Datasheet

( )
P P
RIPPLE
RIPPLE RIPPLE(COUT)
OUT SW
COUT max
P P P P
I
5.8A
V
20mV
V V
8 C f
8 500 F 500kHz
ESR 2.9m
I I 5.8 A
-
- -
æ ö
æ ö
-
-
ç ÷
ç ÷
-
´ ´
´ m ´
è ø è ø
= = = = W
( )
( )
( )
2
TRAN
OUT min
IN OUT UNDER
I L
C
V V V
´
=
- ´
( )
( )
( )
2 2
TRAN
OUT min
OUT OVER
I L 10 320nH
C 320 F
V V 1.0 100mV
´ ´
= = = m
´ ´
( ) ( )
(
)
( )
( )
( )
( )
( )
( ) ( )
( )
2
2
2 2 2 2
1 1 1
RIPPLE OUT P P
L rms L avg
12 12 12
I I I I I 25 5.8 25.06 A
-
= + ´ = + ´ = + ´ =
IN(max) OUT
OUT
OUT IN(max) SW
V V
V
1 14 V 1.0 V 1.0 V 1
L 186nH
0.3 I V f 0.3 25 A 14 V 500kHz
-
-
» ´ ´ = ´ ´ =
´ ´
TPS56221
SLUSAH5B –MARCH 2011–REVISED SEPTEMBER 2012 www.ti.com
Design Procedure
Switching Frequency Selection
To achieve a balance between small size and high efficiency for this design, use switching frequency of 500 kHz.
Inductor Selection
Synchronous buck power inductors are typically sized for between approximately 20% and 40% peak-to-peak
ripple current (I
P-P
).
Using this target ripple current, the required inductor size can be calculated as shown in Equation 3.
(3)
Selecting a standard 320-nH inductor value, I
P-P
= 5.8 A.
The RMS current through the inductor is approximated in Equation 4.
(4)
Output Capacitor Selection
The selection of the output capacitor is typically driven by the output transient response. For applications with
V
IN(min)
> 2 x V
OUT
, use overshoot to calculate the minimum output capacitance and the equation is shown in
Equation 5.
(5)
For applications where V
IN(min)
< 2 × V
OUT
, use undershoot to calculate minimum output capacitance. The
equation is shown in Equation 6.
(6)
In order to meet the low ESR and high-capacitance requirements of this design, 5 100-µF, 1210 ceramic
capacitors are selected. With a minimum capacitance, the maximum allowable ESR is determined by the
maximum ripple voltage and is approximated by Equation 7.
(7)
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