6 8 10 12 SUPPLY VOLTAGE (V) 14 16 0 100 200 300 400 500 600 700 800 OUTPUT CURRENT (mA) 65 60 0 100 200 300 400 500 600 700 800 900 OUTPUT CURRENT (mA) VIN = 11.0V EFFICIENCY (%) 90 85 80 75 70 VIN = 9V VIN = 12V NOTE 2 VIN = 16V 100 95 VIN = 7.0V VIN = 5.0V
MAX763A EFFICIENCY vs. OUTPUT CURRENT
100 95
MAX748A EFFICIENCY vs. OUTPUT CURRENT
VIN = 4.5V VIN = 6V
400
200
0
_________________________________________________________________________________________________ 3
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 3, TA = +25C, VOUT = 3.3V, unless otherwise noted.)
MAX763A QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE
4.5 QUIESCENT SUPPLY CURRENT (mA) QUIESCENT SUPPLY CURRENT (mA) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 0 12 345678 SUPPLY VOLTAGE (V) 9 10 11 12 0 0 2 4 6 8 10 12 SUPPLY VOLTAGE (V) 14 16 0 0 50 100 150 200 250 300 350 400 450 500 550 OUTPUT CURRENT (mA) TA = -55C TA = +25C IOUT = 0mA TA = +125C 5.0 IOUT = OmA 4.0 TA = -55C 3.0 TA = +125C TA = +25C 1.0 PEAK INDUCTOR CURRENT (mA) 800 VIN = 11V
MAX748A QUIESCENT SUPPLY CURRENT vs. SUPPLY VOLTAGE
1000
PEAK INDUCTOR CURRENT vs. OUTPUT CURRENT
600 400 VIN = 6V VIN = 8V
2.0
200
OSCILLATOR FREQUENCY vs. SUPPLY VOLTAGE
275 250 OSCILLATOR FREQUENCY (kHz) SHUTDOWN CURRENT (A) 225 MAX748A 200 175 150 125 100 0 2 4 6 8 10 12 SUPPLY VOLTAGE (V) 14 16 MAX763A 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0
SHUTDOWN CURRENT vs. TEMPERATURE
4.0 NOTES 3, 4 3.5 OUTPUT VOLTAGE (V) 3.0 2.5 2.0 1.5 1.0 IOUT = 0mA V+ = 5V 0.5 0 -60 -40 -20 0 20 40 60 80 100 120 140 2.0 2.5
OUTPUT VOLTAGE vs. SUPPLY VOLTAGE
ILOAD = 50mA ILOAD = 200mA
ILOAD = 500mA ILOAD = 300mA
3.0
3.5
4.0
4.5
5.0
TEMPERATURE (C)
SUPPLY VOLTAGE (V)
OUTPUT VOLTAGE vs. OUTPUT CURRENT
4.0 3.5 3.0 OUTPUT VOLTAGE (V) 2.5 2.0 1.5 1.0 0.5 0 0 50 100 150 200 250 300 350 400 450 500 OUTPUT CURRENT (mA) VIN = 3.5V VIN = 3.0V VIN = 4.0V OSCILATOR FREQUENCY (kHz) 250 200 150 100 50 0 300
OSCILLATOR FREQUENCY vs. TEMPERATURE
IOUT = 100mA V+ = 5V MAX748A
MAX763A
-60 -40 -20
0
20 40 60 80 100 120 140
TEMPERATURE (C)
4
_______________________________________________________________________________________
3.3V, Step-Down, Current-Mode PWM DC-DC Converters
_________________________________Typical Operating Characteristics (continued)
(Circuit of Figure 3, TA = +25C, VOUT = 3.3V, unless otherwise noted.)
MAX748A/MAX763A
SWITCHING WAVEFORMS, CONTINUOUS CONDUCTION
SWITCHING WAVEFORMS, DISCONTINUOUS CONDUCTION
6V A 0V 400mA B 0mA C
6V A 0V 200mA B 0mA
C
2s/div A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +6V B: INDUCTOR CURRENT, 200mA/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div V+ = 6V, IOUT = 250mA
2s/div A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +6V B: INDUCTOR CURRENT, 100mA/div C: OUTPUT VOLTAGE RIPPLE, 50mV/div V+ = 6V, IOUT = 75mA
LINE-TRANSIENT RESPONSE
LOAD-TRANSIENT RESPONSE
A
A
500mA 10V 7V 0V B 0mA B
5ms/div A: VOUT, 50mV/div B: V+, 5V/div, 7.0V TO 10.0V IOUT = 350mA
5ms/div A: VOUT, 50mV/div B: IOUT, 200mA/div, 0mA TO 500mA V+ = 6V
Note 2: Operation beyond the specifications listed in the Electrical Characteristics may exceed the power dissipation ratings of the device. Note 3: Wide temperature range circuit of Figure 5 using Sprague surface-mount capacitors. Note 4: Standby current includes all external component leakage currents. Capacitor leakage currents dominate at TA = +85C.
