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_______________General Description

The MAX746 is a high-efficiency, high-current, step-down

DC-DC power-supply controller that drives external N-chan-

nel 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-Mode

) 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 propri-

etary automatic pulse-skipping control (Idle-Mode

) scheme

that minimizes switching losses by reducing the switching fre-

quency 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 cycle-

by-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 inaseparatedatasheet—

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

____________________________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

* Contact factory for dice specifications.

MAX746

High-Efficiency, PWM, Step-Down,

N-Channel DC-DC Controller

________________________________________________________________

Maxim Integrated Products

GND

V+

HIGH

REF

LBI

LBO

TOP VIEW

MAX746

EXT

AGND

OUT

AV+

SHDN

DIP/SO

__________________Pin Configuration

HIGH

SHDN

LBI

REF

EXT

OUT

V+

AV+

OUTPUT

LOW-BATTERY

DETECTOR OUTPUT

LBO

GND

AGND

ON/OFF

LOW-BATTERY

DETECTOR INPUT

INPUT 6V TO 15V

40mΩ

39µH

440µF

MAX746

__________Typical Operating Circuit

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

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*

MAX746MJE -55°C to +125°C 16 CERDIP

MAX746ESE -40°C to +85°C 16 Narrow SO

MAX746EPE -40°C to +85°C 16 Plastic DIP

19-0192; Rev 1; 11/93

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

Summary of content (16 pages)

