Datasheet

OUT max
IN
IN SW
I 0.25
V
C f
×
D =
×
( )
INmin OUT
OUT
INRMS OUT
INmin INmin
V V
V
I I
V V
-
= × ×
OUT
L
1 1
Co
V
8 fsw
ESR
i
> ×
D
×
-
D
2
OUT
OUT OUT
I L
Co
2 V V
D ×
>
× × D
L
O
Lpeak
i
I I
2
+
D
=
2
OUT INmax OUT
2
INmax
LRMS O
V (V V )
( )
V L fsw
i I
12
× -
× ×
= +
IN OUT OUT
L
O IN
V V V
i
I V fsw
-
D = ×
×
IN OUT OUT
O IN
V V V
L
I LIR V fsw
-
= ×
× ×
TPS5402
www.ti.com SLVSBK4 –SEPTEMBER 2012
(3)
(4)
(5)
(6)
For this design example, use LIR = 0.3 and the inductor is calculated to be 5.40 µH with V
IN
= 12 V. Choose
4.7 µH value for the standard inductor and the peak to peak inductor ripple is about 34% of 1-A DC load current.
Output Capacitor Selection
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 7 gives the minimum output capacitance to meet the transient specification. For this example,
L = 4.7 µH, ΔI
OUT
= 1 A – 0.0 A = 1 A and ΔV
OUT
= 500 mV (10% of regulated 5 V). Using these numbers gives a
minimum capacitance of 1 µF. A standard 22-µF ceramic is chosen in the design.
(7)
The selection of C
O
is driven by the effective series resistance (ESR). Equation 8 calculates the minimum output
capacitance needed to meet the output voltage ripple specification. Where f
SW
is the switching frequency, ΔV
OUT
is the maximum allowable output voltage ripple, and Δi
L
is the inductor ripple current. In this case, the maximum
output voltage ripple is 50 mV (1% of regulated 5 V). From Equation 4, the output current ripple is 1 A. From
Equation 8, the minimum output capacitance meeting the output voltage ripple requirement is 2.5 µF with 3-mΩ
ESR resistance.
(8)
After considering both requirements, for this example, one 22-µF, 6.3-V X7R ceramic capacitor with 3-mΩ ESR
should be used.
Input Capacitor Selection
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 9. For this example, I
OUT
= 1 A, V
OUT
= 5 V, minimum V
INmin
= 9.6 V,
from Equation 9, the input capacitors must support a ripple current of 1-A RMS.
(9)
The input capacitance value determines the input ripple voltage of the regulator. The input voltage ripple can be
calculated using Equation 10. Using the design example values, I
OUTmax
= 1 A, C
IN
= 10 µF, f
SW
= 300 kHz, yields
an input voltage ripple of 83 mV.
(10)
To prevent large voltage transients, a low ESR capacitor sized for the maximum RMS current must be used.
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