SG1842/SG1843 Series

PRODUCT DA T ABOOK 1996/1997

Copyright © 2000

Rev. 1.6a 10/04

10

APPLICA TION INFORMA TION

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.

FIGURE 13 — OSCILLATOR TIMING CIRCUIT

A set of formulas are given to determine the values of R

C

formulas are less accurate for smaller duty cycles or higher

frequencies. This will require trimming of R

this error.)

Given: Frequency

≡ f

Maximum Duty Cycle

≡ D

m

Calculate: R

(

Ω)

where .3 < D

C

(µF)

For Duty-Cycles above 95% use:

Example:

A Flyback power supply requires a maximum of 45% duty

cycle at a switching frequency of 50kHz. What are the values

of R

Given:

f

= 50kHz

D

Calculate: R

= 674

Ω

C

= .025µF













1.86 * 0.45

50000 * 674













1.86 * D

m

f * R

T

V

REF

R

GND

R

T

C

T

.001

1

C

1000

.002

.02

100

10

.005

.01

.05

0.1

R

T

= 680

W

R

T

= 2k

W

R

T

= 5k

W

R

T

= 10k

W

R

T

= 20k

W

R

T

= 30k

W

R

T

= 50k

W

R

T

= 70k

W

R

T

= 100k

W

FIGURE 14 — OSCILLATOR FREQUENCY vs. R

F

≈

where R

1.86

R