Datasheet

OUT
IN
IN SW
I 0.25
5 A 0.25
V 355 mV
C 8.8 F 400 kHz
´
´
D = = =
´ m ´f
( )
( )
( )
(
)
( )
( )
OUT
IN min
OUT
OUT
CI rms
IN min IN min
V V
7 V - 5 V
V
5 V
I I x x 5 A 2.26 A
V V 7 V 7 V
-
= = ´ =
TPS54560
SLVSBN0A –MARCH 2013–REVISED MARCH 2014 www.ti.com
9.2.2.5 Input Capacitor
The TPS54560 requires a high quality ceramic type X5R or X7R input decoupling capacitor with at least 3 μF of
effective capacitance. Some applications will benefit from additional bulk capacitance. The effective capacitance
includes any loss of capacitance due to dc bias effects. The voltage rating of the input capacitor must be greater
than the maximum input voltage. The capacitor must also have a ripple current rating greater than the maximum
input current ripple of the TPS54560. The input ripple current can be calculated using Equation 37.
The value of a ceramic capacitor varies significantly with temperature and the dc bias applied to the capacitor.
The capacitance variations due to temperature can be minimized by selecting a dielectric material that is more
stable over temperature. X5R and X7R ceramic dielectrics are usually selected for switching regulator capacitors
because they have a high capacitance to volume ratio and are fairly stable over temperature. The input capacitor
must also be selected with consideration for the dc bias. The effective value of a capacitor decreases as the dc
bias across a capacitor increases.
For this example design, a ceramic capacitor with at least a 60 V voltage rating is required to support the
maximum input voltage. Common standard ceramic capacitor voltage ratings include 4 V, 6.3 V, 10 V, 16 V, 25
V, 50 V or 100 V. For this example, four 2.2 μF, 100 V capacitors in parallel are used. Table 2 shows several
choices of high voltage capacitors.
The input capacitance value determines the input ripple voltage of the regulator. The input voltage ripple can be
calculated using Equation 38. Using the design example values, I
OUT
= 5 A, C
IN
= 8.8 μF, ƒsw = 400 kHz, yields
an input voltage ripple of 355 mV and a rms input ripple current of 2.26 A.
(37)
(38)
Table 2. Capacitor Types
VALUE (μF) EIA Size VOLTAGE DIALECTRIC COMMENTS
1 to 2.2 100 V
1210 GRM32 series
1 to 4.7 50 V
1 100 V
1206 GRM31 series
1 to 2.2 50 V
1 to 1.8 50 V
2220
1 to 1.2 100 V
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
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
X7R dielectric series
1 to 4.7 50 V
1812
1 to 2.2 100 V
9.2.2.6 Bootstrap Capacitor Selection
A 0.1-μF ceramic capacitor must be connected between the BOOT and SW terminals for proper operation. A
ceramic capacitor with X5R or better grade dielectric is recommended. The capacitor should have a 10 V or
higher voltage rating.
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