AAT1142
800mA Voltage-Scaling Step-Down Converter
rate output regulation. Output regulation is main-
tained until the dropout voltage, or minimum input
voltage, is reached.
sation terminates the transconductance voltage
error amplifier output. Loop stability and fast tran-
sient response are maintained across the entire
input and output voltage range with a small 2.2µH
output inductor and 10µF output capacitor.
The AAT1142 achieves better than ±0.5% output
regulation across the input voltage and output load
range. Maximum continuous load is 800mA. A
current limit of 1A (typical) protects the IC and sys-
tem components from short-circuit damage.
Typical no load quiescent current is 35µA.
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 AAT1142
into a low-power, non-switching state. The total
input current during shutdown is less than 1µA.
The turn-on time from EN to output regulation is
100µs (typical).
Thermal protection completely disables switching
when the maximum junction temperature is detect-
ed. The junction over-temperature threshold is
140°C with 15°C of hysteresis. Once an over-tem-
perature or over-current fault condition is removed,
the output voltage automatically recovers.
Alternatively, the EN/SET pin serves as the input
for S2Cwire single line control. Details of S2Cwire
operation and timing diagrams are provided in the
Applications Information section of this datasheet.
Peak current mode control and optimized internal
compensation provide high loop bandwidth and
excellent response to input voltage and fast load
transient events. The output voltage is stable across
all operating conditions, ensuring fast transitions
with no overshoot or ringing. Soft start eliminates
output voltage overshoot when the enable or the
input voltage is applied. Under-voltage lockout pre-
vents spurious start-up events.
Current Limit and Over-Temperature
Protection
Switching is terminated after entering current limit
for a series of pulses to minimize power dissipation
and stresses under overload and short-circuit con-
ditions. Switching is terminated for seven consecu-
tive clock cycles after a current limit has been
sensed for a series of four consecutive clock cycles.
Control Loop
The AAT1142 is a peak current 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 pro-
tection. 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.
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 condition 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) guarantees
sufficient VIN bias and proper operation of all inter-
nal circuitry prior to activation.
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 compen-
1142.2006.07.1.0
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