Datasheet

L ×(I – I )
Load-Max Load-Min
2 2
V – V
Reg-Max Reg-Min
2 2
C4=
I =
Load-RMS
V (VIN – V )
Reg Max Reg
Ö12×VIN ×f ×L1
Max sw
TPS54262-Q1
www.ti.com SLVS996C –SEPTEMBER 2009–REVISED JUNE 2010
2) Output Capacitor (C4, C12)
The selection of the output capacitor will determine several parameters in the operation of the converter, for
example voltage drop on the output capacitor and the output ripple. The capacitor value also determines the
modulator pole and the roll-off frequency due to the LC output filter double pole. This is expressed in
Equation 15.
The minimum capacitance needed to maintain desired output voltage during high to low load transition and
prevent over shoot is given by Equation 35.
(35)
Where,
L = output inductor
I
Load-Max
= maximum load current
I
Load-Min
= minimum load current
V
Reg-Max
= maximum tolerance of regulated output voltage
V
Reg-Min
= minimum tolerance of regulated output voltage
During a load step from no load to full load or changes in the input voltage, the output capacitor must hold up the
output voltage above a certain level for a specified time and not issue a reset, until the main regulator control
loop responds to the change. The minimum output capacitance required to allow sufficient drop on the output
voltage without issuing a reset is determined by Equation 36.
(36)
Where,
ΔV
Reg
= transient response during load stepping
The minimum capacitance needed for output voltage ripple specification is given by Equation 37.
(37)
Additional capacitance de-ratings for temperature, aging, and DC bias have to be factored in, and so a value of
100 µF with ESR calculated using Equation 38 of less than 100 mΩ should be used on the output stage.
Maximum ESR of the output capacitor is based on output ripple voltage specification in Equation 38 . The output
ripple voltage is a product of the output capacitor ESR and ripple current.
(38)
Output capacitor root mean square (RMS) ripple current is given by Equation 39. This is to prevent excess
heating or failure due to high ripple currents. This parameter is sometimes specified by the manufacturers.
(39)
Filter capacitor (C12) of value 0.1 µF (typical) is used to filter out the noise in the output line.
3) Soft-Start Capacitor (C6)
The soft start capacitor determines the minimum time to reach the desired output voltage during a power up
cycle. This is useful when a load requires a controlled voltage slew rate, and helps to limit the current draw from
the input voltage supply line. A 100 nF capacitor is recommended for startup loads of 1A (max.).
4) Bootstrap Capacitor (C3)
A 0.1µF ceramic capacitor must be connected between the PH and BOOT terminals for the converter to operate
and regulate to the desired output voltage. It is recommended to use a capacitor with X5R or better grade
dielectric material, and the voltage rating on this capacitor of at least 25V to allow for derating.
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