Datasheet

( )
OUT max
IN
IN SW
I 0.25
V
C f
´
D =
´
( )
( )
( )
(
)
( )
-
= ´ ´
OUT
IN min
OUT
OUT
CI rms
IN min IN min
V V
V
I I
V V
TPS54160
SLVS795E –OCTOBER 2008–REVISED SEPTEMBER 2013 www.ti.com
For this example design, a ceramic capacitor with at least a 20 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, so a 25 V capacitor should be selected. For this example, two 2.2 μF, 25 V capacitors in
parallel have been selected. Table 2 shows a selection 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 39.
Using the design example values
• I
OUT(max)
= 1.5 A
• C
IN
= 4.4 μF
• ƒ
SW
= 1200 kHz
yields an input voltage ripple of 71 mV and a rms input ripple current of 0.701A.
(38)
(39)
Table 2. Capacitor Types
VENDOR VALUE (μF) EIA Size VOLTAGE (V) DIALECTRIC COMMENTS
1.0 to 2.2 100
1210 GRM32 series
1.0 to 4.7 50
Murata
1.0 100
1206 GRM31 series
1.0 to 2.2 50
1.0 10 1.8 50
2220
1.0 to 1.2 100
Vishay VJ X7R series
1.0 to 3.9 50
2225
1.0 to 1.8 100
X7R
1.0 to 2.2 100
1812 C series C4532
1.5 to 6.8 50
TDK
1.0. to 2.2 100
1210 C series C3225
1.0 to 3.3 50
1.0 to 4.7 50
1210
1.0 100
AVX X7R dielectric series
1.0 to 4.7 50
1812
1.0 to 2.2 100
Slow Start Capacitor
The slow start capacitor determines the minimum amount of time it will take for the output voltage to reach its
nominal programmed value during power up. This is useful if a load requires a controlled voltage slew rate. This
is also used if the output capacitance is large and would require large amounts of current to quickly charge the
capacitor to the output voltage level. The large currents necessary to charge the capacitor may make the
TPS54160 reach the current limit or excessive current draw from the input power supply may cause the input
voltage rail to sag. Limiting the output voltage slew rate solves both of these problems.
The slow start time must be long enough to allow the regulator to charge the output capacitor up to the output
voltage without drawing excessive current. Equation 40 can be used to find the minimum slow start time, tss,
necessary to charge the output capacitor, Cout, from 10% to 90% of the output voltage, Vout, with an average
slow start current of Issavg. In the example, to charge the 47μF output capacitor up to 3.3V while only allowing
the average input current to be 0.125A would require a 1 ms slow start time.
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