P R O D U C T D A T A B O O K 1 9 9 6 / 1 9 9 7
SG1842/SG1843 Series
C
U R R E N T - M O D E P W M C O N T R O L L E R
P R O D U C T I O N D A T A S H E E T
APPLICATION INFORMATION
OSCILLATOR
The oscillator of the 1842/43 family of PWM's is designed such
that many values of RT and CT will give the same oscillator
frequency, but only one combination will yield a specific duty
cycle at a given frequency.
A set of formulas are given to determine the values of RT and
CT 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 RT or CT to correct for
this error.)
Given: Frequency ≡ f
Example:
Maximum Duty Cycle ≡ Dm
A Flyback power supply requires a maximum of 45ꢀ duty
cycle at a switching frequency of 5ꢁkHz. What are the values
of RT and CT?
(1.76)1/D -1
m
Calculate: R = 267
(Ω)
Given: f = 5ꢁkHz
Dm = ꢁ.45
)/D
(1.76)(1-D
- 1
T
m
m
(1.76)1/.ꢁ45 -1
(1.76).55/.45 - 1
where .3 < Dm < .95
Calculate: RT = 267
= 674Ω
1.86
D
1.86 ꢁ.45
*
R
m
*
CT =
(µF)
CT =
= .ꢁ25µF
f
5ꢁꢁꢁꢁ 674
*
T
*
1000
For Duty-Cycles above 95ꢀ use:
VREF
100
R
T
R / CT
T
10
CT
GND
1
.001
.002
.005
.01
.02
.05
0.1
1.86
RTCT
F ≈
where RT ≥ 5kΩ
CT Value - (µF)
FIGURE 14 — OSCILLATOR FREQUENCY vs. RT FOR VARIOUS CT
FIGURE 13 — OSCILLATOR TIMING CIRCUIT
Copyright © 2000
Rev. 1.6 4/00
1ꢁ
MICROSEMI [ Microsemi ]
