SG1842(2000) 查看數據表（PDF） - Microsemi Corporation
零件编号
产品描述 (功能)
生产厂家
SG1842 Datasheet PDF : 14 Pages
| |||
PRODUCT DATABOOK 1996/1997
SG1842/SG1843 Series
CURRENT-MODE PWM CONTROLLER
PRODUCTION DATA SHEET
APPLICATION INFORMATION
OSCILLATOR
The oscillator of the 1842/43 family of PWM's is designed such
that many values of R and C will give the same oscillator
T
T
frequency, but only one combination will yield a specific duty
cycle at a given frequency.
Given: Frequency ≡ f
Maximum Duty Cycle ≡ Dm
(1.76)1/Dm -1
Calculate:
RT
=
267
(1.76)(1-Dm)/Dm
-
1
(Ω)
where .3 < D < .95
m
A set of formulas are given to determine the values of RT and
C for a given frequency and maximum duty cycle. (Note: These
T
formulas are less accurate for smaller duty cycles or higher
frequencies. This will require trimming of R or C to correct for
T
T
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 RT and CT?
Given: f = 50kHz
D = 0.45
m
(1.76)1/.045 -1
Calculate:
R
T
=
267
(1.76).55/.45
-
1
=
674Ω
CT =
1.86 * Dm
f
*
R
T
(µF)
For Duty-Cycles above 95% use:
VREF
RT
RT/CT
CT
GND
1000
100
10
CT
=
1.86 *
50000
0.45
* 674
= .025µF
R
T
R
T
=
=
680W
2kW
R
T
=
5kW
R
T
=
10kW
R
T
R
T
R
T
R
T
R
T
=
=
=
=
=
20kW
30kW
50kW
70kW
100kW
1.86
F ≈ RTCT where RT ≥ 5kΩ
FIGURE 13 — OSCILLATOR TIMING CIRCUIT
1
.001
.002
.005
.01
.02
.05
0.1
CT Value - (µF)
FIGURE 14 — OSCILLATOR FREQUENCY vs. R FOR VARIOUS C
T
T
10
Copyright © 2000
Rev. 1.6 4/00
Share Link: 