SLLS636L – DECEMBER 2004 – REVISED DECEMBER 2009
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APPLICATION INFORMATION
CALCULATION
t
on
/t
off
(t
on
+ t
off
)
t on
t off
STEP UP
V
out
)
V
F
*
V
in(min)
V
in(min)
*
V
sat
1
f
t
on
)
t
off
STEP DOWN
V
out
)
V
F
V
in(min)
*
V
sat
*
V
out
1
f
t
on
)
t
off
t on
t off
VOLTAGE INVERTING
V
out
)
V
F
V
in
*
V
sat
1
f
t
on
)
t
off
t on
t off
t
off
t
on
C
T
I
pk(switch)
)
1
)
1
)
1
t
on
)
t
off
*
t
off
4
10
*5
t
on
t
on
)
1
t
off
t
on
)
t
off
*
t
off
4
10
*5
t
on
2I
out(max)
0.3
I
pk(switch)
V
in(min)
*
V
sat
*
V
out
I
pk(switch)
I
pk(switch)
t
on
)
t
off
8V
ripple(pp)
1.25(1
)
R2)
R1
See
t
on
)
t
off
*
t
off
4
10
*5
t
on
t
on
)
1
t
off
2I
out(max)
2I
out(max)
R
SC
0.3
I
pk(switch)
V
in(min)
*
V
sat
I
pk(switch)
I
out
t
on
V
ripple(pp)
0.3
I
pk(switch)
V
in(min)
*
V
sat
I
pk(switch)
I
out
t
on
V
ripple(pp)
L
(min)
t
on(max)
t
on(max)
t
on(max)
C
O
9
9
V
out
1.25(1
)
R2)
R1
See
*
1.25(1
)
R2)
R1
See
V
sat
= Saturation voltage of the output switch
V
F
= Forward voltage drop of the chosen output rectifier
The following power-supply parameters are set by the user:
V
in
= Nominal input voltage
V
out
= Desired output voltage
I
out
= Desired output current
f
min
= Minimum desired output switching frequency at the selected values of V
in
and I
out
V
ripple
= Desired peak-to-peak output ripple voltage. The ripple voltage directly affects the line and load
regulation and, thus, must be considered. In practice, the actual capacitor value should be larger than the
calculated value, to account for the capacitor's equivalent series resistance and board layout.
10
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