TPS54140A_15 Datasheet, PDF (28/58 Pages) Texas Instruments – TPS54140A 1.5-A, 42-V Step-Down DC/DC Converter with Eco-mode™

TPS54140A
SLVSB55C – MAY 2012 – REVISED OCTOBER 2015
www.ti.com
R_ESR
=
V_OUT(ripple)
I_RIPPLE
(17)
( ) ^VOUT ^{´ V}IN₍max₎ ^{- V}OUT
I_COUT(rms) = _{12 ´ VIN(max) ´ LO ´ fSW}
(18)
8.2.2.4 Catch Diode
The TPS54140A 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 will be.
Typically, the higher the voltage and current ratings the diode has, the higher the forward voltage will be. Since
the design example has an input voltage up to 18 V, a diode with a minimum of 20 V reverse voltage will be
selected.
For the example design, the B220A 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
B220A 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 19 is used to calculate the total power
dissipation, conduction losses plus ac losses, of the diode.
The B220A has a junction capacitance of 120 pF. Using Equation 19, the selected diode will dissipate 0.632
Watts. This power dissipation, depending on mounting techniques, should produce a 16°C temperature rise in
the diode when the input voltage is 18V and the load current is 1.5 A.
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.
( ) _{PD =}
V_IN(max) - V_OUT ´ I_OUT ´ Vf d _{+ Cj ´ fSW ´ (VIN + Vf d)}²
V_{IN(max )}
2
(19)
8.2.2.5 Input Capacitor
The TPS54140A requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of at least 3 μF 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 TPS54140A.
The input ripple current can be calculated using Equation 20.
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.
28
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