Datasheet

RIPPLE
RIPPLE(total)
RIPPLE(total) RIPPLE(cap)
OUT SW
MAX
RIPPLE RIPPLE
I
V
V V
8 C f
ESR
I I
3.5 A
36mV
8 35 F 1200kHz
7m
3.5 A
æ ö
-
ç ÷
-
´ ´
è ø
= =
æ ö
-
ç ÷
´ m ´
è ø
= = W
( )
2
2
TRAN(max)
OUT(min)
OUT OVER
I L
4 400nH
C 35 F
1.8 100mV
V V
´
´
= = = m
´
´
( )
2
TRAN TRAN TRAN TRAN
UNDER
OUT OUT IN OUT
IN OUT OUT
I I I L I L
V T
C C V V
V V C
´ ´
< ´ D = ´ =
-
- ´
2
TRAN TRAN TRAN TRAN
OVER
OUT OUT OUT OUT OUT
I I I L I L
V T
C C V V C
´ ´
< ´ D = ´ =
´
( ) ( )
2 2 2 2 2 2
1 1 1
RIPPLE OUT RIPPLE
L rms L avg
12 12 12
I I I I I 10 3.5 10.05 A= + = + = + =
( )
( )
OUT
IN max
OUT
OUT SW
IN max
V V
V
1 14 V 1.8 V 1.8 V 1
L 471nH
0.3 I V f 0.3 10A 14 V 1200kHz
-
-
» ´ ´ = ´ ´ =
´ ´
TPS40303, TPS40304, TPS40305
www.ti.com SLUS964A –NOVEMBER 2009–REVISED AUGUST 2012
Inductor Selection (L1)
Synchronous buck power inductors are typically sized for approximately 30% peak-to-peak ripple current (I
RIPPLE
)
Given this target ripple current, the required inductor size can be calculated in Equation 3.
(3)
Selecting a standard 400-nH inductor value, solve for I
RIPPLE
=3.5 A
The RMS current through the inductor is approximated by Equation 4.
(4)
Output Capacitor Selection (C12)
The selection of the output capacitor is typically driven by the output transient response. Equation 5 and
Equation 6 overestimate the voltage deviation to account for delays in the loop bandwidth and can be used to
determine the required output capacitance.
(5)
(6)
If V
IN(min)
> 2 x V
OUT
, use overshoot (Equation 5) to calculate minimum output capacitance. If V
IN(min)
< 2 x V
OUT
,
use undershoot(Equation 6) to calculate minimum output capacitance.
(7)
With a minimum capacitance, the maximum allowable ESR is determined by the maximum ripple voltage and is
approximated by Equation 8.
(8)
Two 0805, 22-µF, 6.3 V, X5R ceramic capacitors are selected to provide more than 35-µF of minimum
capacitance and less than 7 mΩ of ESR (2.5 mΩ each).
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