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SC1531(A)

POWER MANAGEMENT

Revision 1, December 2000

The SC1531(A) is designed to maintain a glitch-free 3.3V

output when at least one of two inputs, 5V (VIN) and

3.3V (VAUX), is present.

Whenever VIN exceeds a predetermined threshold value,

the internal 3.3V linear regulator is enabled, and DR is

pulled high.

When VIN falls below a lower threshold value, DR is pulled

low and the internal linear regulator is turned off. DR has

been designed to drive the gate of an external low

threshold P-channel MOSFET, which can be used to

connect the 3.3V supply directly to the regulator output.

This ensures an uninterrupted 3.3V output even if VIN

falls out of specification. A typical R

recommended (320mW for SC1531A).

When both supplies are simultaneously available, the

drive pin (DR) will be pulled High, turning off the external

PMOS switch.

The internal 5V detector has its upper threshold (for VIN

rising) set to 4.18V (typical) while the lower threshold (for

VIN falling) is at 4.05V (typical) giving a hysteresis of

approximately 130mV.

The SENSE pin, which is connected to the load, connects

internally to the inverting input of the LDO error

amplifier. It enables tight regulation of the load voltage

(while the 5V supply is present) despite variations in load

current.

The SC1531(A) is available in the popular SO-8 surface

mount package.

5V

3.3V

3.3V OUT

Q1

C4

4.7uF

C5

0.1uF

C3

0.1uF

C2

4.7uF

C1

0.1uF

U1

SC1531

1

2

3

4

5

6

7

8

VIN

NC

VAUX

GND

NC

SENSE

VO

DR

Notes for Typical Application Circuit:

(1) External switch (Q1): use Motorola MGSF1P02ELT1

or equivalent (PMOS, typical Gate Threshold Voltage =

1V, typical R

Vishay Si2301DS or equivalent for SC1531A.

(2) Connection of VAUX (pin 3) is optional.

Description

Features

Applications

Typical Application Circuit