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( ) ( )
IN
BR R min
V 1.2 V³ ´
(27)
( ) ( )
( )
D avg OUT max
I I 1 D» ´ -
(28)
( ) ( )
FM
D max D avg
P V I» ´
(29)
Output Capacitor Selection
( )
OUT
2
2
RES
1
C
4 f L
=
´ p ´ ´
(30)
OUT
RIPPLE(tot) RIPPLE(cap) RIPPLE(tot)
(max)
RIPPLE RIPPLE S
V V V
D
ESR
I I f C
-
= = -
´
(31)
TPS54283 , , TPS54286
SLUS749C – JULY 2007 – REVISED OCTOBER 2007
The diode must have reverse breakdown voltage greater than 15.8 V, therefore a 20-V device is used.
The average current in the rectifier diode is estimated by Equation 28 .
For this design, 1.2-A (average) and 2.25 A (peak) is estimated for Channel 1 and 1.5-A (average) and 2.21-A
(peak) for Channel 2.
An MBRS320, 20-V, 3-A diode in an SMC package is selected for both channels. This diode has a forward
voltage drop of 0.4 V at 2 A.
The power dissipation in the diode is estimated by Equation 29 .
For this design, the full load power dissipation is estimated to be 480 mW in D1, and 580 mW in D2.
The TPS54283's internal compensation limits the selection of the output capacitors. From Figure 24 , the internal
compensation has a double zero resonance at about 3 kHz. The output capacitor is selected by Equation 30 .
Solving for C
OUT
using
• f
RES
= 3 kHz
• L = 22 µ H
The resulting is C
OUT
= 128 µ F. The output ripple voltage of the converter is composed of the ripple voltage
across the output capacitance and the ripple voltage across the ESR of the output capacitor. To find the
maximum ESR allowable to meet the output ripple requirements the total ripple is partitioned, and the equation
manipulated to find the ESR.
Based on 128 µ F of capacitance, 300-kHz switching frequency and 50-mV ripple voltage plus rounding up the
ripple current to 0.5 A, and the duty cycle to 50%, the capacitive portion of the ripple voltage is 6.5 mV, leaving a
maximum allowable ESR of 87 m Ω .
To meet the ripple voltage requirements, a low-cost 100- µ F electrolytic capacitor with 400 m Ω ESR (C5, C17)
and two 10- µ F ceramic capacitors (C3 and C4; and C18 and C19) with 2.5-m Ω ESR are selected. From the
datasheets for the ceramic capacitors, the parallel combination provides an impedance of 28 m Ω @ 300 kHz for
14 mV of ripple.
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