Rainbow Electronics MAX763A User Manual

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 0.2µA 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 22µH 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.

___________________________Applications

5V-to-3.3V Converters
Cellular Phones
Portable Instruments
Hand-Held Computers
Computer Peripherals

________________________________Features

♦ Up to 500mA Load Currents
♦ Guaranteed 159kHz to 219.5kHz Current-Mode

PWM

♦ 85% to 90% Efficiencies
♦ 1.7mA Quiescent Current (MAX748A)

1.4mA Quiescent Current (MAX763A)

♦ 0.2µA Shutdown Supply Current
♦ 22µH Preselected Inductor Value;

No Component Design Required

♦ Overcurrent, Soft-Start, and Undervoltage

Lockout Protection

♦ Cycle-by-Cycle Current Limiting
♦ 8-Pin DIP/SO Packages (MAX763A)

_________________Ordering Information

PART TEMP. RANGE PIN-PACKAGE

MAX748ACPA 0°C to +70°C 8 Plastic DIP

MAX748ACWE 0°C to +70°C 16 Wide SO
MAX748AC/D 0°C to +70°C Dice*
MAX748AEPA -40°C to +85°C 8 Plastic DIP
MAX748AEWE -40°C to +85°C 16 Wide SO
MAX748AMJA -55°C to +125°C 8 CERDIP

Ordering Information continued on last page.

* Contact factory for dice specifications.

MAX748A/MAX763A

____________Typical Operating Circuit

INPUT

3.3V TO 16V

100µF

OUTPUT

3.3V

V+

LX

MAX748A



SHDNON/OFF

REF

OUT

CC

GNDSS

_______________________________________________________________________

22µH

____________________Pin Configurations

TOP VIEW

SHDN

1

REF

2

MAX748A

3

SS

MAX763A

4

CC

Pin Configurations continued on last page.

Maxim Integrated Products

Call toll free 1-800-998-8800 for free samples or literature.



DIP

V+

8

LX

7

GND

6

OUT

5

1

3.3V, Step-Down,
Current-Mode PWM DC-DC Converters

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

SHDN

SS, CC,
Peak Switch Current (I
Reference Current (I
Continuous Power Dissipation (T

8-Pin Plastic DIP (derate 6.90mW/°C above +70°C)...552mW

8-Pin SO (derate 5.88mW/°C above +70°C)...............471mW

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.

..........................................-0.3V to (V+ + 0.3V)

).....................................................2.0A

LX

)...................................................2.5mA

REF

= +70°C)

A

ELECTRICAL CHARACTERISTICS

(Circuit of Figure 3, V+ = 5V, I

MAX748A/MAX763A

PARAMETER CONDITIONS

Input Voltage Range 3.3 16.0 3.3 11.0 V

Output Voltage

Line Regulation 0.13 0.13 %/V
Load Regulation I

= 0mA, TA= T

LOAD

C/E temp. ranges,
V+ = 4.0V to 16V, 3.135 3.3 3.465
0mA < I

M temp. range,
V+ = 4.0V to 16V, 3.135 3.3 3.465
0mA < I

C/E temp. ranges,
V+ = 4.75V to 16V, 3.135 3.3 3.465
0mA < I

M temp. range,
V+ = 4.75V to 16V, 3.135 3.3 3.465
0mA < I

C/E temp. ranges,
V+ = 4.0V to 11V, 3.135 3.3 3.465
0mA < I

M temp. range,
V+ = 4.0V to 11V, 3.135 3.3 3.465
0mA < I

C/E temp. ranges,
V+ = 4.75V to 11V, 3.135 3.3 3.465
0mA < I

M temp. range,
V+ = 4.75V to 11V, 3.135 3.3 3.465
0mA < I

LOAD

< 300mA

LOAD

< 250mA

LOAD

< 500mA

LOAD

< 400mA

LOAD

< 300mA

LOAD

< 250mA

LOAD

< 500mA

LOAD

< 400mA

LOAD

= 0mA to 500mA 0.001 0.001 %/mA

MIN

to T

, unless otherwise noted.)

