MAX779LC 查看數據表（PDF） - Maxim Integrated

零件编号

产品描述 (功能)

比赛名单

MAX779LC

Low-Voltage Input, 3V/3.3V/5V/ Adjustable Output, Step-Up DC-DC Converters

Maxim Integrated 

Low-Voltage Input, 3V/3.3V/5V/

Adjustable Output, Step-Up DC-DC Converters

VIN

DELAY

TIMER

tOFF

DELAY

TIMER

tON

22µH

100µF

2

IN

RLIM

1

ILIM LX

SWITCH

DRIVER

ACTIVE RECTIFIER

RECTIFIER

CONTROL

1:N

OUT 6

PGND 4

VOUT

100µF

MAX778L

7 SHDN

SHUTDOWN

CONTROL

VREF 0.2025V

SEL 8

AGND

3

Figure 1. MAX778L Block Diagram

_______________Detailed Description

Operating Principle

The MAX777L/MAX778L/MAX779L combine a switch-

mode regulator with an NPN bipolar switch, current

limit, precision voltage reference, and active rectifier—

all in a single monolithic device. In shutdown mode,

the internal rectifier is completely turned off and dis-

connects the load from the source. Only two external

components are required in addition to the input

bypass capacitor: a 22µH inductor and a 100µF filter

capacitor.

A minimum off-time, current-limited, pulse-frequency-

modulation (PFM) control scheme combines the advan-

tages of pulse width modulation (PWM) (high output

power and efficiency) with those of a traditional PFM

pulse skipper (low quiescent currents).

External conditions (inductor value, load, and input volt-

age) determine the way the converter operates, as follows:

At light loads, the current through the inductor starts at

zero, rises to a peak value, and drops down to zero in

each cycle (discontinuous-conduction mode). In this

case, the switching frequency is governed by a pair of

one-shots that set a maximum on-time inversely propor-

tional to VIN [tON = 8.8/(VIN - 0.25)] and a minimum off-

time (1.3µs for MAX777L/MAX779L and 2.3µs for

MAX778L). With a 22µH inductor, LX’s peak current is

about 400mA and is independent of input voltage.

Efficiency at light loads is improved because of lower

peak currents.

At very light loads, more energy is stored in the coil

than is required by the load in each cycle. The converter

regulates by skipping entire cycles. Efficiency is typically

65% to 75% in the pulse-skipping mode. Pulse-skipping

waveforms can be irregular, and the output waveform

contains a low-frequency component. Larger, low equiv-

alent series resistance (ESR) filter capacitors can help

reduce the ripple voltage if needed.

_______________________________________________________________________________________ 7

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