MAX1642 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1642

High-Efficiency, Step-Up DC-DC Converters for 1V Inputs

Maxim Integrated 

High-Efficiency, Step-Up

DC-DC Converters for 1V Inputs

When the error comparator detects that the output

voltage is too low, it turns on the internal N-channel

MOSFET switch until the on-time is satisfied (see Figure

1 and the Standard Application Circuits, Figures 2 and

3). During the on-time, current ramps up in the induc-

tor, storing energy in a magnetic field. When the MOS-

FET turns off, during the second half of each cycle, the

magnetic field collapses, causing the inductor voltage

to force current through the synchronous rectifier,

transferring the stored energy to the output filter capac-

itor and load. The output filter capacitor stores charge

while current from the inductor is high, then holds up

the output voltage until the second half of the next

switching cycle, smoothing power flow to the load.

Bootstrap DC-DC Block

The bootstrap block contains a low-voltage start-up

oscillator. This oscillator pumps up the output voltage

to approximately 1.7V, where the main DC-DC con-

verter can operate. The oscillator is powered from the

BATT input and drives an NPN switch. During start-up,

the P-channel synchronous rectifier remains off and

either its body diode or an external diode is used as an

output rectifier. Reduce the load as needed to allow

start-up with input voltages below 2V (see Typical

Operating Characteristics).

Shutdown (MAX1642)

Pulling SHDN low places the MAX1642 in shutdown

mode (ISHDN = 2µA typical). In shutdown, the internal

switching MOSFET turns off, PFO goes high-

impedance, and the synchronous rectifier turns off to

prevent reverse current from flowing from the output

back to the input. However, there is still a forward cur-

rent path through the synchronous-rectifier body diode

from the input to the output. Thus, in shutdown, the out-

put remains one diode drop below the battery voltage

(VBATT). To disable the shutdown feature, connect

SHDN (a logic input) to BATT.

BATT

0.5REF

PFI

PFO

OUT

TIMING

TON

TOFF

EN

LOGIC

PDRV

NDRV

P

MAX1642

LX

FB

N

REF

RFRDY

REF

REF

0.5REF

START-UP

OSCILLATOR

GND

SHDN

OUT

1.7V

Figure 1. MAX1642 Functional Diagram

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