9

FN9231.2

March 28, 2008

Block Diagram

Functional Description

The ISL9001 contains all circuitry required to implement a

high performance LDO. High performance is achieved

through a circuit that delivers fast transient response to

varying load conditions. In a quiescent condition, the

ISL9001 adjusts its biasing to achieve the lowest standby

current consumption.

The device also integrates current limit protection, smart

thermal shutdown protection, and soft-start. Smart Thermal

shutdown protects the device against overheating.

Power Control

The ISL9001 has an enable pin (EN) to control power to the

LDO output. When EN is 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 0.1µA.

When the enable pin is 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. Once the

bypass capacitor has been charged, the LDO powers up.

During operation, whenever the VIN voltage drops below

about 1.84V, the ISL9001 immediately disables the LDO

output. When VIN rises back above 2.1V, the device

re-initiates its start-up sequence and LDO operation will

resume automatically.

Reference Generation

The reference generation circuitry includes a trimmed

bandgap, a trimmed voltage reference divider, a trimmed

current reference generator, and an RC noise filter. The filter

includes the external capacitor connected to the CBYP pin.

A 0.01µF capacitor connected to CBYP implements a 100Hz

lowpass filter, and is recommended for most high

performance applications. For the lowest noise application, a

0.1µF CBYP capacitor should be used. This filters the

reference noise to below the 10Hz to 1kHz frequency band,

which is crucial in many noise-sensitive applications.

The bandgap generates a zero temperature coefficient (TC)

voltage for the reference divider. The reference divider

provides the regulation reference, POR detection thresholds,

and other voltage references required for current generation

and over-temperature detection.

The current generator outputs references required for

adaptive biasing as well as references for LDO output

current limit and thermal shutdown determination.

VO

GND

BANDGAP AND

TEMPERATURE

SENSOR

UVLO

VIN

SHORT CIRCUIT,

THERMAL PROTECTION,

SOFT-START

EN

CONTROL

LOGIC

CBYP

1.0V

0.94V

0.9V

POR

GND

POR

DELAY

1.0V

VO

CPOR

VOLTAGE AND

REFERENCE

GENERATOR

+

-

+

-

ISL9001