MAX879E データシートの表示（PDF） - Maxim Integrated

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MAX879E

5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters

Maxim Integrated 

5V/3.3V/3V/Adjustable-Output,

Step-Up/Step-Down DC-DC Converters

VIN

DELAY

TIMER

tOFF

DELAY

TIMER

tON

22µH

22µF

2

IN

RLIM

15

ILIM

LX

SWITCH

DRIVER

ACTIVE RECTIFIER

RECTIFIER

CONTROL

1:N

OUT 6

PGND 4

VOUT

100µF

MAX878

7 SHDN

SHUTDOWN

CONTROL

VREF 0.2025V

SEL 8

AGND

3

Figure 1. MAX878 Block Diagram

With ILIM shorted to IN, the peak switch current of the

internal NPN power switch is set to 1A. It can be set to

a lower value by connecting a resistor between ILIM

and IN (see Current Limit section). This enables the use

of physically smaller inductors with lower saturation-

current ratings. At 1A, the switch voltage drop (VSW) is

about 500mV. VSW decreases to about 250mV at 0.1A.

Conventional PWM converters generate constant-

frequency switching noise, while this architecture

produces variable-frequency switching noise. The out-

put ripple is the product of the peak inductor current

and the output capacitor's ESR. Unlike conventional

pulse-skippers, the MAX877/MAX878/MAX879 peak

currents are scaled down at light loads, resulting in

lower output ripple.

Step-Down Mode and Power Dissipation

In battery-powered applications, for example, where

the input voltage exceeds the output voltage, the

MAX877/MAX878/MAX879 behave as “switched” linear

regulators. If the output voltage starts to drop, the

switch turns on and energy is stored in the coil, as in

normal step-up mode. After the switch turns off, the

voltage at LX flies high. The active rectifier turns on

when LX rises above VIN. As in a linear regulator, the

voltage difference between VIN and VOUT appears

across the rectifier (actually a PNP transistor) until the

current goes to zero and the rectifier turns off. At high

VIN to VOUT differentials, the maximum load current is

limited by the maximum allowable power dissipation in

the package (see Typical Operating Characteristics).

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