PRODUCT DATASHEET
AAT1161
TM
SwitchReg
13.2V Input, 3A Step-Down Converter
voltage droop during the three switching cycles to the
output capacitance can be estimated by:
FOSC is the switching frequency and COUT is based on the
output capacitor calculation. The CCOMP value can be
determined from the following equation:
3 · ∆ILOAD
DROOP · FOSC
4
COUT
=
CCOMP (C) =
V
FOSC
10
2πRCOMP (R5) ·
Once the average inductor current increases to the DC
load level, the output voltage recovers. The above equa-
tion establishes a limit on the minimum value for the
output capacitor with respect to load transients. The
internal voltage loop compensation also limits the mini-
mum output capacitor value to 22μF. This is due to its
effect on the loop crossover frequency (bandwidth),
phase margin, and gain margin. Increased output capac-
itance will reduce the crossover frequency with greater
phase margin.
Schottky Diode Selection
Power dissipation is the limiting factor when choosing a
diode. The worst-case average power can be calculated
as follows:
VOUT
⎛
PDIODE = 1 -
⎝
⎞
⎠
⋅ IOUT ⋅ VF
VIN
The maximum output capacitor RMS ripple current is
given by:
where VF is the voltage drop across the diode at the
given output current IOUTMAX. The total power dissipation
of the diode is the combined totaI of forward power dis-
sipation, reverse power dissipation and switching loss.
Ensure that the selected diode will be able to dissipate
the power based on the equation:
1
V
OUT · (VIN(MAX) - VOUT
)
IRMS(MAX)
=
·
L · FOSC · VIN(MAX)
2 · 3
Dissipation due to the RMS current in the ceramic output
capacitor ESR is typically minimal, resulting in less than
a few degrees rise in hot-spot temperature.
TJ(MAX) = TAMB + ΘJA · PDIODE
Where:
Compensation
θJA = Package Thermal Resistance (°C/W)
TJ(MAX) = Maximum Device Junction Temperature (°C)
TA = Ambient Temperature (°C)
The AAT1161 step-down converter uses peak current
mode control with slope compensation scheme to main-
tain stability with lower value inductors for duty cycles
greater than 50%. The regulation feedback loop in the
IC is stabilized by the components connected to the
COMP pin, as shown in Figure 1.
For reliable operation over the input voltage range,
ensure that the reverse-repetitive maximum voltage is
greater than the maximum input voltage (VRRM>VINMAX).
The diode’s forward-current specification must meet or
exceed the maximum output current (IF(AV)>=IOUTMAX).
See Table 3 for recommended diodes for different IOUT
conditions.
To optimize the compensation components, the following
equations can be used. The compensation resistor RCOMP
(R5) is calculated using the following equation:
2πVOUT · COUT
10GEA · GCOMP · VFB
·
FOSC
RCOMP (R5)=
Where VFB = 0.6V, GCOMP = 40.1734 and GEA = 9.091 ·
10-5.
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1161.2008.03.1.0
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