10
FN9219.5
March 10, 2008
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 0.01µF capacitor connected CBYP implements a 100Hz
lowpass filter, and is recommended for most high
performance applications. For the lowest noise application, a
0.1µF or greater 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 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 1µF to 10µF output
capacitor that has a tolerance better than 20% and ESR less
than 200mΩ. 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
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
