9

FN9220.0

September 27, 2005

Block Diagram

Functional Description

The ISL9012 contains all circuitry required to implement two

high performance LDO’s. High performance is achieved

through a circuit that delivers fast transient response to

varying load conditions. In a quiescent condition, the

ISL9012 adjusts its biasing to achieve the lowest standby

current consumption.

The device also integrates current limit protection, smart

thermal shutdown protection, staged turn-on and soft-start.

Smart Thermal shutdown protects the device against

overheating. Staged turn-on and soft-start minimize start-up

input current surges without causing excessive device turn-

on time.

Power Control

The ISL9012 has two separate enable pins, EN1 and EN2,

to individually control power to each of the LDO outputs.

When both EN1 and EN2 are low, the device is in shutdown

mode. During this condition, all on-chip circuits are off, and

the device draws minimum current, typically less than 0.1

µA.

When one or both of the enable pins are asserted, the

device first polls the output of the UVLO detector to ensure

that VIN voltage is at least about 2.1V. Once verified, the

device initiates a start-up sequence. During the start-up

sequence, trim settings are first read and latched. Then,

sequentially, the bandgap, reference voltage and current

generation circuitry power up. Once the references are

stable, a fast-start circuit quickly charges the external

reference bypass capacitor (connected to the CBYP pin) to

the proper operating voltage. After the bypass capacitor has

been charged, the LDO’s power up.

If EN1 is brought high, and EN2 is goes high before the VO1

output stablizes, the ISL9012 delays the VO2 turn-on until

the VO1 output reaches its target level. This minimizes input

current surge due to concurrent turn-on.

LDO

ERROR

AMPLIFIER

IS2

1V

QEN2

LDO-2

LDO-1

POR

COMPARATOR

VOK2

POR

VREF

TRIM

VIN

VO1

VO2

POR

CPOR

CBYP

GND

EN2

EN1

CONTROL

LOGIC

POR

DELAY

VOLTAGE

REFERENCE

GENERATOR

BANDGAP AND

TEMPERATURE

SENSOR

UVLO

VOK2

1.00V

0.94V

0.90V

VO2

~1.0V

ISL9012