AAT1145_08 Datasheet, PDF (9/17 Pages) Advanced Analogic Technologies – 1.5A Step-Down Converter

SwitchReg^TM
until either the inductor current starts to reverse, as
indicated by the current reversal comparator, I_ZERO, or
the beginning of the next clock cycle.
Control Loop
The AAT1145 is a peak current mode step-down con-
verter. The current through the P-channel MOSFET (high
side) is sensed for current loop control, as well as short
circuit and overload protection. A slope compensation
signal is added to the sensed current to maintain stabil-
ity 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 transconduc-
tance voltage error amplifier output. For fixed voltage
versions, the error amplifier reference voltage is inter-
nally set to program the converter output voltage. For
the adjustable output, the error amplifier reference is
fixed at 0.6V.
Soft Start / Enable
Soft start limits the current surge seen at the input and
eliminates output voltage overshoot. The enable pin is
active high. When pulled low, the enable input (EN)
forces the AAT1145 into a low-power, non-switching
state. The total input current during shutdown is less
than 1μA.
Current Limit and
Over-Temperature Protection
For overload conditions, the peak input current is limited
to 2.5A. To minimize power dissipation and stresses
under current limit and short-circuit conditions, switch-
ing is terminated after entering current limit for a series
of pulses. The termination lasts for seven consecutive
clock cycles after a current limit has been sensed during
a series of four consecutive clock cycles.
PRODUCT DATASHEET
AAT1145
1.5A Step-Down Converter
Thermal protection completely disables switching when
internal dissipation becomes excessive. The junction
over-temperature threshold is 170°C with 10°C of hys-
teresis. Once an over-temperature or over-current fault
conditions is removed, the output voltage automatically
recovers.
Dropout Operation
When the battery input voltage decreases near the value
of the output voltage, the AAT1145 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:
D = T_ON · F_OSC · 100% ≈
V_OUT
V_IN
· 100%
Where T_ON is the main switch on time and F_OSC is the
oscillator frequency. 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 R_DS(ON)
of the P-channel MOSFET increases, and the efficiency of
the converter decreases. Caution must be exercised to
ensure the heat dissipated does not exceed the maxi-
mum junction temperature of the IC.
Maximum Load Current
The AAT1145 will operate with an input supply voltage as
low as 2.5V, however, the maximum load current
decreases at lower input voltages due to a 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.
1145.2008.01.1.2
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