MAX

16-Pin Wide SO (derate 9.52mW/°C above +70°C)....762mW

8-Pin CERDIP (derate 8.00mW/°C above +70°C).......640mW

Operating Temperature Ranges:

MAX7__AC__ ....................................................0°C to +70°C

MAX7__AE__ .................................................-40°C to +85°C

MAX7__AMJA__ ..........................................-55°C to +125°C

Junction Temperatures:

MAX7__AC/E ..............................................................+150°C

MAX7__AM.................................................................+175°C

Storage Temperature Range ............................-65°C to +160°C

Lead Temperature (soldering, 10sec) ............................+300°C

MAX748A MAX763A

MIN TYP MAX MIN TYP MAX

UNITS

V

2 ________________________________________________________________________________________________

3.3V, Step-Down,

Current-Mode PWM DC-DC Converters

ELECTRICAL CHARACTERISTICS (continued)

(Circuit of Figure 3, V+ = 5V, I

PARAMETER CONDITIONS

Efficiency V+ = 5V

Supply Current Includes switch current 1.7 3.0 1.4 2.5 mA
Shutdown Current

Shutdown Input
Threshold

Shutdown Input
Leakage Current

Short-Circuit Current 1.2 1.2 A
Undervoltage Lockout V+ falling 2.7 3.0 2.7 3.0 V
LX On Resistance ILX= 500mA 1.0 1.0 Ω
LX Leakage Current V+ = 12V, LX = 0 10 10 nA
Reference Voltage TA= +25°C 1.15 1.22 1.30 1.15 1.22 1.30 V
Reference Drift TA= T
Oscillator Frequency 159 180 212.5 159 200 212.5 kHz
Compensation Pin

Impedance

Note 1: The standby current typically settles to 10µA (over temperature) within 2 seconds; however, to decrease test time, the part
is guaranteed at a 100µA maximum value.

= 0mA, TA= T

LOAD

MIN

to T

, unless otherwise noted.)

MAX

MAX748A MAX763A

MIN TYP MAX MIN TYP MAX

= 300mA 88 88

I

LOAD

I

= 100mA 90 90

LOAD

SHDN

= 0V (Note 1) 0.2 100.0 0.2 100.0 µA

V

IH

V

IL

2.0 2.0

0.25 0.25

1.0 1.0 µA

MIN

to T

MAX

50 50 ppm/°C

7500 7500 Ω

UNITS

%

V

MAX748A/MAX763A

________________________________________________T ypical Operating Characteristics

(Circuit of Figure 3, TA = +25°C, V

MAXIMUM OUTPUT CURRENT

vs. SUPPLY VOLTAGE

1000

800

600

400

200

MAXIMUM OUTPUT CURRENT (mA)

0

MAX748A

MAX763A

0246810121416

SUPPLY VOLTAGE (V)

_________________________________________________________________________________________________

= 3.3V, unless otherwise noted.)

OUT

100

95
90

85
80
75

EFFICIENCY (%)

70

NOTE 2

65
60

0 100 200 300 400 500 600 700 800

EFFICIENCY vs. OUTPUT CURRENT

MAX763A

VIN = 7.0V

VIN = 5.0V

VIN = 11.0V

NOTE 2

OUTPUT CURRENT (mA)

EFFICIENCY vs. OUTPUT CURRENT

MAX748A

100

VIN = 4.5V

95
90
85
80
75

EFFICIENCY (%)

70

VIN = 12V

65
60

0 100 200 300 400 500 600 700 800 900

VIN = 6V

VIN = 16V

VIN = 9V

NOTE 2

OUTPUT CURRENT (mA)

3

3.3V, Step-Down,
Current-Mode PWM DC-DC Converters

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 3, TA = +25°C, V

MAX763A 

QUIESCENT SUPPLY CURRENT

vs. SUPPLY VOLTAGE

4.5

4.0

I

= 0mA

OUT

3.5

3.0

2.5

2.0

1.5

1.0

QUIESCENT SUPPLY CURRENT (mA)

0.5
0

MAX748A/MAX763A

0 1 2 3 4 5 6 7 8 9 10 11 12

275
250

225
200
175

MAX763A

150

OSCILLATOR FREQUENCY (kHz)

125

100

0 2 4 6 810121416

TA = +125°C

TA = -55°C

TA = +25°C

SUPPLY VOLTAGE (V)

OSCILLATOR FREQUENCY

vs. SUPPLY VOLTAGE

MAX748A

SUPPLY VOLTAGE (V)

= 3.3V, unless otherwise noted.)

