
11
FN9219.1
October 13, 2005
If EN2 is brought high, and EN1 goes high before the VO2 
output stablizes, the ISL9011 delays the VO1 turn-on until 
the VO2 output reaches its target level. 
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
μ
F capacitor connected CBYP implements a 100Hz 
lowpass filter, and is recommended for most high 
performance applications. For the lowest noise application, a 
0.1
μ
F 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
μ
F to 10
μ
F output 
capacitor that has a tolerance better than 20% and ESR less 
than 200m
. The design is performance-optimized for a 1
μ
F 
capacitor. Unless limited by the application, use of an output 
capacitor value above 4.7
μ
F 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
μ
s/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