AAT1161 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – 13.2V Input, 3A Step-Down Converter

SwitchReg^TM
voltage droop during the three switching cycles to the
output capacitance can be estimated by:
C_OUT
=
3 · ∆I_LOAD
V_DROOP · F_OSC
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.
The maximum output capacitor RMS ripple current is
given by:
I = _RMS(MAX)
1
2·
·
3
V_OUT · (V_IN(MAX) - V_OUT)
L · F_OSC · V_IN(MAX)
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.
Compensation
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.
To optimize the compensation components, the following
equations can be used. The compensation resistor R_COMP
(R5) is calculated using the following equation:
R_COMP (R5)=
2πV_OUT · C_OUT · F_OSC
10G_EA · G_COMP · V_FB
Where V_FB = 0.6V, G_COMP = 40.1734 and G_EA = 9.091 ·
10^-5.
PRODUCT DATASHEET
AAT1161
13.2V Input, 3A Step-Down Converter
F_OSC is the switching frequency and C_OUT is based on the
output capacitor calculation. The C_COMP value can be
determined from the following equation:
4
C_COMP (C) =
2πR_COMP (R5) ·
F_OSC
10
Schottky Diode Selection
Power dissipation is the limiting factor when choosing a
diode. The worst-case average power can be calculated
as follows:
P_DIODE
=
⎛⎝_{1 -}
V_OUT⎞
V_IN ⎠
⋅ I_OUT ⋅ V_F
where V_F is the voltage drop across the diode at the
given output current I_OUTMAX. 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:
T_J(MAX) = T_AMB + Θ_JA · P_DIODE
Where:
θ_JA = Package Thermal Resistance (°C/W)
T_J(MAX) = Maximum Device Junction Temperature (°C)
T_A = Ambient Temperature (°C)
For reliable operation over the input voltage range,
ensure that the reverse-repetitive maximum voltage is
greater than the maximum input voltage (V_RRM>V_INMAX).
The diode’s forward-current specification must meet or
exceed the maximum output current (I_F(AV)>=I_OUTMAX).
See Table 3 for recommended diodes for different I_OUT
conditions.
12
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1161.2008.03.1.0