Datasheet

( )
IN OUT
OUT
INRMS OUT
IN IN
V -V
V
I = I × ×
V V
OUT(max)
INRIPPLE
IN SW
I × 0.25
V =
C × f
IN(max) OUT
OUT
OUT(min)
OUT IND IN(max) SW
V V
V
L
I K V f
-
= ´
´ ´
IN(min) OUT
OUT
OUT(max)
IN(min) SW
V V
V
L
30mA V f
-
= ´
´
TPS5401
www.ti.com SLVSAB0 –DECEMBER 2010
Using the design example values, I
OUT(max)
= 0.5 A, C
IN
= 4.4 mF,and f
SW
= 700 kHz, yields an input voltage ripple
of 40.6 mV and a maximum rms input ripple current of 0.25 A when V
IN
is equal to 10 V, which is 2 times the
output voltage of 5 V.
(13)
(14)
Table 4. Capacitor Types
VENDOR VALUE (mF) EIA Size VOLTAGE DIELECTRIC COMMENTS
1 to 2.2 100 V
1210 GRM32 series
1 to 4.7 50 V
Murata
1 100 V
1206 GRM31 series
1 to 2.2 50 V
1 10 1.8 50 V
2220
1 to 1.2 100 V
Vishay VJ X7R series
1 to 3.9 50 V
2225
1 to 1.8 100 V
X7R
1 to 2.2 100 V
1812 C series C4532
1.5 to 6.8 50 V
TDK
1. to 2.2 100 V
1210 C series C3225
1 to 3.3 50 V
1 to 4.7 50 V
1210
1 100 V
AVX X7R dielectric series
1 to 4.7 50 V
1812
1 to 2.2 100 V
Output Inductor Selection
To calculate the minimum value of the output inductor, use Equation 15.
(15)
K
IND
is a coefficient that represents the amount of inductor ripple current relative to the maximum output current.
The inductor ripple current is filtered by the output capacitor. Therefore, choosing high inductor ripple currents
impacts the selection of the output capacitor, because the output capacitor must have a ripple-current rating
equal to or greater than the inductor ripple current. In general, the inductor ripple value is at the discretion of the
designer; however, the following guidelines may be used.
For designs using low-ESR output capacitors such as ceramics, a value as high as K
IND
= 0.4 may be used.
When using higher-ESR output capacitors, K
IND
= 0.2 yields better results. Because the inductor ripple current is
part of the PWM control system, the TPS5401 requires ripple current that is always greater than 30 mA for
dependable operation. In a wide-input voltage regulator, it is best to choose an inductor ripple current on the
larger side. This allows the inductor to still have a measurable ripple current with the input voltage at its
minimum. The maximum value of the input inductance is limited by Equation 16.
(16)
For this design example, use K
IND
= 0.3, and the minimum inductor value is calculated to be 42 mH. The nearest
standard value was chosen: 47 mH. For the output-filter inductor, it is important that the root-mean-square (rms)
current and saturation current ratings not be exceeded. The rms and peak inductor current can be found from
Equation 17 to Equation 19.
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