1MHz 3A Step-Down DC/DC Converter
Functional Block Diagram
VCC
VP = 2.7V to 5.5V
AAT1154
REF
FB
OP. AMP
CMP
DH
LOGIC
LX
OSC
Temp.
Sensing
GND
EN
Applications Information
Main Control Loop
The AAT1154 is a peak current mode step-down
converter. The inner wide bandwidth loop controls
the inductor peak current. The inductor current is
sensed as it flows through the internal P-channel
MOSFET. A fixed slope compensation signal is
then added to the sensed current to maintain sta-
bility for duty cycles greater than 50%. The inner
loop appears as a voltage-programmed current
source in parallel with the output capacitor.
The voltage error amplifier output programs the
current loop for the necessary inductor current to
force a constant output voltage for all load and line
conditions. The feedback resistive divider is inter-
nal, dividing the output voltage to the error amplifi-
er reference voltage of 1V. The error amplifier has
a limited DC gain. This eliminates the need for
external compensation components, while still pro-
viding sufficient DC loop gain for good load regula-
tion. The crossover frequency and phase margin
are set by the output capacitor value.
Duty cycle extends to 100% as the input voltage
approaches the output voltage. Thermal shutdown
protection disables the device in the event of a
short-circuit or overload condition.
Soft Start/Enable
Soft start controls the current limit when the input
voltage or enable is applied. It limits the current
surge seen at the input and eliminates output volt-
age overshoot.
When pulled low, the enable input forces the device
into a low-power, non-switching state. The total
input current during shutdown is less than 1µA.
Power and Signal Source
Separate small signal ground and power supply
pins isolate the internal control circuitry from
switching noise. In addition, the low pass filter R1
and C3 (shown in Figure 1) filters noise associated
with the power switching.
Current Limit and Over-Temperature
Protection
Over-temperature and current limit circuitry protects
the AAT1154 and the external Schottky diode dur-
ing overload, short-circuit, and excessive ambient
temperature conditions. The junction over-tempera-
ture threshold is 140°C nominal and has 15°C of
hysteresis. Typical graphs of the over-temperature
load current vs. input voltage and ambient temper-
ature are shown in the Typical Characteristics sec-
tion of this document.
9
1154.2006.09.1.6
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