OUT

QUIESCENT SUPPLY CURRENT

5.0

4.0

3.0

2.0

1.0

QUIESCENT SUPPLY CURRENT (mA)

3.5

3.0

2.5

2.0

1.5

1.0

SHUTDOWN CURRENT (µA)

0.5

vs. SUPPLY VOLTAGE

I

= OmA

OUT

TA = +125°C

0

0 2 4 6 8 10 12 14 16

NOTES 3, 4

0

-60 -40 -20 0 20 40 60 80 100 120 140

MAX748A

TA = -55°C

TA = +25°C

SUPPLY VOLTAGE (V)

SHUTDOWN CURRENT

vs. TEMPERATURE

I

= 0mA

OUT

V+ = 5V

TEMPERATURE (°C)

PEAK INDUCTOR CURRENT

vs. OUTPUT CURRENT

1000

800

600

400

200

PEAK INDUCTOR CURRENT (mA)

4.0

3.5

3.0

2.5

2.0

1.5

OUTPUT VOLTAGE (V)

1.0

0.5

VIN = 11V

VIN = 8V

0

0 50 100 150 200 250 300 350 400 450 500 550

OUTPUT CURRENT (mA)

OUTPUT VOLTAGE

vs. SUPPLY VOLTAGE

I

= 50mA

LOAD

I

= 200mA

LOAD

I

LOAD

I

= 300mA

LOAD

0

2.0 2.5 3.0 3.5 4.0 4.5 5.0
SUPPLY VOLTAGE (V)

VIN = 6V

= 500mA

OUTPUT VOLTAGE

4.0

3.5

3.0

2.5

2.0

1.5

OUTPUT VOLTAGE (V)

1.0

0.5

vs. OUTPUT CURRENT

VIN = 4.0V

VIN = 3.5V

VIN = 3.0V

0

0 50 100 150 200 250 300 350 400 450 500

OUTPUT CURRENT (mA)

OSCILATOR FREQUENCY (kHz)

OSCILLATOR FREQUENCY

vs. TEMPERATURE

300

I

= 100mA

OUT

V+ = 5V

250

200

150

100

50

0

-60 -40 -20 0 20 40 60 80 100 120 140
TEMPERATURE (°C)

MAX748A

MAX763A

4 _______________________________________________________________________________________

3.3V, Step-Down,

Current-Mode PWM DC-DC Converters

_________________________________T ypical Operating Characteristics (continued)

(Circuit of Figure 3, TA = +25°C, V

SWITCHING WAVEFORMS,
CONTINUOUS CONDUCTION

2µs/div

A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +6V
B: INDUCTOR CURRENT, 200mA/div
C: OUTPUT VOLTAGE RIPPLE, 50mV/div

V+ = 6V, I

OUT

= 250mA

LINE-TRANSIENT RESPONSE

= 3.3V, unless otherwise noted.)

OUT

6V
0V

400mA

0mA

SWITCHING WAVEFORMS,

DISCONTINUOUS CONDUCTION

A

B B

C

2µs/div

A: SWITCH VOLTAGE (LX PIN), 5V/div, 0V TO +6V
B: INDUCTOR CURRENT, 100mA/div
C: OUTPUT VOLTAGE RIPPLE, 50mV/div

V+ = 6V, I

OUT

= 75mA

6V
0V

200mA

0mA

LOAD-TRANSIENT RESPONSE

A

C

MAX748A/MAX763A

10V
7V

0V

5ms/div

A: V

, 50mV/div

OUT

B: V+, 5V/div, 7.0V TO 10.0V
I

= 350mA

OUT

Note 2: Operation beyond the specifications listed in the

A

B

A: V

, 50mV/div

OUT

B: I

, 200mA/div, 0mA TO 500mA

OUT

V+ = 6V

Electrical Characteristics

5ms/div

may exceed the power dissipation ratings of

A

500mA

B

0mA

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= +85°C.

_______________________________________________________________________________________

5

3.3V, Step-Down,
Current-Mode PWM DC-DC Converters

_______________________________________________________________________Pin Description

PIN #

8-PIN DIP/SO

12

2 3 REF

37SS

48CC

5 9 OUT

6 10, 11 GND Ground*

MAX748A/MAX763A

7 12, 13, 14 LX Drain of internal P-channel power MOSFET*

8 1,15,16 V+ electrolytic capacitor in parallel. The 1µF capacitor must be as close

*16-pin wide SO package: All pins sharing the same name must be connected together externally.

16-PIN WIDE SO NAME FUNCTION

(MAX748A)

SHDN

4, 5, 6 N.C. No Connect—no internal connections to these pins.

Shutdown—active low. Connect to ground to power down chip; tie to V+ for normal
operation. Output voltage falls to 0V when

Reference Voltage Output (+1.23V) supplies up to 100µA for external loads.
Bypass to GND with a 0.047µF 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.

Supply Voltage Input. Bypass to GND with 1µF ceramic and large-value

to the GND and V+ pins as possible.*

SHDN

is low.

