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Design Procedure
Duty Cycle Estimation
OUT FD
max
IN(min) FD
V V
D
V V
+
»
+
(21)
OUT FD
min
IN(max) FD
V V
D
V V
+
»
+
(22)
Inductor Selection
IN(max) OUT
min min
LRIP(m ax) SW
V V
1
L D
I f
-
» ´ ´
(23)
IN(max) OUT
RIPPLE min
SW
V V
1
I D
L f
-
» ´ ´
(24)
( )
( )
( )
2
2
L(avg) RIPPLE
L rms
1
I I I
12
= +
(25)
( )
OUT(max) RIPPLE
L peak
1
I I I
2
» +
(26)
Rectifier Diode Selection
TPS54283 , , TPS54286
SLUS749C – JULY 2007 – REVISED OCTOBER 2007
The first step is to estimate the duty cycle of each switching FET.
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).
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 .
The inductor values are
• L1 = 18.3 µ H
• L2 = 15.3 µ H
The next higher standard inductor value of 22 µ H is used for both inductors.
The resulting ripple currents are :
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 .
and is approximately 2.0 A for both channels.
The peak inductor current is found using:
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- µ H, 6.8-A inductor is selected.
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:
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