Control Scheme

The MAX1947 is a bootstrapped design. Upon turn-on,

a startup oscillator brings the output voltage high

enough to allow the main DC-DC circuitry to run. Once

the output voltage reaches 1.62V (typ) the main DC-DC

circuitry turns on and boosts the output voltage to the

final regulation point.

The unique minimum off-time, current-limited control

scheme is the key to the MAX1947’s low operating cur-

rent and high efficiency over a wide load range. The

architecture combines the high output power and effi-

ciency of a pulse-width modulation (PWM) device with

the ultra-low quiescent current of a traditional pulse-

skipping controller. The switching frequency can be as

high as 2MHz and depends upon the load current and

input voltage. The MAX1947 is designed to operate

using low-ESR ceramic capacitors, so output voltage

Track Mode

The MAX1947 enters track mode when BATT is greater

than the output-voltage regulation point. Track mode

can only be entered under the following conditions:

minimum off-time expires. During track mode, the syn-

chronous rectifier is turned on 100% of the time and the

output voltage tracks the battery voltage. Track mode is

Synchronous Rectification

The internal synchronous rectifier eliminates the need

for an external Schottky diode, reducing cost and board

space. During the cycle off-time, the p-channel MOSFET

turns on and shunts the MOSFET body diode. As a

result, the synchronous rectifier significantly improves

efficiency without the addition of an external compo-

nent. Conversion efficiency can be as high as 94%.

RESET

The MAX1947 features an active-low push-pull RESET

output for use with a microcontroller (µC). It signals the

µC when the MAX1947 output voltage is within operat-

ing limits. During startup, RESET is held low. When the

RESET threshold (90% of the output regulation voltage)

is reached, a 75ms (min) timer begins counting. RESET

is switched high once the timer expires.

Low Input/Output Voltage

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7

Functional Diagram

STARTUP

OSCILLATOR

SHUTDOWN

CIRCUITRY

MAX TIMER

CONTROL

LOGIC AND

GATE DRIVERS

BATT

SHDN

0.9 x REF

REF

ERROR

AMPLIFIER

ZERO-CROSSING

AMPLIFIER

CURRENT-LIMIT

AMPLIFIER

P*

N

LX

PGND

OUT

RESET

75ms TIMER

MIN TIMER

1.25V REFERENCE

(REF)

GND

MAX1947

REF

*BODY DIODE CONTROL