Datasheet

DI
L
+ Vout
1 *
Vout
Vin
L ƒ
I
Lmax
+ I
outmax
)
DI
L
2
C
z
+
1
2 p 8 kHz R1
+
1
2 p 8 kHz 2kW
+ 9.9 nF [ 10 nF
(Example for the 3.3-V output)
V
out
+ 1.213 V
ǒ
1 )
R1
R2
Ǔ
TPS65161
TPS65161A, TPS65161B
SLVS617E –APRIL 2006–REVISED MARCH 2013 www.ti.com
Step-Down Converter Design Procedure
Setting the Output Voltage
The step-down converter uses an external voltage divider to set the output voltage. The output voltage is
calculated as:
with R2 as 1.2 kΩ, and internal reference voltage V
(ref)
typ = 1.213 V
At load current <1 mA, the device operates in discontinuous conduction mode. When the load current is reduced
to zero, the output voltage rises slightly above the nominal output voltage. At zero load current, the device skips
clock cycles but does not completely stop switching; thus, the output voltage sits slightly higher than the nominal
output voltage. Therefore, the lower feedback resistor is selected to be around 1.2 kΩ to always have around 1-
mA minimum load current.
Selecting the Feedforward Capacitor
The feedforward capacitor across the upper feedback resistor divider sets a zero in the converter loop transfer
function. For a 15-μH inductor, f
z
= 8 kHz and when a 22-μH inductor is used, f
z
= 17 kHz.
Usually a capacitor value closest to the calculated value is selected.
Inductor Selection (Step-Down Converter)
The TPS65161 typically operates with a 15-μH inductor value. For high efficiencies, the inductor should have a
low dc resistance to minimize conduction losses. This needs to be considered when selecting the appropriate
inductor. In order to avoid saturation of the inductor, the inductor should be rated at least for the maximum output
current of the converter, plus the inductor ripple current that is calculated as:
With:
f = Switching frequency (750 kHz, 500 kHz minimal)
L = Inductor value (typically 15 μH)
ΔI
L
= Peak-to-peak inductor ripple current
I
Lmax
= Maximum inductor current
The highest inductor current occurs at maximum V
IN
. A more conservative approach is to select the inductor
current rating just for the typical switch current of 3.2 A.
Table 5. Inductor Selection (Step-Down Converter)
INDUCTOR VALUE COMPONENT SUPPLIER DIMENSIONS in mm Isat/DCR
15 μH Wuerth 7447789115 7,3 × 7,3 × 3,2 1.75 A/100 mΩ
15 μH Sumida CDRH8D28-150 8,3 × 8,3 × 3,0 1.9 A/53 mΩ
15 μH Sumida CDRH8D38-150 8,3 × 8,3 × 4,0 2.3 A/53 mΩ
15 μH Coilcraft MSS1038-153NX 10,2 × 10,2 × 3,6 2.7 A/50 mΩ
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