Rainbow Electronics MAX746 User Manual

19-0192; Rev 1; 11/93

High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

_______________General Description

The MAX746 is a high-efficiency, high-current, step-down
DC-DC power-supply controller that drives external N-chan­nel FETs. It provides 93% to 96% efficiency from a 6V supply
voltage with load currents ranging from 50mA up to 3A. It
uses a pulse-width-modulating (PWM) current-mode control
scheme to provide precise output regulation and low output
noise. The MAX746's 4V to 15V input voltage range, fixed
5V/adjustable (Dual-ModeTM) output, and adjustable current
limit make this device ideal for a wide range of applications.

High efficiency is maintained with light loads due to a propri­etary automatic pulse-skipping control (Idle-ModeTM) scheme
that minimizes switching losses by reducing the switching fre­quency at light loads. The low 950µA quiescent current and
ultra-low 1.4µA shutdown current further extend battery life.

External components are protected by the MAX746's cycle­by-cycle current limit. The MAX746 also features a 2V ±1.5%
reference, a comparator for low-battery detection or level
translating, and soft-start and shutdown capability.

The MAX747—discussed in aseparatedata sheet—
functions similarly to the MAX746, but drives P-channel logic
level FETs.

________________________Applications

5V-to-3.3V Green PC Applications
Notebook/Laptop Computers
Personal Digital Assistants
Battery-Operated Equipment
Cellular Phones

__________Typical Operating Circuit

INPUT 6V TO 15V

V+

AV+

40mΩ

OUTPUT

39µH

440µF

LOW-BATTERY
DETECTOR OUTPUT

5V

ON/OFF

LOW-BATTERY

DETECTOR INPUT

CP
HIGH
SHDN

LBI
REF

SS

MAX746

CC

FB

AGND

GND

CS

EXT

OUT
LBO

____________________________Features

♦ 93% to 96% Efficiency for 50mA to 3A

Output Currents

♦ 4V to 15V Input Voltage Range
♦ Low 950µA Supply Current
♦ 1.4µA Shutdown Current
♦ Drives External N-Channel FETs
♦ Fixed-Frequency Current-Mode PWM (Heavy Loads)
♦ Idle-Mode PFM (Light Loads)
♦ Cycle-by-Cycle Current Limiting
♦ 2V ±1.5% Accurate Reference Output
♦ Adjustable Soft-Start
♦ Undervoltage Lockout
♦ Precision Comparator for Power-Fail or

Low-Battery Warning

______________Ordering Information

PART TEMP. RANGE PIN-PACKAGE

MAX746CPE 0°C to +70°C 16 Plastic DIP
MAX746CSE 0°C to +70°C 16 Narrow SO
MAX746C/D 0°C to +70°C Dice*
MAX746EPE -40°C to +85°C 16 Plastic DIP
MAX746ESE -40°C to +85°C 16 Narrow SO
MAX746MJE -55°C to +125°C 16 CERDIP

* Contact factory for dice specifications.

__________________Pin Configuration

TOP VIEW

LBO

REF

SHDN

AV+

1

LBI

2

SS

3

MAX746

4
5

FB

6

CC

7
8

GND

16

V+

15

CP

14

HIGH

13

EXT

12

AGND

11

CS

10

OUT

9

MAX746

DIP/SO

™Dual-Mode and Idle-Mode are trademarks of Maxim Integrated Products.

________________________________________________________________

Maxim Integrated Products

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

1

High-Efficiency, PWM, Step-Down,
N-Channel DC-DC Controller

ABSOLUTE MAXIMUM RATINGS

Supply Voltage V+, AV+ to GND..............................-0.3V to 17V

HIGH, EXT to GND....................................................-0.3V to 21V

AGND to GND..........................................................-0.3V to 0.3V

All Other Pins................................................-0.3V to (V+ + 0.3V)

Reference Current (I
Continuous Power Dissipation (T

Plastic DIP (derate 10.53mW/°C above +70°C) ..........842mW

MAX746

Narrow SO (derate 8.70mW/°C above +70°C) ............696mW

CERDIP (derate 10.00mW/°C above +70°C)...............800mW

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.

)....................................................±2mA

REF

= +70°C)

A

ELECTRICAL CHARACTERISTICS

(V+ = 10V, I

Input Voltage 415V

Output Voltage

Feedback Voltage V

Line Regulation

Load Regulation 1.3 2.5 %

Efficiency 94 %

OUT Leakage Current 50 80
FB Input Logic Low 40 mV
FB Input Leakage Current 1 100 nAFB = 2V

Reference Voltage V

Reference Load Regulation 920mV
Soft-Start Source Current 0.5 1.0 1.5
Soft-Start Fault Current (Note 1) 100 500

Supply Current (Note 2) I

Oscillator Frequency f

= 0A, I

LOAD

PARAMETER SYMBOL MIN TYP MAX UNITS

= 0µA, TA= T

REF

to T

MIN

V+

V

OUT

FB

REF

SUPP

OSC

, unless otherwise noted.)

