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-

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

across the rectifier (actually a PNP transistor) until the

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

limited by the maximum allowable power dissipation in

the package (see

Typical Operating Characteristics).

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

Step-Up/Step-Down DC-DC Converters

_______________________________________________________________________________________

7

MAX878

DELAY

TIMER

DELAY

TIMER

VREF

0.2025V

SHUTDOWN

CONTROL

SHDN

7

ACTIVE RECTIFIER

RECTIFIER

CONTROL

OUT

PGND

AGND

3

SEL 8

4

6

100

µF

22

µF

22

µH

SWITCH

DRIVER

LX

ILIM

IN

21

5

1 : N

Figure 1. MAX878 Block Diagram