_______________________________________________________________________________________
5
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
_______________________________________________________________________Pin Description
PIN # 16-PIN WIDE SO 8-PIN DIP/SO (MAX748A) 1 2 3 4 5 6 7 8 2 3 7 8 9 10, 11 12, 13, 14 1,15,16 4, 5, 6 NAME SHDN REF SS CC OUT GND LX V+ N.C. FUNCTION Shutdown--active low. Connect to 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 external loads. Bypass to GND with a 0.047F capacitor. Soft-Start. Capacitor between SS and GND provides soft-start and short-circuit protection. Compensation Capacitor Input externally compensates the outer (voltage) feedback loop. Connect to OUT with a 330pF capacitor. Output-Voltage Sense Input provides regulation feedback sensing. Connect to +3.3V output. Ground* Drain of internal P-channel power MOSFET* Supply Voltage Input. Bypass to GND with 1F ceramic and large-value electrolytic capacitor in parallel. The 1F capacitor must be as close to the GND and V+ pins as possible.* No Connect--no internal connections to these pins.
*16-pin wide SO package: All pins sharing the same name must be connected together externally.
_________________Detailed Description
The MAX748A/MAX763A switch-mode regulators use a current-mode pulse-width-modulation (PWM) control system in a step-down (buck) regulator topography. They convert an unregulated DC input voltage from 4V to 11V (MAX763A) or from 4V to 16V (MAX748A) to a regulated 3.3V output at 300mA. For loads less than 300mA, V+ may be less than 4.0V (see the Output Voltage vs. Supply Voltage graph in the Typical Operating Characteristics). The current-mode PWM architecture provides cycle-by-cycle current limiting, improved load-transient response, and simpler outerloop 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-by-cycle current limiting, truncating the power transistor on-time 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.
6
Programmable Soft-Start
Figure 2 shows a capacitor connected to the soft-start (SS) pin to ensure orderly power-up. A typical value is 0.047F. 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. Table 1 lists timing characteristics for selected capacitor values and circuit conditions. The overcurrent comparator trips when the load exceeds approximately 1.2A. When either an undervoltage or overcurrent fault condition is detected, an SS cycle is actively initiated, which triggers an internal transistor to discharge the SS capacitor to ground. An SS cycle is also enabled at power-up and when coming out of shutdown mode.
Overcurrent Limiting
The overcurrent comparator triggers when the load current exceeds approximately 1.2A. 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 for overcurrent protection to function properly. A typical value is 0.047F.
_______________________________________________________________________________________
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
MAX748A 3.3V to 16.0V MAX763A 3.3V to 11.0V VIN SHDN OVERCURRENT COMPARATOR SLOPE COMPENSATION BIAS GEN RAMP GEN RSENSE ERROR AMP CC 1.23V BANDGAP REF C6 1000pF 1M 35% SS C1 0.047F VUVLO GND SS CLAMP UNDERVOLTAGE LOCKOUT R S PWM COMPARATOR 200kHz OSC F/F Q LX CURRENT SENSE AMP L1 22H D1 1N5817 VOUT 3.3V C4 150F V+ C2 1.0F C3 150F
OUT C5 330pF
MAX748A MAX763A
Figure 1. Detailed Block Diagram with External Components
Table 1. Typical Soft-Start Times
FROM SHDN
(Circuit of Figure 3, C4 = 150F) Circuit Cond. Soft-Start Time (ms) vs. C1 (F) V+ (V) IOUT (mA) C1 = 0.01 C1 = 0.047 C1 = 0.1 C1 = 0.47 8 12* 8 12*
1M 35%
SS CLAMP
0 0 200 200 300 300
1 1 10 7 13 8
4 2 33 17 44 25
7 3 50 20 65 35
12 6 200 80 325 140
SS
8 12*
C1 1.23V
* MAX748A only MAX748A MAX763A
Figure 2. Soft-Start Circuitry Block Diagram
_______________________________________________________________________________________ 7
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
Table 3. External Component Suppliers
