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Example: TPS54386 Buck Converter Operating at 12-V Input, 3.3-V Output and 400-mA
(P-P)
Ripple Current
V
OUT DIODE
+ V
V
IN
+ V
DIODE
3.3 0.5+
12 0.5+
d =
=
=30%
(10)
IN OUT
S
L
V V
12 3.3 1
L T 0.3 10.9 H
I 0.4 600000
-
-
= ´ d ´ = ´ ´ = m
D
(11)
( )
( )
2 2
6
RES
1 1
C 70 F
10 10 2 3.14 6000
L 2 f
-
= = = m
´ ´ ´ ´
´ ´ p´
(12)
( )
f
ESR
6
RES
1 1
R 40 m
2 10 C
2 3.14 10 6000 68 10
-
< = » W
´ p´ ´ ´
´ ´ ´ ´ ´
(13)
100 1M1k 100k10k
-20
20
40
80
0
60
-180
-90
-45
45
90
-135
0
180
135
f Frequency Hz- -
Gain
Phase
-10
10
50
30
70
Gain dB-
Phase -
°
TPS54383 , , TPS54386
SLUS774B – AUGUST 2007 – REVISED OCTOBER 2007
First, the steady state duty cycle is calculated. Assuming the rectifier diode has a voltage drop of 0.5 V, the duty
cycle is approximated using Equation 10 .
The filter inductor is then calculated; see Equation 11 .
A custom-designed inductor may be used for the application, or a standard value close to the calculated value
may be used. For this example, a standard 10- µ H inductor is used. Using Figure 28 , find the 30% duty cycle
curve. The 30% duty cycle curve has a down slope from low frequency and rises at approximately 6 kHz. This
curve is the resonant frequency that must be compensated. Any frequency wthin an octave of the peak may be
used in calculating the capacitor value. In this example, 6 kHz is used.
A 68- µ F capacitor should be used as a bulk capacitor, with up to 10 µ F of ceramic bypass capacitance. To
ensure the ESR zero does not significantly impact the loop response, the ESR of the bulk capacitor should be
placed a decade above the resonant frequency.
The resulting loop gain and phase are shown in Figure 31 . Based on measurement, loop crossover is 45 kHz
with a phase margin of 60 degrees.
GAIN AND PHASE
vs
FREQUENCY
Figure 31. Example Loop Result
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