Datasheet

-
> ´
-
2 2
OH OL
OUT OUT
2 2
FIN INI
I I
C L
V V
OUT
ORIPPLE
SW
ESR
LRIPPLE
1 1
C
V
8 f
R
I
> ´
´
-
( )
OUT IN(max) OUT
CO(RMS)
IN(max) OUT SW
V V V
I
12 V L f
´ -
=
´ ´ ´
TPS5401
www.ti.com SLVSAB0 –DECEMBER 2010
inductor, I
OH
is the output current under heavy load, I
OL
is the output under light load, V
FIN
is the final peak output
voltage, and V
INI
is the initial capacitor voltage. For this example, the worst-case load step is from 0.5 A to 0 A.
The output voltage increases during this load transition, and the stated maximum in our specification is 4% of the
output voltage. This makes V
FIN
= 1.04 × 5 V = 5.2 V. V
INI
is the initial capacitor voltage, which is the nominal
output voltage of 5 V. Using these numbers in Equation 21 yields a minimum capacitance of 5.76 mF.
(21)
Equation 22 calculates the minimum output capacitance needed to meet the output-voltage ripple specification,
where f
SW
is the switching frequency, V
ORIPPLE
is the maximum allowable output voltage ripple, and I
LRIPPLE
is the
inductor ripple current. Equation 22 shows the ESR of the output capacitor must be less than V
ORIPPLE
/I
LRIPPLE
to
meet the output-voltage ripple requirement. Low-ESR capacitors are preferred to keep the output-voltage ripple
low. If a high-ESR electrolytic capacitor is used, a small ESR ceramic capacitor is recommended to be in parallel
with the electrolytic capacitor to minimize the output voltage ripple. In this application, an aluminum electrolytic
capacitor is chosen as the output capacitor. It has 260 mΩ ESR. Equation 22 yields 1.44 µF.
(22)
The most stringent criterion for the output capacitor is 20.5 µF of capacitance to keep the output voltage in
regulation during a load transient in this example.
Additional capacitance de-ratings for aging, temperature and dc bias should be factored in, which increases this
minimum value. For this example, a 220 µF electrolytic capacitor with 260 mΩ of ESR can be used for low cost
target.
Capacitors generally have limits to the amount of ripple current they can handle without failing or producing
excess heat. An output capacitor that can support the inductor ripple current must be specified. Some capacitor
data sheets specify the root-mean-square (rms) value of the maximum ripple current. Equation 23 can be used to
calculate the rms ripple current the output capacitor must support. For this application, Equation 23 yields 37.6
mA.
(23)
Catch Diode
The TPS5401 requires an external catch diode between the PH pin and GND. The selected diode must have a
reverse voltage rating equal to or greater than V
IN(MAX)
. The peak current rating of the diode must be greater than
the maximum inductor current. The diode should also have a low forward voltage. Schottky diodes are typically a
good choice for the catch diode, due to their low forward voltage. The lower the forward voltage of the diode, the
higher the efficiency of the regulator.
Typically, when the voltage and current ratings for the diode are higher, then the forward voltage is higher.
Because the design example has an input voltage up to 42 V, a diode with a minimum of 42-V reverse voltage is
selected.
For the example design, the B160A Schottky diode is selected for its lower forward voltage, and it comes in a
larger package size which has good thermal characteristics over small devices. The typical forward voltage of the
B160A is 0.5 V.
The diode must also be selected with an appropriate power rating. The diode conducts the output current during
the off-time of the internal power switch. The off-time of the internal switch is a function of the maximum input
voltage, the output voltage, and the switching frequency. The output current during the off-time is multiplied by
the forward voltage of the diode, which equals the conduction losses of the diode. At higher switch frequencies,
the ac losses of the diode must be taken into account. The ac losses of the diode are due to the charging and
discharging of the junction capacitance and reverse recovery. Equation 24 is used to calculate the total power
dissipation, conduction losses plus ac losses, of the diode.
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