TPS54260
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SLVSA86 –MARCH 2010
V
ORIPPLE
R
<
ESR
I
RIPPLE
(35)
(36)
Vout ´ (Vin max - Vout)
12 ´ Vin max ´ Lo ´ ¦sw
Icorms =
Catch Diode
The TPS54260 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 Vinmax. 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, the higher the voltage and current ratings the diode has, the higher the forward voltage will be.
Although the design example has an input voltage up to 13.2V, a diode with a minimum of 60V reverse voltage is
selected to allow input voltage transients up to the rated voltage of the TPS54260.
For the example design, the B360B-13-F 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 B360B-13-F is 0.70 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 B360B-13-F has a junction capacitance of 200 pF. Using Equation 37, the selected diode will dissipate 1.32
Watts.
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.
2
Cj ´ ƒsw ´ Vin + Vƒd
(
)
(Vin max - Vout) ´ Iout ´ Vƒd
Pd =
+
2
Vin max
(37)
Input Capacitor
The TPS54260 requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of at least 3 mF of
effective capacitance and in some applications a bulk capacitance. The effective capacitance includes any dc
bias effects. The voltage rating of the input capacitor must be greater than the maximum input voltage. The
capacitor must also have a ripple current rating greater than the maximum input current ripple of the TPS54260.
The input ripple current can be calculated using Equation 38.
The value of a ceramic capacitor varies significantly over temperature and the amount of dc bias applied to the
capacitor. The capacitance variations due to temperature can be minimized by selecting a dielectric material that
is stable over temperature. X5R and X7R ceramic dielectrics are usually selected for power regulator capacitors
because they have a high capacitance to volume ratio and are fairly stable over temperature. The output
capacitor must also be selected with the dc bias taken into account. The capacitance value of a capacitor
decreases as the dc bias across a capacitor increases.
For this example design, a ceramic capacitor with at least a 60V voltage rating is required to support the
maximum input voltage. Common standard ceramic capacitor voltage ratings include 4V, 6.3V, 10V, 16V, 25V,
50V or 100V so a 100V capacitor should be selected. For this example, two 2.2mF, 100V capacitors in parallel
have been selected. Table 2 shows a selection of high voltage capacitors. The input capacitance value
determines the input ripple voltage of the regulator. The input voltage ripple can be calculated using Equation 39.
Using the design example values, Ioutmax = 2.5 A, Cin = 4.4mF, ƒsw = 300 kHz, yields an input voltage ripple of
206 mV and a rms input ripple current of 1.15 A.
Copyright © 2010, Texas Instruments Incorporated
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