Production Method Sumida CD105 series Surface Mount Coiltronics CTX series Coilcraft DT series High Performance/ Miniature Through-Hole Through-Hole Phone and FAX Numbers: Coilcraft Coiltronics Matsuo Nichicon USA: USA: USA: Japan: USA: Japan: (708) 639-6400, (305) 781-8900, (714) 969-2491, (06) 332-0871 (708) 843-7500, (03) 3607-5111, FAX: (708) 639-1469 FAX: (305) 782-4163 FAX: (714) 960-6492 FAX: (708) 843-2798 FAX: (03) 3607-5428 Renco Sanyo Sprague Elec. Co. Sumida USA: USA: USA: USA: (516) 586-5566, (0720) 70-1005, (603) 224-1961, (708) 956-0666, FAX: FAX: FAX: FAX: (516) 586-5562 (0720) 70-1174 (603) 224-1430 (708) 956-0702 Sumida RCH895 series Renco RL1284 series Sanyo OS-CON series (very low ESR) Nichicon PL series (low ESR) Inductors Matsuo 267 series Sprague 595D/293D series Capacitors
Undervoltage Lockout
The undervoltage lockout feature monitors the supply voltage at V+ and allows operation to start when V+ rises above 2.95V. When V+ falls, operation continues until the supply voltage falls below 2.7V (typ). 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 2.95V, at which time an SS cycle begins. When the input voltage exceeds the undervoltage lockout threshold, switching action will occur, but the output will not be regulated until the input voltage exceeds 3.3V (no load). The exact input voltage required for regulation depends on load conditions (see the Output Voltage vs. Supply Voltage graph in the Typical Operating Characteristics).
normal operation, connect SHDN to V+. Coming out of shutdown mode initiates an SS cycle.
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 MAX748A/MAX763A 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 MAX748A/MAX763A to deliver maximum load current, and is also slightly less noisy than DCM, because it doesn't exhibit the ringing that occurs when the inductor current reaches zero.
Shutdown Mode
The MAX748A/MAX763A are held in shutdown mode 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 supply current in shutdown mode is 0.2A. The actual design limit for shutdown current is much less than the 100A specified in the Electrical Characteristics. However, testing to tighter limits is prohibitive because the current takes several seconds to settle to a final value. For
8
Internal Reference
The +1.23V bandgap reference supplies up to 100A at REF. A 1000pF bypass capacitor from REF to GND is required.
Oscillator
The MAX748A/MAX763A's internal oscillator is guaranteed to operate in the 159kHz to 212.5 kHz range over temperature for V+ = 5V. Temperature stability over the military temperature range is about 0.04%/C.
_______________________________________________________________________________________
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
INPUT MAX748A 3.3V TO 16.0V V MAX763A 3.3V TO 11.0V IN 8 1 V+ SHDN LX 7 D1 1N5817 L1 22H OUTPUT 3.3V C5 330pF C4 150F OPTIONAL 21kHz LOWPASS OUTPUT FILTER C2 1.0F C3 150F
MAX748A MAX763A
OUT
5
L2 25H OUTPUT C7 2.2F
FILTER OUTPUT
3 SS CC
4
C1 0.047F
GND 6 2
REF C6 1000pF
Figure 3. Standard 3.3V Step-Down Application Circuit Using Through-Hole Components (commercial temperture range)
Table 2. Component Table for Wide Temperature Applications
C1(F) C2(F) C3(F) C4(F) C5(pF) C6(pF) L1(H)
ThroughHole 0.047 SO 0.047
1.0 1.0
150* 68**
220* 100***
330 330
1000 1000
22 22
* Sanyo OS-CON Series (very low ESR) ** 16V or greater maximum voltage rating. *** 6.3V or greater maximum voltage rating.
input voltages of 4.75V to 16V with 800mA typical output currents. The MAX763A delivers a guaranteed 300mA for input voltages of 4V to 11V, a guaranteed 500mA for input voltages of 4.75V to 11V, and has 700mA typical output currents. The MAX748A/ MAX763A operate from an input down to 3V (the upper limit of undervoltage lockout), but with some reduction in output voltage and maximum output current.
Inductor Selection
The MAX748A/MAX763A 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 single 22H inductor. The 22H inductor's incremental saturation current rating should be greater than 1A for 500mA load operation. Table 3 lists inductor types and suppliers for various applications. The surface-mount inductors have nearly equivalent efficiencies to the larger through-hole inductors.