MAX

V+ = 6V to 15V, 0V < (V+ - CS) < 0.125V,
FB = 0V (includes line and load regulation)

(V+ - CS) = 0V,
external feedback mode

V+ = 6V to 15V, FB = 0V 0.05
V+ = 4V to 15V, external feedback mode
0V < (V+ - CS) < 0.125V

Circuit of Figure 1, I
V+ = 6V

V

= 5V

OUT

For dual-mode switchover

I

= 0µA

REF

I

= 0µA to 100µA

REF

SS = 0V
SS = 2V

Operating, V+ = 15V

Operating, V+ = 10V
Shutdown mode

Operating Temperature Ranges:

MAX746C_E........................................................0°C to +70°C

MAX746E_E.....................................................-40°C to +85°C

MAX746MJE ..................................................-55°C to +125°C

Junction Temperatures:

MAX746C_E/E_E..........................................................+150°C

MAX746MJE.................................................................+175°C

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

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

CONDITIONS

4.85 5.08 5.25 V

MAX746C 1.96 2.00 2.04
MAX746E/M 1.95 2.00 2.05

0.1

= 0.5A to 2.5A,

LOAD

MAX746C
MAX746E/M

MAX746C 1.1 1.4
MAX746E/M

MAX746E/M

1.97 2.00 2.03

1.96 2.00 2.04

1.7

0.95

1.4 20
85 100 115MAX746C
80 100 120

V

%/V

µA

V

µA
µA

mA

µA

kHz

2 _______________________________________________________________________________________

High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

ELECTRICAL CHARACTERISTICS (continued)

(V+ = 10V, I

Maximum Duty Cycle 91 96 %
Charge-Pump Output Voltage
Current-Sense Amplifier

Current-Limit Threshold
EXT Output High V
EXT Output Low
EXT Sink Current 160 mA
EXT Source Current 270 mA
Compensation Pin Impedance 24

LBI Threshold Voltage

LBO Output Voltage Low V
LBI Input Leakage Current nA
LBO Output Leakage Current 1
SHDN Input Voltage Low V
SHDN Input Voltage High V
SHDN Input Leakage Current

Note 1: The soft-start fault current is the current sink capability of SS when V
Note 2: I

= 0A, I

LOAD

PARAMETER

is the supply current drawn by V+, which includes the current drawn by the charge pump. The charge pump

SUPP

doubles the current drawn by HIGH from the V+ input, so I

= 0µA, TA= T

REF

to T

MIN

SYMBOL MIN TYP MAX UNITS

V

HIGH

V

LIMIT

OL

IL
IH

, unless otherwise noted.)

MAX

CONDITIONS

V+ = 6V
I

= 0mA to 10mA

HIGH

V+ – CS

V

forced to 15V, I

HIGH

V

forced to 15V, I

HIGH

V

= 15V, V

HIGH

V

HIGH

LBI falling

I

= 0.5mA

SINK

LBI = 2.5V
V+ = 15V, LBO = 15V, LBI = 2.5V

SHDN = 10V

= 15V, V

EXT
EXT

EXT
EXT

= 12.5V
= 2.5V

SUPP

= -1mA
= 1mA

REF

= IV++ 2I

V+ + 4 V+ + 5 V+ + 6 V

125 150 175 mV

- 0.1 V

HIGH

MAX746C 1.97 2.00 2.03
MAX746E/M

< 1V or when the device is in shutdown.

HIGH

1.96 2.00 2.04

2.0 V

0.1 100

.

0.25 V

0.4 V

100

0.4 V

MAX746

kΩ

V

µA

nA

_______________________________________________________________________________________ 3

High-Efficiency, PWM, Step-Down,
N-Channel DC-DC Controller

__________________________________________Typical Operating Characteristics

(Circuit of Figure 1a, TA = +25°C, unless otherwise noted.)

