MC34163, MC33163
http://onsemi.com
14
Calculation
Step−Down Step−Up Voltage−Inverting
t
on
t
off
(Notes 1, 2, 3)
V
out
) V
F
V
in
* V
sat
* V
out
V
out
) V
F
V
in
V
in
V
sat
|V
out
| ) V
F
V
in
* V
sat
t
on
ƒ
t
on
t
off
ǒ
t
on
t
off
) 1
Ǔ
ƒ
t
on
t
off
ǒ
t
on
t
off
) 1
Ǔ
ƒ
t
on
t
off
ǒ
t
on
t
off
) 1
Ǔ
C
T
ƒ
32.143 · 10
6
ƒ
32.143 · 10
6
ƒ
32.143 · 10
6
I
L(avg)
I
out
I
out
ǒ
t
on
t
off
) 1
Ǔ
I
out
ǒ
t
on
t
off
) 1
Ǔ
I
pk (Switch)
I
L(avg)
)
DI
L
2
I
L(avg)
)
DI
L
2
I
L(avg)
)
DI
L
2
R
SC
0.25
I
pk (Switch)
0.25
I
pk (Switch)
0.25
I
pk (Switch)
L
ǒ
V
in
* V
sat
* V
out
DI
L
Ǔt
on
ǒ
V
in
* V
sat
DI
L
Ǔt
on
ǒ
V
in
* V
sat
DI
L
Ǔt
on
V
ripple(pp)
DI
L
ǒ
1
8C
O
Ǔ
2
) (ESR)
2
ƒ
[
t
on
I
out
C
O
[
t
on
I
out
C
O
V
out
V
ref
ǒ
R
2
R
1
) 1
Ǔ
V
ref
ǒ
R
2
R
1
) 1
Ǔ
V
ref
ǒ
R
2
R
1
) 1
Ǔ
V
in
−
V
out
−
I
out
−
DI
L
−
p −
V
ripple(pp)
−
Nominal operating input voltage.
Desired output voltage.
Desired output current.
Desired peak−to−peak inductor ripple current. For maximum output current it is suggested that
DI
L
be chosen to be less
than 10% of the average inductor current I
L(avg)
. This will help prevent I
pk
(Switch)
from reaching the current limit
threshold set by R
SC
. If the design goal is to use a minimum inductance value, let DI
L
= 2(I
L(avg)
). This will
proportionally reduce converter output current capability.
Maximum output switch frequency.
Desired peak−to−peak output ripple voltage. For best performance the ripple voltage should be kept to a low value
since it will directly affect line and load regulation. Capacitor C
O
should be a low equivalent series resistance (ESR)
electrolytic designed for switching regulator applications.
The following Converter Characteristics must be chosen:
NOTES: 1. V
sat
− Saturation voltage of the output switch, refer to Figures 8 and 9.
NOTES: 2. V
F
− Output rectifier forward voltage drop. Typical value for 1N5822 Schottky barrier rectifier is 0.5 V.
NOTES: 3. The calculated t
on
/t
off
must not exceed the minimum guaranteed oscillator charge to discharge ratio of 8, at the minimum
NOTES: 3. operating input voltage.
Figure 28. Design Equations
