Datasheet

zZ
Z
RF
C
´´´
=
1
2
1
p
zP
P
RF
C
´´´
=
1
2
1
p
-6 6
2 25000 3.3 54 10 8 10
Rz = = 29.2 k
12 800 0.8
p´ ´ ´ ´ ´ ´ ´
W
´ ´
1
= = 928 pF
2 6010 29200
Cz
p´ ´ ´
1
= = 51 pF
2 103900 29200
Cp
p´ ´ ´
TPS54331
SLVS839D –JULY 2008– REVISED JANUARY 2012 www.ti.com
(26)
(27)
For this design, the two 47-μF output capacitors are used. For ceramic capacitors, the actual output capacitance
is less than the rated value when the capacitors have a dc bias voltage applied. This is the case in a dc/dc
converter. The actual output capacitance may be as low as 54 μF. The combined ESR is approximately .001 Ω.
Using Equation 19 and Equation 20, the output stage gain and phase loss are equivalent as:
Gain = –2.26 dB
and
PL - –83.52 degrees
For 70 degrees of phase margin, Equation 21 requires 63.52 degrees of phase boost.
Equation 22, Equation 23, and Equation 24 are used to find the zero and pole frequencies of:
F
Z1
= 5883 Hz
And
F
P1
= 106200 Hz
R
Z
, C
Z
, and C
P
are calculated using Equation 25, Equation 26, and Equation 27:
(28)
(29)
(30)
Using standard values for R3, C6, and C7 in the application schematic of Figure 15:
R3 = 29.4 kΩ
C6 = 1000 pF
C7 = 47 pF
BOOTSTRAP CAPACITOR
Every TPS54331 design requires a bootstrap capacitor, C4. The bootstrap capacitor must be 0.1 μF. The
bootstrap capacitor is located between the PH pins and BOOT pin. The bootstrap capacitor should be a
high-quality ceramic type with X7R or X5R grade dielectric for temperature stability.
CATCH DIODE
The TPS54331 is designed to operate using an external catch diode between PH and GND. The selected diode
must meet the absolute maximum ratings for the application: Reverse voltage must be higher than the maximum
voltage at the PH pin, which is VINMAX + 0.5 V. Peak current must be greater than IOUTMAX plus on half the
peak to peak inductor current. Forward voltage drop should be small for higher efficiencies. It is important to note
that the catch diode conduction time is typically longer than the high-side FET on time, so attention paid to diode
parameters can make a marked improvement in overall efficiency. Additionally, check that the device chosen is
capable of dissipating the power losses. For this design, a Diodes, Inc. B340A is chosen, with a reverse voltage
of 40 V, forward current of 3 A, and a forward voltage drop of 0.5 V.
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