_________________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

Operating Characteristics

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 _______________________________________________________________________________________

). The current-mode

Typical

Figure 2 shows a capacitor connected to the soft-start
(SS) pin to ensure orderly power-up. A typical value is

0.047µF. 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.

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 0.01µF for overcurrent protection to function prop­erly. A typical value is 0.047µF.

Programmable Soft-Start

Overcurrent Limiting

3.3V, Step-Down,

Current-Mode PWM DC-DC Converters

MAX748A/MAX763A

SHDN

SLOPE COMPENSATION

BIAS
GEN

Σ

PWM

COMPARATOR

SS CLAMP

330pF

1000pF

C5

C6

C1

0.047µF

OUT

CC

REF

SS

BANDGAP

1M
±35%

ERROR AMP

1.23V

GND

Figure 1. Detailed Block Diagram with External Components

FROM SHDN

SS CLAMP

SS

1M

C1

±35%

1.23V

MAX748A
MAX763A

OVERCURRENT COMPARATOR

RAMP

GEN

CURRENT

F/F

RQ

200kHz

OSC

SENSE AMP

S

Table 1. Typical Soft-Start Times

(Circuit of Figure 3, C4 = 150µF)

Circuit Cond. Soft-Start Time (ms) vs. C1 (µF)

V+ (V) I

12* 0 1 2 3 6

12* 200 7 17 20 80

12* 300 8 25 35 140

* MAX748A only

MAX748A 3.3V to 16.0V
MAX763A 3.3V to 11.0V

R

SENSE

V

IN

V+

LX

C2

1.0µF

22µH

D1
1N5817

C3

150µF

L1

V

OUT

3.3V

C4
150µF

MAX748A
MAX763A

UNDERVOLTAGE

LOCKOUT

V

UVLO

(mA) C1 = 0.01 C1 = 0.047 C1 = 0.1 C1 = 0.47

OUT

80 1 4 712

8 200 10 33 50 200

8 300 13 44 65 325

Figure 2. Soft-Start Circuitry Block Diagram

_______________________________________________________________________________________ 7

3.3V, Step-Down,
Current-Mode PWM DC-DC Converters

Table 3. External Component Suppliers

Production Method Inductors Capacitors

Sumida Matsuo
CD105 series 267 series

Surface Mount Coiltronics Sprague

High Performance/ Sumida Sanyo
Miniature Through-Hole RCH895 series OS-CON series (very low ESR)

Through-Hole

Phone and FAX Numbers:

Coilcraft USA: [phone], FAX: [phone] Renco USA: [phone], FAX: [phone]
Coiltronics USA: [phone], FAX: [phone] Sanyo USA: (0720) 70-1005, FAX: [phone]
Matsuo USA: [phone], FAX: [phone] Sprague Elec. Co. USA: [phone], FAX: [phone]

MAX748A/MAX763A

Nichicon USA: [phone], FAX: [phone]

Japan: [phone] Sumida USA: [phone], FAX: [phone]

Japan: [phone], FAX: [phone]

CTX series 595D/293D series
Coilcraft

DT series

Renco Nichicon
RL1284 series PL series (low ESR)

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

Shutdown Mode

The MAX748A/MAX763A are held in shutdown mode

Undervoltage Lockout

by keeping
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 0.2µA. The actual design
limit for shutdown current is much less than the 100µA
specified in the
testing to tighter limits is prohibitive because the cur­rent takes several seconds to settle to a final value. For

8 _______________________________________________________________________________________

SHDN

at ground. In shutdown mode, the

Electrical Characteristics

. However,

normal operation, connect
shutdown mode initiates an SS cycle.

SHDN

to V+. Coming out of

Continuous-/Discontinuous-

Conduction 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.

Internal Reference

The +1.23V bandgap reference supplies up to 100µA
at REF. A 1000pF bypass capacitor from REF to GND
is required.

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.

Oscillator

3.3V, Step-Down,

Current-Mode PWM DC-DC Converters

INPUT
MAX748A 3.3V TO 16.0V
MAX763A 3.3V TO 11.0V

1

3

C1

0.047µF

Figure 3. Standard 3.3V Step-Down Application Circuit Using Through-Hole Components (commercial temperture range)

SHDN

MAX748A
MAX763A

GND

62

V

IN

8

V+

LX

OUT

CCSS

REF

C6
1000pF

C2

1.0µF
C3

150µF

7

D1

L1

1N5817

22µH

5

C5

4

330pF

OUTPUT

3.3V

C4
150µF

OPTIONAL 21kHz LOWPASS OUTPUT FILTER

OUTPUT

L2

25µH

2.2µF

FILTER

OUTPUT

C7

MAX748A/MAX763A

Table 2. Component Table for
Wide Temperature Applications

C1(µF) C2(µF) C3(µF) C4(µF) C5(pF) C6(pF) L1(µH)

Through­Hole 0.047 1.0 150* 220* 330 1000 22

SO 0.047 1.0 68** 100*** 330 1000 22

* Sanyo OS-CON Series (very low ESR)
** 16V or greater maximum voltage rating.
*** 6.3V or greater maximum voltage rating.

