AAT2786 Datasheet, PDF (15/27 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter and 150mA LDO

SystemPower^TM
PRODUCT DATASHEET
AAT2786
1.5A Step-Down Converter and 150mA LDO
At dropout, the step-down converter duty cycle increases
to 100% and the output voltage tracks the input voltage
minus the R_DS(ON) drop of the P-channel high side MOSFET
(plus the DC drop of the external inductor). The device
integrates extremely low R_DS(ON) MOSFETs to achieve low
dropout voltage during 100% duty cycle operation. This is
advantageous in applications requiring high output volt-
ages (typically > 2.5V) at low input voltages.
The integrated low-loss MOSFET switches can provide
greater than 95% efficiency at full load. Light-Load
operation maintains high efficiency under light load con-
ditions (typically <150mA). The MODE/SYNC pin allows
optional “PWM only” mode. This maintains constant fre-
quency and low output ripple across all load conditions.
Alternatively, the IC can be synchronized to an external
clock via the MODE/ SYNC input. External synchroniza-
tion is maintained between 0.6MHz and 3.0MHz.
In battery-powered applications, as V_IN decreases, the
converter dynamically adjusts the operating frequency
prior to dropout to maintain the required duty cycle and
provide accurate output regulation. Output regulation is
maintained until the dropout voltage, or minimum input
voltage, is reached. At 1.5A output load, dropout voltage
headroom is approximately 200mV.
The step-down converter in the AAT2786 typically
achieves better than ±0.5% output regulation across the
input voltage and output load range. A current limit of
2.0A (typical) protects the IC and system components
from short-circuit damage. Typical no load quiescent cur-
rent is 42µA.
Thermal protection completely disables switching when
the maximum junction temperature is detected. The
junction over-temperature threshold is 140°C with 15°C
of hysteresis. Once an over-temperature or over-current
fault condition is removed, the output voltage automati-
cally recovers.
Peak current mode control and optimized internal com-
pensation provide high loop bandwidth and excellent
response to input voltage and fast load transient events.
Soft start eliminates output voltage overshoot when the
step-down converter is enabled. Under-voltage lockout
prevents spurious start-up events.
Control Loop
The step-down converter in the AAT2786 is a peak cur-
rent mode step-down converter. 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 compensation signal is added to the sensed
current to maintain stability for duty cycles greater than
50%. The peak current mode loop appears as a voltage-
programmed current source in parallel with the output
capacitor.
The output of the voltage error amplifier programs the
current mode loop for the necessary peak switch current
to force a constant output voltage for all load and line
conditions. 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 VIN and elim-
inates output voltage overshoot. When EN_BUCK input is
pulled low the step-down converter is forced into a low-
power, non-switching state. The total input current dur-
ing shutdown is less than 1µA.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current in the
step-down converter is limited. To minimize power dis-
sipation and stresses under current limit and short-cir-
cuit conditions, switching is terminated after entering
current limit for a series of pulses. Switching is termi-
nated for seven consecutive clock cycles after a current
limit has been sensed for a series of four consecutive
clock cycles. Thermal protection completely disables
switching when internal dissipation becomes excessive.
The junction over-temperature threshold is 140°C with
15°C of hysteresis. Once an over-temperature or over-
current fault conditions is removed, the output voltage
automatically recovers.
Under-Voltage Lockout
Internal bias of all circuits is controlled via the VIN input.
Under-voltage lockout (UVLO) in the step-down con-
verter guarantees sufficient VIN bias and proper opera-
tion of all internal circuitry prior to activation.
2786.2008.04.1.0
www.analogictech.com
15