ISL78302A
9
FN7932.2
December 22, 2015
Submit Document Feedback
Functional Description
The ISL78302A contains two high-performance LDOs. High
performance is achieved through a circuit that delivers fast
transient response to varying load conditions. In a quiescent
condition, the ISL78302A adjusts its biasing to achieve the
lowest standby current consumption.
The device also integrates current limit protection, smart thermal
shutdown protection, staged turn-on, and soft-start. Smart
thermal shutdown protects the device against overheating.
Staged turn-on and soft-start minimize start-up input current
surges without causing excessive device turn-on time.
Power Control
The ISL78302A has two separate enable pins (EN1 and EN2) to
individually control power to each of the LDO outputs. When both
EN1 and EN2 are 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 0.3µA.
When one or both of the enable pins are 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. After the bypass capacitor has
been charged, the LDOs power up in their specified sequence.
Soft-start circuitry integrated into each LDO limits the initial
ramp-up rate to about 30µs/V to minimize current surge.
If EN1 is brought high, and EN2 goes high before the VO1 output
stabilizes, the ISL78302A delays the VO2 turn-on until the VO1
output reaches its target level.
If EN2 is brought high, and EN1 goes high before VO2 starts its
output ramp, then VO1 turns on first, and the ISL78302A 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 ISL78302A 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 ISL78302A immediately disables both LDO outputs.
When VIN rises back above 2.1V, the device re-initiates 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 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 or greater CBYP capacitor
should be used. This filters the reference noise 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 provides the 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
ISL78302A 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 normally needed as
LDO performance improvement is minimal.
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.
Power-On Reset Generation
Each LDO has a separate power-on reset (POR) signal generation
circuit that outputs to the respective POR
pins. The POR signal is
generated as follows.
A POR comparator continuously monitors the output of each
LDO. The LDO enters a power-good state when the output voltage
is above 94% of the expected output voltage for a period
exceeding the LDO PGOOD entry delay time. In the power-good
state, the open-drain PORx
output is in a high-impedance state.
An internal 100kΩ pull-up resistor pulls the pin up to the
respective LDO output voltage. An external resistor can be added
between the PORx
output and the LDO output for a faster rise
time; however, the PORx
output should not connect through an
external resistor to a supply greater than the associated LDO
voltage.
For the 1.5V regulated output option, it has been found that the
internal pull-ups on POR output does not always function correctly
above V
IN
= 6V. For this reason, it is recommended to use an
external 100k pull-up resistor for the POR pin that is associated
to the 1.5V output. For outputs higher than 1.5V, no external
resistor is required over the full input range from 2.3V to 6.5V.
The power-good state is exited when the LDO output falls below
90% of the expected output voltage for a period longer than the
PGOOD exit delay time. While power-good is false, the
ISL78302A pulls the respective POR
pin low.
For LDO-1, the PGOOD entry delay time is fixed at about 2ms
while the PGOOD exit delay is about 25µs. For LDO-2, the PGOOD
entry and exit delays are determined by the value of the external
capacitor connected to the CPOR pin. For a 0.01µF capacitor, the