N0-LOAD SUPPLY CURRENT

1.2

MAX746

1.1

1.0

0.9

NO-LOAD SUPPLY CURRENT (mA)

0.8
579111315

EFFICIENCY vs. OUTPUT CURRENT

100

90

EFFICIENCY (%)

80

vs. SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)



VIN = 6V

VIN = 9V

VIN = 12V

4

MAX746-01

3

2

1

NO-LOAD SUPPLY CURRENT (mA)

0

-75

100

MAX746-07

CIRCUIT OF FIGURE 1b
V
V+ = 5V

90

EFFICIENCY (%)

80

NO-LOAD SUPPLY CURRENT 

V+ = 9V

= 5V

V

OUT

-50 -25 0 25 50 75 100 125

EFFICIENCY vs. OUTPUT CURRENT

= 3.3V

OUT



vs. TEMPERATURE

ENTIRE
CIRCUIT

SCHOTTKY DIODE
LEAKAGE EXCLUDED

TEMPERATURE (°C)



MAX746-02

MAX746-08

CONTINUOUS-CONDUCTION MODE
BOUNDARY AND CORRESPONDING

PEAK INDUCTOR CURRENT

15

DISCONTINUOUS-

CONDUCTION REGION

13

PEAK

INDUCTOR

CURRENT

11

9

SUPPLY VOLTAGE (V)

7

5

0.9 1.1 1.3 1.5 1.7

0.7
OUTPUT CURRENT (A)

EFFICIENCY vs. OUTPUT CURRENT

100

CIRCUIT OF FIGURE 1c
V

= 5V

OUT

90

EFFICIENCY (%)

80

VIN = 6V

VIN = 12V

CONTINUOUS-

CONDUCTION

REGION



MAX746-09

MAX746-06

CIRCUIT OF FIGURE 1a

= 5V

V

70

0.01

4

CIRCUIT OF FIGURE 1a
V

3

2

1

PEAK INDUCTOR CURRENT (A)

VIN = 9V

0

0.01

OUT

0.1 1 10

OUTPUT CURRENT (A)

PEAK INDUCTOR CURRENT

vs. OUTPUT CURRENT

= 5V

OUT

VIN = 12V

VIN = 6V

0.1 1 10

OUTPUT CURRENT (A)

70

0.01

4

MAX746-03

CIRCUIT OF FIGURE 1b

V

OUT

3

V+ = 5V

2

1

PEAK INDUCTOR CURRENT (A)

0

0.01

0.1 1 10

OUTPUT CURRENT (A)

PEAK INDUCTOR CURRENT

vs. OUTPUT CURRENT

= 3.3V

0.1 1 10

OUTPUT CURRENT (A)

MAX746-05

PEAK INDUCTOR CURRENT (A)

70

0.01

1.5

1.0

0.5

0

0.01

0.1 1 10

OUTPUT CURRENT (A)

PEAK INDUCTOR CURRENT

vs. OUTPUT CURRENT

CIRCUIT OF FIGURE 1c
V

= 5V

OUT

VIN = 12V

VIN = 6V

0.1 1

OUTPUT CURRENT (A)

4 _______________________________________________________________________________________

MAX746-03

10

High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

____________________________Typical Operating Characteristics (continued)

(Circuit of Figure 1a, TA = +25°C, unless otherwise noted.)

LOAD-TRANSIENT RESPONSE

200µs/div

A: LOAD CURRENT, 0.1A TO 1.5A, 1A/div
B: V

RIPPLE, 50mV/div, AC-COUPLED

OUT

V+ = 10V

CONTINUOUS-CONDUCTION MODE

WAVEFORMS

LOAD-TRANSIENT RESPONSE

A

B

A: LOAD CURRENT, 0.1A TO 1.5A, 1A/div
B: V

RIPPLE, 50mV/div, AC COUPLED

OUT

V+ = 10V

DISCONTINUOUS-CONDUCTION

IDLE-MODE WAVEFORMS

A

B

0V

C

A

B

1ms/div

A

B

C

LINE-TRANSIENT RESPONSE

8V

500ms/div

A: V+ = 8V TO 10V, 2V/div

RIPPLE, 100mV/div

B: V

OUT

I

= 3A

OUT

MODERATE-LOAD, IDLE-MODE

WAVEFORMS

10V
A

B

MAX746

A

B

C

5µs/div

A : EXT VOLTAGE, 20V/div
B : INDUCTOR CURRENT 1A/div
C : V

RIPPLE, 50mV/div

OUT

V+ = 10V, I

= 3A

OUT

_______________________________________________________________________________________

20µs/div

A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 500mA/div
C: V

RIPPLE, 50mV/div, AC-COUPLED

OUT

V+ = 10V, I

OUT

= 75mA

20µs/div

A: EXT VOLTAGE, 10V/div
B: INDUCTOR CURRENT, 500mA/div
C: V

RIPPLE, 50mV/div, AC-COUPLED

OUT

V+ = 6V, I

= 480mA

OUT

5