PAGE 1
9-0192; Rev 1; 11/93 High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller ____________________________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.
PAGE 2
MAX746 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 (IREF) ....................................................
PAGE 3
High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller (V+ = 10V, ILOAD = 0A, IREF = 0µA, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SYMBOL Maximum Duty Cycle Charge-Pump Output Voltage Current-Sense Amplifier Current-Limit Threshold CONDITIONS V+ = 6V VHIGH VLIMIT IHIGH = 0mA to 10mA MIN TYP 91 96 V+ + 4 V+ – CS MAX % V+ + 5 V+ + 6 125 150 UNITS 175 VHIGH - 0.
PAGE 4
__________________________________________Typical Operating Characteristics (Circuit of Figure 1a, TA = +25°C, unless otherwise noted.) 0.9 2 ENTIRE CIRCUIT 1 7 9 11 13 SUPPLY VOLTAGE (V) -75 -50 -25 15 0 25 50 75 5 0.7 100 125 MAX746-07 VIN = 6V VIN = 9V VIN = 12V 80 1.1 1.3 1.5 1.7 EFFICIENCY vs. OUTPUT CURRENT 100 CIRCUIT OF FIGURE 1c VOUT = 5V VIN = 6V EFFICIENCY (%) CIRCUIT OF FIGURE 1b VOUT = 3.3V V+ = 5V 0.9 OUTPUT CURRENT (A) EFFICIENCY vs.
PAGE 5
High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller LOAD-TRANSIENT RESPONSE LINE-TRANSIENT RESPONSE LOAD-TRANSIENT RESPONSE 10V A 8V A A B 200µs/div B 500ms/div 1ms/div A: LOAD CURRENT, 0.1A TO 1.5A, 1A/div B: VOUT RIPPLE, 50mV/div, AC-COUPLED V+ = 10V A: V+ = 8V TO 10V, 2V/div B: VOUT RIPPLE, 100mV/div IOUT = 3A A: LOAD CURRENT, 0.1A TO 1.
PAGE 6
MAX746 High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller ______________________________________________________________Pin Description PIN NAME FUNCTION 1 LBO Low-battery output is an open-drain output that goes low when LBI is less than 2V. Connect to V+ through a pull-up resistor. Leave floating if not used. LBO is disabled in shutdown mode. 2 LBI Input to the low-battery comparator. Tie to V+ or GND if not used. 3 SS Soft-start limits start-up surge currents.
PAGE 7
High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller MAX746 V IN 6V TO 15V C3 0.1µF C2 100µF R2 2 R1 D2 1N914 * 15 V+ LBI CP HIGH C5 0.1µF 3 C6 1.0µF 4 6 5 11 AV+ SS REF C9 4.7µF D4 1N5817 MAX746 CS 14 D3 1N914 * C8 0.1µF 13 8 R SENSE 40mΩ 10 Q1 Si9410DY EXT 12 FB CC SHDN AGND OUT LB0 7 C7 2.7nF N D1 NSQ03A03 L1 39µH 5V AT 3A C1 430µF 9 C4 0.1µF 1 R3 100k GND 16 * SEE TABLE 2 FOR DIODE SELECTION. Figure 1a.
PAGE 8
MAX746 High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller V IN 4.5V TO 6V C3 0.1µF C2 100µF R2 2 R1 D2 D3 D5 1N914 1N914 1N914 15 V+ LBI CP HIGH C5 0.1µF 3 C6 1µF 4 AV+ SS REF MAX746 CS 14 C8 0.1µF C9 1µF D6 1N914 11 CC SHDN 13 8 R SENSE 40mΩ 10 Q1 Si9410DY FB LB0 N 7 OUT 9 AGND C11 1µF C10 0.1µF EXT 12 5 D4 1N5817 R5 13k (1%) 6 C7 2nF 1 R4 20k (1%) L1 * 22µH D1 NSQ03A03 3.3V AT 3A C4 0.
PAGE 9
High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller MAX746 VIN 6V TO 15V C3 0.1µF C2 47µF R2 2 R1 D2 1N914 * 15 V+ LBI CP HIGH C5 0.1µF 3 C6 1µF 4 6 5 11 AV+ SS REF C9 4.7µF D4 1N5817 MAX746 CS 14 D3 1N914 * C8 0.1µF 13 8 R SENSE 75mΩ 10 Q1 Si9410DY EXT 12 FB CC SHDN AGND OUT LB0 7 C7 1nF N D1 NSQ03A03 L1 ** 82µH 5V AT 1.5A C1 220µF 9 C4 0.1µF 1 R3 100k GND 16 * SEE TABLE 2 FOR DIODE SELECTION. ** SUMIDA CDR125 SURFACE-MOUNT INDUCTOR. Figure 1c.
PAGE 10
MAX746 High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller LBO HIGH EXT V+ PUMP FROM AV+ LBI LOW-BATTERY COMPARATOR CHARGE-PUMP CONTROL COMPARATOR 4.
PAGE 11
High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller C8 0.1µF D4 1N5817 RSENSE = 50mΩ PEAK CURRENT LIMIT (A) 15 V+ D3 1N914 C9 1µF MAX746 T FLIPT FLOP Q CLK MAX746-FG03 3 D2 1N914 V IN MAX746 PEAK CURRENT LIMIT vs. SOFT-START VOLTAGE 3a. CHARGE-PUMP VOLTAGE DOUBLER CP 14 2 V+ - VCS = 150mV 1 HIGH 13 RSENSE = 100mΩ 100kHz OSCILLATOR 0 0 1 2 3 SOFT-START VOLTAGE (V) 5V Figure 4. Peak Current Limit vs. Soft-Start Voltage 3b.
PAGE 12
MAX746 High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller VIN 15 EXT V+ 12 N L R5 FB 6 VOUT C7* MAX746 R4 OUT OUT 9 9 16 R4 = 10kΩ TO 60kΩ VOUT R5 = R4 -1 VREF VREF = 2.0V NOMINAL ( C1 FB 6 MAX746 GND V OUT D1 R5 26.1k (1%) ) * SEE COMPENSATION CAPACITOR SECTION. SELECT WITH FET OFF: VOUT = VREF SELECT WITH FET OFF: VOUT = VREF R5 +1 ) ( R4a R4a 17.4k (1%) 5V/3.3V N C7 R4b 22.6k (1%) ( R4a R5+ R4b +1 ) VREF = 2.0V NOMINAL Figure 6.
PAGE 13
High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller To ensure the external N-FET is turned on hard, use logic-level or low-threshold N-FETs. Three important parameters to note when selecting the N-FET are the total gate charge (Qg), on resistance (rDS(ON)), and reverse transfer capacitance (CRSS). Qg includes all capacitances associated with charging the gate.
PAGE 14
MAX746 High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller Table 1. Measured Output Voltage Sag with 30mA to 3A Load Step* OUTPUT FILTER CAPACITOR C1 (mF) VIN =6V VIN=6.5V VIN=7V VIN=9V VIN =10V 440 400 250 210 140 90 660 260 190 160 70 50 880 200 100 90 40 25 OUTPUT VOLTAGE SAG (mV) FOR VARIOUS INPUT VOLTAGES *Circuit of Figure 1a. For load currents up to 3A, 100µF (C2) in parallel with 0.1µF (C3) is adequate.
PAGE 15
High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller Setting the Low-Battery Detector Voltage Select R1 between 10kΩ and 1MΩ. Determine R2 using the following equation: R2 = R1 (VTRIP - VREF) ________________ VREF ( ) MAX746 Increasing C7 by up to 50% enhances outer-loop stability by adding stability to the inductor current waveform.
PAGE 16
MAX746 High-Efficiency, PWM, Step-Down, N-Channel DC-DC Controller or equal to the N-FET’s V GS threshold voltage. Once the switch is off, the voltage at EXT is pulled to GND and the N-FET source voltage is a Schottky diode drop below GND. However, this is not always the case in the “in-between” mode, due to the changing duty cycle inherent with DCM.