9

FN6300.2

April 16, 2007

Functional Description

The ISL9008A 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 ISL9008A 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. Soft-start

minimizes start-up input current surges without causing

excessive device turn-on time.

Power Control

The ISL9008A has an enable pin, EN, to control power to the

LDO output. When EN is low, the device is in shutdown mode.

In this condition, all on-chip circuits are off, and the device

draws minimum current, typically less than 0.3

μA. When the

EN pin goes high, the device first polls the output of the UVLO

detector to ensure that VIN voltage is at least 2.1V (typical).

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 turn on. Once the references are stable, the

LDO powers up.

During operation, whenever the VIN voltage drops below about

1.84V, the ISL9008A immediately disables the LDO output.

When VIN rises back above 2.1V (assuming the EN pin is

high), the device re-initiates its start-up sequence and LDO

operation resumes 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 bandgap generates a zero temperature coefficient (TC)

voltage for the regulator reference and other voltage references

required for current generation and over-temperature detection.

A current generator provides references required for adaptive

biasing as well as references for LDO output current limit and

thermal shutdown determination.

LDO Regulation and Programmable Output Divider

The LDO Regulator is implemented with a high-gain

operational amplifier driving a PMOS pass transistor. The

design of the ISL9008A provides a regulator that has low

quiescent current, fast transient response, and overall stability

across all operating and load current conditions. LDO stability is

guaranteed for a 1

μF to 4.7μF output capacitor that has a

design is performance-optimized for a 1

μF capacitor. Unless

limited by the application, use of an output capacitor value

above 4.7

μF is not recommended as LDO performance

improvement is minimal.

Soft-start circuitry integrated into each LDO limits the initial

ramp-up rate to about 30

μs/V to minimize current surge. The

ISL9008A provides short-circuit protection by limiting the output

current to about 265mA (typ).

The LDO uses an independently trimmed 1V reference as its

input. An internal resistor divider drops the LDO output voltage

down to 1V. This is compared to the 1V reference for regulation.

The resistor division ratio is programmed in the factory.

Overheat Detection

The bandgap outputs a proportional-to-temperature current that

is indicative of the temperature of the silicon. This current is

compared with references to determine if the device is in

danger of damage due to overheating. When the die

temperature reaches about +140°C, the LDO momentarily

shuts down until the die cools sufficiently. In the overheat

condition, if the LDO sources more than 50mA it will be shut off.

Once the die temperature falls back below about +110°C, the

disabled LDO is re-enabled and soft-start automatically takes

place.

ISL9008A