TPS54383, TPS54386
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SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
Design Procedure
Duty Cycle Estimation
The first step is to estimate the duty cycle of each switching FET.
V
+ V
FD
OUT
D
»
max
V
+ V
FD
IN(min)
(21)
(22)
V
+ V
FD
OUT
D
»
min
V
+ V
FD
IN(max)
Using an assumed forward drop of 0.5 V for a schottky rectifier diode, the Channel 1 duty cycle is approximately
40.1% (minimum) to 48.7% (maximum) while the Channel 2 duty cycle is approximately 27.7% (minimum) to
32.2% (maximum).
Inductor Selection
The peak-to-peak ripple is limited to 30% of the maximum output current. This places the peak current far
enough from the minimum overcurrent trip level to ensure reliable operation.
For both Channel 1 and Channel 2, the maximum inductor ripple current is 600 mA. The inductor size is
estimated in Equation 23.
V
- V
OUT
1
IN(max)
L
»
´ D
´
min
min
I
f
SW
LRIP(max)
(23)
The inductor values are
•
•
L1 = 18.3 µH
L2 = 15.3 µH
The next higher standard inductor value of 22 µH is used for both inductors.
The resulting ripple currents are :
V
- V
OUT
1
IN(max)
I
»
´D
´
RIPPLE
min
L
f
SW
(24)
Peak-to-peak ripple currents of 0.498 A and 0.416 A are estimated for Channel 1 and Channel 2 respectively.
The RMS current through an inductor is approximated by Equation 25.
2
1
2
)
I
=
I
(L(avg) )
+
I
(
RIPPLE
L rms
(
)
12
(25)
(26)
and is approximately 2.0 A for both channels.
The peak inductor current is found using:
1
IL peak » IOUT(max)
+
IRIPPLE
(
)
2
An inductor with a minimum RMS current rating of 2.0 A and minimum saturation current rating of 2.25 A is
required. A Coilcraft MSS1278-223ML 22-µH, 6.8-A inductor is selected.
Rectifier Diode Selection
A schottky diode is selected as a rectifier diode for its low forward voltage drop. Allowing 20% over VIN for
ringing on the switch node, the required minimum reverse break-down voltage of the rectifier diode is:
34
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