AAT2550
Total Power Solution for Portable Applications
grammed current source in parallel with the output
capacitor.
Step-Down Converter
Functional Description
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.
The AAT2550 step-down converter is a high per-
formance 600mA 1.4MHz monolithic power supply.
It has been designed with the goal of minimizing
external component size and optimizing efficiency
over the complete load range. Apart from the small
bypass input capacitor, only a small L-C filter is
required at the output. Typically, a 4.7µH inductor
and a 4.7µF ceramic capacitor are recommended
(see Table 5).
The fixed output version requires only three exter-
nal power components (CIN, COUT, and L). The
adjustable version can be programmed with exter-
nal feedback to any voltage, ranging from 0.6V to
the input voltage. An additional feed-forward
capacitor can also be added to the external feed-
back with a 10µF output capacitor for improved
transient response (see C10 and C11 in Figure 4).
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 AAT2550
into a low-power, non-switching state. The total
input current during shutdown is less than 1µA.
At dropout, the converter duty cycle increases to
100% and the output voltage tracks the input volt-
age minus the RDS(ON) drop of the P-channel high-
side MOSFET.
Current Limit and Over-Temperature
Protection
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.
The input voltage range is 2.7V to 5.5V. The con-
verter efficiency has been optimized for all load
conditions, ranging from no load to 600mA.
The internal error amplifier and compensation pro-
vides excellent transient response, load, and line
regulation. Soft start eliminates any output voltage
overshoot when the enable or the input voltage is
applied.
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.
Control Loop
The AAT2550 step-down converter is a peak cur-
rent mode control 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-pro-
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.
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2550.2006.07.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
