
(
)
Vin min
Vout
Icirms = Iout
Vin min
-
(38)
Iout max
0.25
Vin =
Cin
sw
(39)
Slow Start Capacitor
Cout
Vout
0.8
Tss >
Issavg
(40)
Bootstrap Capacitor Selection
www.ti.com................................................................................................................................................................................................... SLVS918 – MARCH 2009
Table 1. Capacitor Types
VENDOR
VALUE (
F)
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
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
TPS54040 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 47
F output capacitor up to 5.0V while only allowing
the average input current to be 0.125A would require a 1.5 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 47
F which does not
require much current to charge to 5.0V. The example circuit has the slow start time set to an arbitrary value of
3.2ms which requires a 0.01
F capacitor.
A 0.1-
F ceramic capacitor must be connected between the BOOT and PH pins for proper operation. It is
recommended to use a ceramic capacitor with X5R or better grade dielectric. The capacitor should have a 10V
or higher voltage rating.
Copyright 2009, Texas Instruments Incorporated
33