PRODUCT DATABOOK 1996/1997
SG1842/SG1843 Series
C
URRENT
-M
ODE
PWM C
ONTROLLER
P
R O D U C T I O N
D
A T A
S
H E E T
A P P L I C AT I O N I N F O R M AT I O N
OSCILLATOR
The oscillator of the 1842/43 family of PWM's is designed such
that many values of R
T
and C
T
will give the same oscillator
frequency, but only one combination will yield a specific duty
cycle at a given frequency.
Given: Frequency
≡
f
Maximum Duty Cycle
≡
D
m
A set of formulas are given to determine the values of R
T
and
C
T
for a given frequency and maximum duty cycle. (Note: These
formulas are less accurate for smaller duty cycles or higher
frequencies. This will require trimming of R
T
or C
T
to correct for
this error.)
Example:
A Flyback power supply requires a maximum of 45% duty
cycle at a switching frequency of 50kHz. What are the values
of R
T
and C
T
?
Given:
f
=
50kHz
D
m
=
0.45
(1.76)
1/D
m
-1
Calculate: R
T
= 267
(Ω)
(1-D
m
)/D
m
- 1
(1.76)
where .3 < D
m
< .95
C
T
=
1.86 * D
m
f * R
T
(µF)
1000
(1.76)
1/.045
-1
Calculate: R
T
= 267
(1.76)
.55/.45
- 1
= 674Ω
C
T
=
1.86 * 0.45
= .025µF
50000 * 674
For Duty-Cycles above 95% use:
V
REF
f - (kHz)
100
R
T
R
T
/C
T
C
T
GND
10
R
T
=
680
W
R
T
=
2k
W
R
T
=
5k
W
R
T
=
10k
W
R
T
=
20
R
T
= k
W
30k
W
R
T
=
50k
W
R
T
=
70k
W
R
T
=
100
k
W
F
≈
1.86
R
T
C
T
where R
T
≥
5kΩ
1
.001
.002
.005
.01
.02
.05
0.1
C
T
Value - (µF)
FIGURE 14
— OSCILLATOR FREQUENCY vs. R
T
FOR VARIOUS C
T
FIGURE 13
— OSCILLATOR TIMING CIRCUIT
10
Copyright © 2000
Rev. 1.6 4/00
MICROSEMI [ MICROSEMI CORPORATION ]
