TPS54383, TPS54386
www.ti.com
SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
Example: TPS54386 Buck Converter Operating at 12-V Input, 3.3-V Output and 400-mA(P-P) Ripple Current
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.
VOUT + VDIODE
3.3 + 0.5
d =
=
= 30%
V
IN + VDIODE
12 + 0.5
(10)
The filter inductor is then calculated; see Equation 11.
V
- V
OUT
12 - 3.3
1
IN
L =
´ d´ T =
´ 0.3´
= 10.9mH
S
DI
0.4
600000
L
(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-µH 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.
1
1
C =
=
= 70mF
2
)
2
)
L ´ 2´ p´ f
(
10´10-6 ´ 2´3.14´ 6000
(
RES
(12)
A 68-µF capacitor should be used as a bulk capacitor, with up to 10 µF 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.
1
1
RESR
<
=
» 40 mW
-6
2´ 3.14´10´ 6000´ 68´ 10
( )
2´ p´10´ fRES ´ C
(13)
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
80
70
60
50
40
180
135
90
Phase
45
0
30
20
10
0
-45
-90
Gain
-135
-10
-20
-180
100
1 k
10 k
f - Frequency - Hz
100 k
1 M
Figure 31. Example Loop Result
Copyright © 2007, Texas Instruments Incorporated
Submit Documentation Feedback
23
Product Folder Link(s): TPS54383 TPS54386
TI [ TEXAS INSTRUMENTS ]
