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TPS54341 Datasheet(PDF) 29 Page - Texas Instruments

Part # TPS54341
Description  4.5-V to 42-V Input, 3.5-A Step-Down DC-DC Converter With Soft-Start and Eco-mode?
PDF  48 Pages
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

TPS54341 Datasheet(HTML) 29 Page - Texas Instruments

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2
OUT
OUT
IN max
j
SW
IN
D
IN max
2
V
V
I
V d
C
V
V d
P
V
2
42 V - 3.3 V
3.5 A x 0.55 V
90 pF x 600 kHz x (42 V
0.55 V)
2.27 W
42 V
2
-
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´
m
´
OUT
OUT
IN max
COUT(rms)
O
SW
IN max
V
V
V
3.3 V
42 V - 3.3 V
I
261 mA
12
V
L
12
42 V
5.6 H
600 kHz
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=
=
W
ORIPPLE
ESR
RIPPLE
V
16.5 mV
R
18 m
I
0.905 A
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SW
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RIPPLE
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11.4
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16.5 mV
8
8 x 600 kHz
V
0.905 A
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2.625 A
- 0.875 A
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5.6
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38.6
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3.432 V
3.3 V
V
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OUT
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SW
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1.75 A
C
44.9
F
V
600 kHz x 0.13 V
TPS54341
www.ti.com
SLVSC61 – NOVEMBER 2013
Capacitance de-ratings for aging, temperature and dc bias increases this minimum value. For this example, a
100-
μF ceramic capacitor with 5 mΩ of ESR is used. The derated capacitance is 70 µF, well above the minimum
required capacitance of 44.9 µF.
Capacitors are generally rated for a maximum ripple current that can be filtered without degrading capacitor
reliability. Some capacitor data sheets specify the Root Mean Square (RMS) value of the maximum ripple
current. Equation 39 can be used to calculate the RMS ripple current that the output capacitor must support. For
this example, Equation 39 yields 261 mA.
(35)
(36)
(37)
(38)
(39)
Catch Diode
The TPS54341 device requires an external catch diode between the SW 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. 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, diodes with higher voltage and current ratings have higher forward voltages. A diode with a minimum of
42 V reverse voltage is preferred to allow input voltage transients up to the rated voltage of the TPS54341
device.
For the example design, the PDS560 Schottky diode is selected for its lower forward voltage and good thermal
characteristics compared to smaller devices. The typical forward voltage of the PDS560 is 0.55 V at 3.5 A.
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 to calculate the instantaneous conduction losses of the diode. At higher
switching 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 charge. Equation 40 is
used to calculate the total power dissipation, including conduction losses and AC losses of the diode.
The PDS560 diode has a junction capacitance of 90 pF at 42 V input voltage. Using Equation 40, the total loss in
the diode is 2.27 W.
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.
(40)
Copyright © 2013, Texas Instruments Incorporated
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