Primary Current Limiting to turn Power The UCC3751 directly drives primary side switches used to implement a
switches located on the secondary of the converter. For normal ring signal
鈥?/div>
Operates from a Single 12V Supply
generation, the primary switching frequency and secondary sampling fre-
quency are precisely offset from each other by the ringing frequency to pro-
duce a high voltage low frequency alias signal at the output. The off-hook
condition is detected by sensing the AC current and when AC limit is ex-
ceeded, the sampling frequency is set to be equal to the primary switching
frequency to produce a DC output.
The drive signal frequencies are derived from a high frequency (3579545
Hz) crystal. The primary switching frequency is 89.488 kHz and the sam-
pling frequency is 20, 25 or 50 Hz less depending on the status of fre-
quency select pins FS0 and FS1.
The circuits described in this datasheet are covered under US Patent #5,663,878 and other patents pending.
TYPICAL APPLICATIONS CIRCUIT
D1
L
IN
T1
V
IN
12V
L
R
C
R2
SAMPLING
CIRCUIT
L
R
C
R1
N:1
12V
C
BYP1
9
VS12
Q1
11
DRV1
UCC3751
12
DRVS
2
RINGEN
6
OHD
4
VDD
ENABLE
10
C
BYP2
AC SIGNAL
C
F
V
OUT
C
DC
DC SIGNAL
V
1
R
SENSE
5
DCLIM
DELAY
1
Q2
13
DRV2
GND
3
PGND
14
FS0
7
FS1
8
XTAL2
XTAL1
16
15
3.579545MHz
UDG-98047
APRIL 1999 - REVISED AUGUST 2000 - SLUS267B