9

FN9217.3

August 2, 2006

Block Diagram

Functional Description

The ISL9000 contains 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 ISL9000 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 ISL9000 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 in their specified

sequence.

Soft-start circuitry integrated into each LDO limits the initial

ramp-up rate to about 30

μs/V to minimize current surge.

VO2

LDO

ERROR

AMPLIFIER

IS1

1V

QEN1

LDO-1

LDO-2

POR

COMPARATOR

VOK1

POR1

VREF

TRIM

VIN

VO1

VO2

POR2

POR1

GND

EN2

EN1

CONTROL

LOGIC

POR2

DELAY

POR1

DELAY

VOLTAGE

REFERENCE

GENERATOR

BANDGAP AND

TEMPERATURE

SENSOR

UVLO

VOK2

VOK1

1.00V

0.94V

0.90V

VO1

VO2

CPOR

CBYP

VO1

~1.0V

VOK2

POR2

ISL9000