SG3843D MICROSEMI [Microsemi Corporation], SG3843D Datasheet - Page 10

• Manufacturer Part Number: SG3843D
• Description: CURRENT MODE PWM CONTROLLER
• Manufacturer: MICROSEMI [Microsemi Corporation]

SG1842/SG1843 Series
10
OSCILLATOR
The oscillator of the 1842/43 family of PWM's is designed such
that many values of R
frequency, but only one combination will yield a specific duty
cycle at a given frequency.
Given: Frequency ≡ f
Calculate: R
For Duty-Cycles above 95% use:
Maximum Duty Cycle ≡ D
C
FIGURE 13 — OSCILLATOR TIMING CIRCUIT
T
T
= 267
=
F ≈
1.86 * D
R
f * R
GND



where .3 < D
R
1.86
T
(1.76)
V
/C
T
T
C
REF
(1.76)
T
T
and C
T
m
(1-D
where R
1/D
m
(µF)
)/D
T
P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
m
m
will give the same oscillator
m
-1
m
- 1
< .95
T
C



≥ 5kΩ
U R R E N T
(Ω)
P
A P P L I C A T I O N I N F O R M A T I O N
R
C
R O D U C T I O N
T
T
- M
O D E
1000
P W M C
100
10
1
.001
D
C
formulas are less accurate for smaller duty cycles or higher
frequencies. This will require trimming of R
this error.)
Example:
cycle at a switching frequency of 50kHz. What are the values
of R
Given:
Calculate: R
FIGURE 14 — OSCILLATOR FREQUENCY vs. R
A T A
T
for a given frequency and maximum duty cycle. (Note: These
A set of formulas are given to determine the values of R
A Flyback power supply requires a maximum of 45% duty
T
and C
O N T R O L L E R
.002
f = 50kHz
D
S
m
C
T
?
H E E T
= 0.45
T
T
= 267
=
50000 * 674
1.86 * 0.45



.005
(1.76)
(1.76)
C
T
.55/.45
Value - (µF)
1/.045
= .025µF
.01
- 1
-1



= 674Ω
.02
T
FOR VARIOUS C
T
or C
T
to correct for
Copyright © 2000
.05
Rev. 1.6a 10/04
T
T
and
0.1