Rainbow-electronics MAX1774 Manual do Utilizador descarregar pdf (Página 12)

each cycle (see Typical Operating Characteristics). The

switch waveform may exhibit ringing, which occurs at

the resonant frequency of the inductor and stray capaci-

tance, due to the residual energy trapped in the core

when the rectifier MOSFET turns off. This ringing is nor-

mal and does not degrade circuit performance.

When delivering medium-to-high output currents, the

MAX1774 operates in PWM continuous-conduction

mode. In this mode, current always flows through the

inductor and never ramps to zero. The control circuit

adjusts the switch duty cycle to maintain regulation

without exceeding the peak switching current set by

the current-sense resistor.

100% Duty Cycle and Dropout

The MAX1774 operates with a duty cycle up to 100%,

extending the input voltage range by turning the MOS-

FET on continuously when the supply voltage ap-

proaches the output voltage. This services the load

when conventional switching regulators with less than

MAX1774

Dual, High-Efficiency, Step-Down

Converter with Backup Battery Switchover

12 ______________________________________________________________________________________

PGNDC

FBC

LXC

INC

FBM

CS-

CS+

PGND

PDRV

NDRV

CVH

MAIN

1µF

5µH

MAIN

47µF

10µF

CORE

22µF

R10

R11

5.4µH

1µF

1MΩ

GND

REF

CVL

LXB

BIN

DBI

LBI

BACKUP

BATTERY

MAIN

BATTERY

IN_AC

10µF

0.22µF

1µF

EP05Q

03L

22µH

BKOFF

BKUP

LBO

ACO

SHDNC

SHDNM

MDRV

MAX1774

OFF

1.0V

5.5V

LXB2(QFN ONLY)

0.9V

5.5V

1MΩ

ACI

INS

FDS8928A

1.25V

5.5V

NDS356AP

NSD03A10

2.7V

5.5V

2.7V

5.5V

NOTE: FOR INPUT VOLTAGES

TO 28V SEE FIGURE 4

AND FIGURE 5

Figure 1. Typical Application Circuit For Low-Input Voltage Applications

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 ... 20 21

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