9
FN9231.2
March 28, 2008
Block Diagram
Functional Description
The ISL9001 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
ISL9001 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.
Power Control
The ISL9001 has an enable pin (EN) to control power to the
LDO output. When EN is low, the device is in shutdown
mode. During this condition, all on-chip circuits are off, and
the device draws minimum current, typically less than 0.1µA.
When the enable pin is asserted, the device first polls the
output of the UVLO detector to ensure that VIN voltage is at
least about 2.1V. 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 power-up.
Once the references are stable, a fast-start circuit quickly
charges the external reference bypass capacitor (connected
to the CBYP pin) to the proper operating voltage. Once the
bypass capacitor has been charged, the LDO powers up.
During operation, whenever the VIN voltage drops below
about 1.84V, the ISL9001 immediately disables the LDO
output. 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 to CBYP implements a 100Hz
lowpass filter, and is recommended for most high
performance applications. For the lowest noise application, a
0.1µF 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, POR detection thresholds,
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.
VO
GND
BANDGAP AND
TEMPERATURE
SENSOR
UVLO
VIN
SHORT CIRCUIT,
THERMAL PROTECTION,
SOFT-START
EN
CONTROL
LOGIC
CBYP
1.0V
0.94V
0.9V
POR
GND
POR
DELAY
1.0V
VO
100k
CPOR
VOLTAGE AND
REFERENCE
GENERATOR
+
-
+
-
ISL9001
