10

FN9219.4

June 21, 2006

If EN2 is brought high, and EN1 goes high before VO2 starts

its output ramp, then VO1 turns on first and the ISL9011

delays the VO2 turn-on until the VO1 output reaches its

target level.

If EN2 is brought high, and EN1 goes high after VO2 starts

its output ramp, then the ISL9011 immediately starts to ramp

up the VO1 output.

If both EN1 and EN2 are brought high at the same time, the

VO1 output has priority, and is always powered up first.

During operation, whenever the VIN voltage drops below

about 1.8V, the ISL9011 immediately disables both LDO

outputs. 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 0.01

µF capacitor connected CBYP implements a 100Hz

lowpass filter, and is recommended for most high

performance applications. For the lowest noise application, a

0.1

µF or greater CBYP capacitor should be used. This filters

the reference noise to below the 10Hz – 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 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.

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 ISL9011 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 10µF output

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

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

ISL9011 provides short-circuit protection by limiting the

output current to about 475mA.

Each LDO uses an independently trimmed 1V reference. 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 145°C, one or both of the

LDO’s momentarily shut down until the die cools sufficiently.

In the overheat condition, only the LDO sourcing more than

50mA will be shut off. This does not affect the operation of

the other LDO. If both LDOs source more than 50mA and an

overheat condition occurs, both LDO outputs are disabled.

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

disabled LDO(s) are re-enabled and soft-start automatically

takes place.

ISL9011