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FN6300.5

June 27, 2014

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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 0.3µA.

When the EN pin goes high, the device first polls the output

of the UVLO detector to ensure that the 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.

about 1.84V, the ISL9008A immediately disables the LDO

pin is high), the device reinitiates 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 1µF to 4.7µF output

capacitor that has a tolerance better than 20% and ESR less

than 200mW. The design is performance-optimized for a 1µF

capacitor. Unless limited by the application, use of an output

capacitor value above 4.7µF is not recommended as LDO

performance improvement is minimal.

Soft-start circuitry integrated into each LDO limits the initial

ramp-up rate to about 30µs/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.

Exposed Thermal Pad

The ISL9008A with µTDFN package has an exposed

thermal pad at the bottom side of the package. The PCB

layout should connect the exposed pad to some copper on

the component layer for a good thermal conductivity. Since

the copper area on the component layer is limited by the

surrounding pins of the package, it is more effective to use

some thermal vias to conduct the heat to other copper layers

if possible.

Electrically, the copper and vias connecting to the exposed

pad should be isolated from any other pin connection, they

are strictly for thermal enhancement purpose.

ISL9008A