Datasheet

( )
´ ´
>
OUT OUT
SS
SS avg
C V 0.8
t
I
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OUT max
IN
IN SW
I 0.25
V
C f
´
D =
´
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(
)
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-
= ´ ´
OUT
IN min
OUT
OUT
CI rms
IN min IN min
V V
V
I I
V V
TPS54140
www.ti.com SLVS889B –OCTOBER 2008–REVISED SEPTEMBER 2013
For this example design, a ceramic capacitor with at least a 20V voltage rating is required to support the
maximum input voltage. Common standard ceramic capacitor voltage ratings include 4V, 6.3V, 10V, 16V, 25V,
50V or 100V so a 25V capacitor should be selected. For this example, two 2.2μF, 25V 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, Ioutmax = 1.5 A, Cin = 4.4μF, ƒsw = 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 DIALECTRIC COMMENTS
1.0 to 2.2 100 V
1210 GRM32 series
1.0 to 4.7 50 V
Murata
1.0 100 V
1206 GRM31 series
1.0 to 2.2 50 V
1.0 10 1.8 50 V
2220
1.0 to 1.2 100 V
Vishay VJ X7R series
1.0 to 3.9 50 V
2225
1.0 to 1.8 100 V
X7R
1.0 to 2.2 100 V
1812 C series C4532
1.5 to 6.8 50 V
TDK
1.0. to 2.2 100 V
1210 C series C3225
1.0 to 3.3 50 V
1.0 to 4.7 50 V
1210
1.0 100 V
AVX X7R dielectric series
1.0 to 4.7 50 V
1812
1.0 to 2.2 100 V
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 very 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
TPS54140 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.
Once the slow start time is known, the slow start capacitor value can be calculated using Equation 6. For the
example circuit, the slow start time is not too critical since the output capacitor value is 47μF which does not
require much current to charge to 3.3V. The example circuit has the slow start time set to an arbitrary value of
1ms which requires a 3.3 nF capacitor.
(40)
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