Datasheet

max 0.25Iout
Vin
Cin fsw
×
D =
×
( )
min
min min
Vin Vout
Vout
Icirms Iout
Vin Vin
-
= × ×
1 1
8
Co
Vripple
fsw
Iripple
> ×
×
2
OUT o
out
I L
Co
V Vout
D ×
>
×D
2
Iripple
ILpeak Iout= +
2
2
1 ( max )
12 max
Vo Vin Vo
ILrms Io
Vin Lo fsw
æ ö
× -
= + ×
ç ÷
× ×
è ø
Vin Vout Vout
Iripple
Lo Vin fsw
-
= ×
×
ind
Vin Vout Vout
Lo
Io K Vin fsw
-
= ×
× ×
TPS65250
www.ti.com SLVSAA3C –JUNE 2010–REVISED OCTOBER 2012
For this design example, use KIND = 0.2 and the inductor value is calculated to be 5.4 µH. For this design, a
nearest standard value was chosen: 4.7 µH. For the output filter inductor, it is important that the RMS current
and saturation current ratings not be exceeded. The RMS and peak inductor current can be found from
Equation 13 and Equation 14.
(12)
(13)
(14)
(15)
Output Capacitor
There are two primary considerations for selecting the value of the output capacitor. The output capacitors are
selected to meet load transient and output ripple’s requirements.
Equation 16 gives the minimum output capacitance to meet the transient specification. For this example, L
O
= 4.7
µH, ΔI
OUT
= 2 A – 0.5 A = 1.5 A and ΔV
OUT
= 165 mV. Using these numbers gives a minimum capacitance of
19.4 µF. A standard 22-µF ceramic capacitor is chose in the design.
(16)
Equation 17 calculates the minimum output capacitance needed to meet the output voltage ripple specification.
Where fsw is the switching frequency, V
RIPPLE
is the maximum allowable output voltage ripple, and I
RIPPLE
is the
inductor ripple current. In this case, the maximum output voltage ripple is 30 mV. From Equation 13, the output
current ripple is 0.46 A. From Equation 17, the minimum output capacitance meeting the output voltage ripple
requirement is 1.74 µF.
(17)
After considering both requirements, for this example, one 22-µF, 6.3-V X7R ceramic capacitor with 3 mΩ of
ESR will be used.
Input Capacitor
A minimum 10-µF X7R/X5R ceramic input capacitor is recommended to be added between VIN and GND. These
capacitors should be connected as close as physically possible to the input pins of the converters as they handle
the RMS ripple current shown in Equation 18. For this example, I
OUT
= 2 A, V
OUT
= 3.3 V, V
INmin
= 9.6 V, from
Equation 18, the input capacitors must support a ripple current of 1.81 A RMS.
(18)
The input capacitance value determines the input ripple voltage of the regulator. The input voltage ripple can be
calculated using Equation 19. Using the design example values, I
OUTmax
= 2 A, C
IN
= 10 µF, f
SW
= 1100 kHz,
yields an input voltage ripple of 45 mV.
(19)
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