____________Applications Information

Fixed +3.3V Step-Down

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

_______________________________________________________________________________________ 9

Converter Application

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

Characteristics

with high efficiency using a single

Electrical

22µH inductor. The 22µH 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.

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.

3.3V, Step-Down,
Current-Mode PWM DC-DC Converters

INPUT
MAX748A 3.3V TO 16.0V
MAX763A 3.3V TO 11.0V

1

SHDN

V

IN

8

V+

LX

C2

1.0µF
C3*

150µF
(16V)

7

D1
1N5817

MAX748A
MAX763A

3

C1

0.047µF

MAX748A/MAX763A

GND

62

5

OUT

4

CCSS

REF

C6
1000pF

*OS-CON Series (very low ESR)

C5
330pF

Figure 4. Standard 3.3V Step-Down Application Circuit Using
Through-Hole Components (all temperature ranges)

The capacitor’s ESR should be less than 0.25Ω to keep
the output ripple less than 50mV

p-p

over the entire cur­rent range (using a 22µH inductor). Capacitor ESR
usually rises at low temperatures, but OS-CON capaci­tors provide very low ESR below 0°C. 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.

Printed Circuit Layouts

A good layout is essential for stable, low-noise opera­tion. The layouts and component placement diagrams

L1
22µH

OUTPUT

3.3V

C4*
220µF
(10V)

INPUT
MAX748A 3.3V TO 16.0V
MAX763A 3.3V TO 11.0V

1

SHDN

3

C1

0.047µF

GND

V

IN

8

V+

MAX748A
MAX763A

62

OUT

CCSS

REF

C6
1000pF

*Sprague 293D or 595D Series-16V. See Table 3
for alternative suppliers.

C2

1.0µF
C3*

68µF
(16V)

7

LX

D1

L1

1N5817

22µH

5

C5
330pF

4

OUTPUT

3.3V

C4* 
100µF
(6.3V)

Figure 5. Standard 3.3V Step-Down Application Circuit Using
Surface-Mount Components (Commercial and Extended
Industrial Temperature Ranges)

in Figures 6-9 have been tested successfully over a
wide range of operating conditions. The 1µF 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 5mV

p-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.

.

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 8. DIP PC Layout, Solder Side (1X Scale) Figure 9. DIP PC Layout, Drill Guide (1X Scale)

Figure 7. DIP PC Layout, Component Side (1X Scale)

______________________________________________________________________________________ 11

3.3V, Step-Down,
Current-Mode PWM DC-DC Converters

_____Pin Configurations (continued)

TOP VIEW

SHDN

REF

1
2

MAX748A

3

SS

MAX763A



4

CC

V+

8

LX

7

GND

6

OUT

5

SO

V+

1



SHDN

2

REF

3

MAX748A/MAX763A

N.C.
N.C.
N.C.

MAX748A

4



5
6

SS

7

CC

8

V+

16

V+

15

LX

14

LX

13

LX

12

GND

11

GND

10

OUT

9

Wide SO

_Ordering Information (continued)

____________________Chip Topographies

0.131"

3.327mm

SS

CC

OUT

GND

GND

TRANSISTOR COUNT: 298
SUBSTRATE CONNECTION: V+

SHDN

MAX748A

0.072"

1.829mm
V+

LXLX LX

V+ V+

REF

SHDN

V+

V+
V+
V+

0.116"

2.946mm

PART TEMP. RANGE PIN-PACKAGE

REF

MAX763ACPA 0°C to +70°C 8 Plastic DIP
MAX763ACSA 0°C to +70°C 8 SO
MAX763AC/D 0°C to +70°C Dice*
MAX763AEPA -40°C to +85°C 8 Plastic DIP
MAX763AESA -40°C to +85°C 8 SO
MAX763AMJA -55°C to +125°C 8 CERDIP

* Contact factory for dice specifications.

SS

OUTCC

MAX763A

TRANSISTOR COUNT: 281
SUBSTRATE CONNECTION: V+

12 ______________________________________________________________________________________

LX

LX

LX

0.116"

2.946mm

GND