Datasheet

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TPS54040
www.ti.com SLVS918A –MARCH 2009–REVISED SEPTEMBER 2013
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 36 can be used
to calculate the RMS ripple current the output capacitor needs to support. For this application, Equation 36 yields
39 mA.
(32)
(33)
(34)
(35)
(36)
Catch Diode
The TPS54040 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 Vin 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. Since
the design example has an input voltage up to 42V, a diode with a minimum of 42V reverse voltage will be
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 0.50 volts.
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 need to 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 37 is used to calculate the total power
dissipation, conduction losses plus ac losses, of the diode.
The B160A has a junction capacitance of 110pF. Using Equation 37, the selected diode will dissipate 0.290
Watts. This power dissipation, depending on mounting techniques, should produce a 5.9°C temperature rise in
the diode when the input voltage is 42V and the load current is 0.5A.
If the power supply spends a significant amount of time at light load currents or in sleep mode consider using a
diode which has a low leakage current and slightly higher forward voltage drop.
(37)
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