MAX1674 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1674

High-Efficiency, Low-Supply-Current, Compact, Step-Up DC-DC Converters

Maxim Integrated 

High-Efficiency, Low-Supply-Current,

Compact, Step-Up DC-DC Converters

Detailed Description

The MAX1674/MAX1675/MAX1676 compact, step-up

DC-DC converters start up with voltages as low as 0.9V

and operate with an input voltage down to 0.7V.

Consuming only 16µA of quiescent current, these

devices offer a built-in synchronous rectifier that

reduces cost by eliminating the need for an external

diode and improves overall efficiency by minimizing

losses in the circuit (see Synchronous Rectification sec-

tion for details). The internal MOSFET resistance is typi-

cally 0.3Ω, which minimizes losses. The current limit of

the MAX1674 and MAX1675 are 1A and 0.5A, respec-

tively. The MAX1675’s lower current limit allows the use

of a physically smaller inductor in space-sensitive

applications. The MAX1676 features a circuit that elimi-

nates noise due to inductor ringing. In addition, the

MAX1676 offers a selectable current limit (0.5A or 1A)

for design flexibility.

PFM Control Scheme

A unique minimum-off-time, current-limited, pulse-fre-

quency-modulation (PFM) control scheme is a key fea-

ture of the MAX1674/MAX1675/MAX1676. This scheme

combines the high output power and efficiency of a

pulse-width-modulation (PWM) device with the ultra-low

quiescent current of a traditional PFM (Figure 1). There

is no oscillator; a constant-peak-current limit in the

switch allows the inductor current to vary between this

peak limit and some lesser value. At light loads, the

switching frequency is governed by a pair of one-shots

that set a typical minimum off-time (1µs) and a typical

maximum on-time (4µs). The switching frequency

depends upon the load and the input voltage, and can

range up to 500kHz. The peak current of the internal N-

channel MOSFET power switch is fixed at 1A

(MAX1674), at 0.5A (MAX1675), or is selectable

(MAX1676). Unlike conventional pulse-skipping DC-DC

converters (where ripple amplitude varies with input

voltage), ripple in these devices does not exceed the

product of the switch current limit and the filter-capaci-

tor equivalent series resistance (ESR).

Synchronous Rectification

The internal synchronous rectifier eliminates the need

for an external Schottky diode, thus reducing cost and

board space. During the cycle off-time, the P-channel

MOSFET turns on and shunts the MOSFET body diode.

SHDN

MINIMUM

OFF-TIME

ONE-SHOT

Q TRIG

ONE-SHOT

CLSEL

(MAX1676)

VIN VOUT

R3

R2

100k

S F/F Q

R

MAXIMUM

ON-TIME

ONE-SHOT

TRIG Q

ONE-SHOT

ERROR

AMPLIFIER

CURRENT-LIMIT

AMPLIFIER

LBO

R4

LBI

LOW-BATTERY

COMPARATOR

ZERO

EN

CROSSING

AMPLIFIER

OUT 0.1µF 47µF

MAX1674

MAX1675

MAX1676

P

LX

22µH

N

GND

BATT

(MAX1676)

DAMPING

SWITCH

FB

R1

200Ω

R5

REFERENCE

R6

REF

0.1µF

VOUT

VIN

47µF

Figure 1. Simplified Functional Diagram

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