AAT1106
600mA Step-Down Converter
is 150°C with 15°C of hysteresis. Once an over-
temperature or over-current fault conditions is
removed, the output voltage automatically recovers.
Control Loop
The AAT1106 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. An adaptive 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 cur-
rent 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 compen-
sation terminates the transconductance voltage
error amplifier output. For fixed voltage versions,
the error amplifier reference voltage is internally set
to program the converter output voltage. For the
adjustable output, the error amplifier reference is
fixed at 0.6V.
Dropout Operation
When the input voltage decreases toward the value
of the output voltage, the AAT1106 allows the main
switch to remain on for more than one switching
cycle and increases the duty cycle until it reaches
100%.
The duty cycle D of a step-down converter is
defined as:
VOUT
VIN
D = TON · FOSC · 100% ≈
· 100%
Where TON is the main switch on time and FOSC is
the oscillator frequency (1.5MHz).
The output voltage then is the input voltage minus
the voltage drop across the main switch and the
inductor. At low input supply voltage, the RDS(ON) of
the P-channel MOSFET increases and the efficien-
cy of the converter decreases. Caution must be
exercised to ensure the heat dissipated does not
exceed the maximum junction temperature of the IC.
Enable
The enable pin is active high. When pulled low, the
enable input forces the AAT1106 into a low-power,
non-switching state. The total input current during
shutdown is less than 1ꢀA.
Current Limit and Over-Temperature
Protection.
Maximum Load Current
The AAT1106 will operate with an input supply volt-
age as low as 2.5V; however, the maximum load
current decreases at lower input due to the large IR
drop on the main switch and synchronous rectifier.
The slope compensation signal reduces the peak
inductor current as a function of the duty cycle to
prevent sub-harmonic oscillations at duty cycles
greater than 50%. Conversely, the current limit
increases as the duty cycle decreases.
For overload conditions, the peak input current is
limited. 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 protection completely dis-
ables switching when internal dissipation becomes
excessive. The junction over-temperature threshold
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1106.2007.07.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
