TPS54260
SLVSA86 –MARCH 2010
www.ti.com
Vin min - Vout
(
)
Vout
Icirms = Iout ´
´
Vin min
Vin min
(38)
(39)
Iout max ´ 0.25
Cin ´ ¦sw
ΔVin =
Table 2. Capacitor Types
VENDOR
VALUE (mF)
EIA Size
VOLTAGE
100 V
50 V
DIALECTRIC
COMMENTS
1.0 to 2.2
1.0 to 4.7
1.0
1210
GRM32 series
GRM31 series
Murata
100 V
50 V
1206
2220
2225
1812
1210
1210
1812
1.0 to 2.2
1.0 10 1.8
1.0 to 1.2
1.0 to 3.9
1.0 to 1.8
1.0 to 2.2
1.5 to 6.8
1.0. to 2.2
1.0 to 3.3
1.0 to 4.7
1.0
50 V
100 V
50 V
Vishay
TDK
VJ X7R series
100 V
100 V
50 V
X7R
C series C4532
C series C3225
100 V
50 V
50 V
100 V
50 V
AVX
X7R dielectric series
1.0 to 4.7
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 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
TPS54260 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 effective output capacitance of 72.4 µF up to 3.3V
while only allowing the average output current to be 1 A would require a 0.19 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 2 x 47mF 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
3.5 ms which requires a 8.75 nF slow start capacitor. For this design, the next larger standard value of 10 nF is
used.
Cout ´ Vout ´ 0.8
Tss >
Issavg
(40)
Bootstrap Capacitor Selection
A 0.1-mF 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.
34
Submit Documentation Feedback
Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): TPS54260
TI [ TEXAS INSTRUMENTS ]
