________________Detailed Description

Operating Principle

The MAX761/MAX762 BiCMOS step-up switch-mode

power supplies provide fixed outputs of 12V and 15V,

respectively. They have a unique control scheme that

combines the advantages of pulse-frequency modulation

(low supply current) and pulse-width modulation (high

efficiency at high loads). The internal N-channel power

MOSFET allows 1A peak currents, increasing the output

current capability over previous pulse-frequency-modu-

lation (PFM) devices. Figure 1 shows the MAX761/

MAX762 block diagram.

The MAX761/MAX762 offer three main improvements

over prior solutions: (1) the converters operate with tiny

surface-mount inductors (less than 5mm diameter)

because of their 300kHz switching frequency, (2) the

current-limited PFM control scheme allows 86% efficien-

cies over a wide range of load currents, and (3) the max-

imum supply current is only 110µA.

Bootstrapped/Non-Bootstrapped Modes

Figures 2 and 3 show the standard application circuits

for bootstrapped and non-bootstrapped modes. In boot-

strapped mode, the IC is powered from the output

bootstrapped circuit is different from the V+ current the

chip consumes. The voltage applied to the gate of the

providing more switch-gate drive and increasing the effi-

ciency of the DC-DC converter compared with non-boot-

strapped operation.

12V/15V or Adjustable, High-Efficiency,

_______________________________________________________________________________________

7

N

N

N

LBI

LBI

LBO

V+

FB

DUAL-MODE

™

COMPARATOR

REF

1.5V

REFERENCE

UNDER VOLTAGE

COMPARATOR

LOW INPUT

VOLTAGE

OSCILLATOR

CURRENT CONTROL

CIRCUITRY

CURRENT

COMPARATOR

Q

S

R

Q

TRIG

ONE-SHOT

Q

TRIG

ONE-SHOT

0.1V

0.2V

2.5V

100mV

V+

LX

GND

ERROR

COMPARATOR

MAX761

MAX762

Figure 1. Simple Block Diagram