____________Applications Information
Fixed +3.3V Step-Down Converter Application
Figure 3 shows the standard 3.3V step-down circuit with components shown for commercial temperature range applications. Figures 4, 5, and Table 2 suggest external component values for both SO and through-hole wide temperature range applications. These circuits are useful in systems that require high current and high efficiency and are powered by an unregulated supply, such as a battery or wall-plug AC-DC adapter. The MAX748A delivers a guaranteed 300mA for input voltages of 4V to 16V, and a guaranteed 500mA for
Output Filter Capacitor Selection
The primary criterion for selecting the output filter capacitor is low effective 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. Minimize the output filter capacitor's ESR to maintain AC stability.
9
_______________________________________________________________________________________
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
INPUT MAX748A 3.3V TO 16.0V MAX763A 3.3V TO 11.0V VIN C2 1.0F C3* 150F (16V) V+ SHDN LX 7 D1 1N5817 L1 22H OUTPUT 3.3V 3 C4* 220F (10V) SS CC 1 SHDN INPUT MAX748A 3.3V TO 16.0V V MAX763A 3.3V TO 11.0V IN C2 1.0F C3* 68F (16V) V+ LX 7 D1 1N5817 L1 22H OUTPUT 3.3V
8 1
8
MAX748A MAX763A
OUT
5 C5 330pF
MAX748A MAX763A
OUT
5 C5 330pF 4
3 SS CC
4
C4* 100F (6.3V)
C1 0.047F
GND 6 2
REF C6 1000pF
C1 0.047F
GND 6 2
REF C6 1000pF
*OS-CON Series (very low ESR)
*Sprague 293D or 595D Series-16V. See Table 3 for alternative suppliers.
Figure 4. Standard 3.3V Step-Down Application Circuit Using Through-Hole Components (all temperature ranges)
Figure 5. Standard 3.3V Step-Down Application Circuit Using Surface-Mount Components (Commercial and Extended Industrial Temperature Ranges)
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 22H inductor). Capacitor ESR usually rises at low temperatures, but OS-CON capacitors provide very low ESR below 0C. Table 3 lists capacitor suppliers.
Other Components
The catch diode should be a Schottky or high-speed silicon rectifier with a peak current rating of at least 1.0A for full-load (500mA) operation. The 1N5817 is a good choice. The 330pF outer-loop compensation capacitor provides the widest input voltage range and best transient characteristics.
in Figures 6-9 have been tested successfully over a wide range of operating conditions. The 1F input bypass capacitor 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 ground to the input and output filter capacitors and to the catch diode must be short to reduce inductance. 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, minimize the filter inductor's resistance so the voltage drop across it is not excessive.
Printed Circuit Layouts
A good layout is essential for stable, low-noise operation. The layouts and component placement diagrams
10
______________________________________________________________________________________
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
Figure 6. DIP PC Layout, Through-Hole Component Placement Diagram (1X Scale)
Figure 7. DIP PC Layout, Component Side (1X Scale)
Figure 8. DIP PC Layout, Solder Side (1X Scale)
Figure 9. DIP PC Layout, Drill Guide (1X Scale)
______________________________________________________________________________________
11
3.3V, Step-Down, Current-Mode PWM DC-DC Converters MAX748A/MAX763A
_____Pin Configurations (continued)
TOP VIEW
SHDN 1 REF 2 8 7 V+ LX GND OUT 0.116" 2.946mm CC
____________________Chip Topographies
0.131" 3.327mm SS REF
SS 3 CC 4
MAX748A MAX763A
6 5
SHDN
SO
V+ V+
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 OUT GND GND LX LX LX
V+ V+
MAX748A
13 LX 12 LX 11 GND 10 GND 9 OUT
MAX748A TRANSISTOR COUNT: 298 SUBSTRATE CONNECTION: V+
0.072" 1.829mm
Wide SO
SHDN
V+
V+ V+
_Ordering Information (continued)
PART MAX763ACPA MAX763ACSA MAX763AC/D MAX763AEPA MAX763AESA MAX763AMJA 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
LX LX REF LX
* Contact factory for dice specifications.
0.116" 2.946mm SS GND
CC
OUT
MAX763A TRANSISTOR COUNT: 281 SUBSTRATE CONNECTION: V+ 12 ______________________________________________________________________________________

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