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AAT2782 Datasheet(PDF) 14 Page - Advanced Analogic Technologies

Part No. AAT2782
Description  Triple Output PMIC: Dual Buck with Low-VIN LDO
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Maker  ANALOGICTECH [Advanced Analogic Technologies]
Homepage  http://www.analogictech.com
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AAT2782 Datasheet(HTML) 14 Page - Advanced Analogic Technologies

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AAT2782
Triple Output PMIC: Dual Buck with Low-VIN LDO
SystemPowerTM
PRODUCT DATASHEET
14
2782.2008.05.1.0
www .analogictech.com
Functional Description
The AAT2782 is a 3-channel high performance power
management IC. Channel 1 is a 1200mA step-down con-
verter. Channel 2 is a 600mA step-down converter.
Channel 3 is a 400mA LDO regulator with low-input volt-
age capability and low output noise for sensitive analog
applications.
The low input voltage capability of the LDO regulator
allows the step-down converter to be tied directly to the
LDO input. This configuration provides the efficiency
benefits of a switching converter plus the low noise ben-
efits of a LDO (low drop-out) regulator.
Channel 1 and 2: 1200/600mA
Step-Down (Buck) Converters
The AAT2782 Channel 1 and 2 step-down converters are
peak current mode PWM converters operating at 1.3MHz
frequency. The input voltage range is 2.7V to 5.5V. The
output voltage range is 0.6V to VIN and is adjustable with
an external resistor divider. The converters provide
internal compensation. Power devices are sized for 1A
output current while maintaining over 85% efficiency at
full load. Peak efficiency is above 95%. Light load effi-
ciency is maintained at greater than 80% down to 85%
of full load current. Channel 2 has excellent transient
response and load and line regulation. Transient response
time is typically less than 20µs.
The enable inputs, when pulled low, force the converter
into a low power non-switching state consuming less
than 1µA of current.
For overload conditions, the peak input current is limit-
ed. Also, thermal protection completely disables switch-
ing if internal dissipation becomes excessive, thus pro-
tecting the device from damage. The junction over-
temperature threshold is 130°C with 20°C of hysteresis.
Under-voltage lockout (UVLO) guarantees sufficient VIN
bias and proper operation of all internal circuits prior to
activation.
Control Loop
The AAT2782 is a peak current mode step-down con-
verter. The current through the P-channel MOSFET (high
side) is sensed for current loop control, as well as short-
circuit and overload protection. A fixed slope compensa-
tion signal is added to the sensed current to maintain
stability for duty cycles greater than 50%. The peak cur-
rent mode loop appears as a voltage-programmed cur-
rent source in parallel with the output capacitor. The
output of the voltage error amplifier programs the cur-
rent mode loop for the necessary peak switch current to
force a constant output voltage for all load and line con-
ditions. Internal loop compensation terminates the
transconductance voltage error amplifier output. The
reference voltage is internally set to program the con-
verter output voltage greater than or equal to 0.6V.
Soft Start/Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. When pulled low,
the enable input forces the AAT2782 into a low-power,
non-switching state. The total input current during shut-
down is less than 1µA.
Low Dropout Operation
For conditions where the input voltage drops to the out-
put voltage level, the converter duty cycle increases to
100%. As the converter approaches the 100% duty
cycle, the minimum off-time initially forces the high side
on-time to exceed the 1.3MHz clock cycle and reduces
the effective switching frequency. Once the input drops
below the level where the converter can regulate the
output, the high side P-channel MOSFET is enabled con-
tinuously for 100% duty cycle. At 100% duty cycle the
output voltage tracks the input voltage minus the I*R
drop of the high side P-channel MOSFET.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limit-
ed. To minimize power dissipation and stresses under
current limit and short-circuit conditions, switching is
terminated after entering current limit for a series of
pulses. Switching is terminated for seven consecutive
clock cycles after a current limit has been sensed for a
series of four consecutive clock cycles. Thermal protec-
tion completely disables switching when internal dissipa-
tion becomes excessive. The junction over-temperature
threshold is 130°C with 20°C of hysteresis. Once an
over-temperature or over-current fault conditions is
removed, the output voltage automatically recovers.


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