Datasheet
( ) ( )
1 1
RIPPLE
L1 peak OUT1 max
2 2
I I I 3.0 A 0.661A 3.3 A» + = + =
(32)
( ) ( )
1 1
RIPPLE
L2 peak OUT2 max
2 2
I I I 3.0 A 0.547 A 3.3 A» + = + =
(33)
( ) ( )
( )
( )
IN max
BR R min IN max
V
V 1.25 V 1.25 13.2 V 16.5 V
0.8
³ = ´ = ´ =
(34)
( ) ( )
( )
MIN1
D1 avg OUT1 max
I I 1 D 3.0 A (1 0.397) 1.81A» ´ - = ´ - =
(35)
( ) ( )
( )
MIN2
D2 avg OUT2 max
I I 1 D 3.0 A (1 0.272) 2.18 A» ´ - = ´ - =
(36)
( ) ( )
D peak L peak
I I=
(37)
( ) ( )
FM
D1 max D1 avg
P V I 0.4 V 1.81 0.72 W» ´ » ´ =
(38)
D2(max) FM D2(avg)
P V I 0.4V 2.18 0.87W» ´ » ´ =
(39)
( )
( )
(
)
( )
( )
2
2
TRAN MAX
OUT1 min
OUT1 OVER
I L
1A 8.2 H
C 8.2 F
5.0 V 0.2 V
V V
´
´ m
= = = m
´
´
(40)
( )
( )
(
)
( )
( )
2
2
TRAN MAX
OUT2 min
OUT2 OVER
I L
1A 8.2 H
C 12.4 F
3.3 V 0.2 V
V V
´
´ m
= = = m
´
´
(41)
TPS55383 , , TPS55386
SLUS818 – SEPTEMBER 2008 .........................................................................................................................................................................................
www.ti.com
A DC current with 30% peak to peak ripple has an RMS current approximately 0.4% above the average current.
The peak inductor current is estimated by:
An 8.2- µ H inductor with a minimum RMS current rating of 3.0 A and minimum saturation current rating of 3.3 A
must be selected. A Coilcraft MSS1048-822ML 8.2- µ H, 4.38-A inductor is chosen for both outputs.
Rectifier Diode Selection
A low forward voltage drop schottky diode is used as a rectifier diode to minimize power dissipation and
maximize efficiency.
Allowing 20% over VIN for ringing on the switch node, the rectifier diode ’ s minimum reverse break-down voltage
is given by:
Reviewing 20-V and 30-V schottky diodes, the MBRS330T3, 30-V, 3-A diodes in an SMC package are selected
for both channels. This diode has a forward voltage drop of 0.4 V at 3 A, so the conduction power dissipation is:
For this design, the maximum power dissipation is estimated as 0.72 W and 0.87 W respectively.
Output Capacitor Selection
Output capacitors are selected to support load transients and output ripple current. The minimum output
capacitance to meet the transient specification is given by:
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