Datasheet
5
UCC2750
UCC3750
The UCC3750 provides complete control and protection
functions for a four quadrant flyback converter used to
generate ring signals for telephone circuits. A typical ap
-
plication circuit for a 15 REN ring generator is shown in
Fig. 1.
As shown, the flyback converter takes a DC input (typi
-
cally 48V) and provides an isolated output with a pro
-
grammable frequency (and amplitude) AC signal
superimposed on a programmable DC offset. The power
path consists of a primary side PWM switch Q1, primary
return rectifier DR1, a 4-winding transformer T1, output
rectifiers DR2 and DR3, synchronous/PWM switches Q2
and Q3, and output filter CF. Resistor R
SENSE provides
the output current sensing for protection circuits.
Different operating modes of the converter are depicted
in Table 2. Fig. 2 shows the output voltage and current
waveforms for a purely capacitive load and identifies the
four operating modes. Fig. 3 shows the PWM waveforms
for the circuit and Fig. 4a - 4d show the equivalent cir
-
cuits under the operating modes. The addition of Q2, Q3
and primary diode facilitates true four quadrant operation
where both the output voltage and power transfer can be
bi-directional. Mode 1 is similar to the commonly used
DC-DC converter operation where Q1 is modulated with
the PWM signal and rectification is provided through the
Q2, DR2 path to provide a positive output proportional to
the increasing, positive reference voltage. The
pulse-width is controlled by the error amplifier output to
increase or decrease the output as dictated by the refer
-
ence. The maximum duty cycle is limited to 50% to pre
-
vent DR1 from turning on prior to Q2/DR2.
In mode 2, the reference begins to decrease, necessitat
-
ing that the power transfer back to the input. For this
mode, switch Q3 needs to be modulated while DR1 acts
as the rectifier back to the input. The UCC3750 has
mode decoding circuitry which automatically directs the
PWM signal to Q3 and turns off Q1.
APPLICATION INFORMATION
18
21
22
15
FULLWAVE
RECTIFIER
MAG
SIGN
OUT2
MODE SIGN OUT1 OUT2 OUT3
0 0 PWM OFF RECIRC
01 PWMOFF
OFF OFF PWM
OFF
1
1
0
1 PWM RECIRC OFF
CONTROL LOGIC
+
–
REF
ERROR
AMP
V
CM
NEG2
17 16
OUT1
+
–
REF
DC OFFSET AMP
V
CM
NEG1
2
28
26
V
CP
V
CP
V
CP
GD1
GD2
GD3
+
–
MODE
SIGN
PWM
+
–
DC
CURRENT
LIMIT
SINE REFERENCE
GENERATOR
REF
10
SINFLT
9
SINREF
20
FS0
8
SWRLY
19
FS1
XTAL
OSC
14
13
12
11
5
4
7
6
CHARGE
PUMP
INTERNAL
REFERENCE
REF=7.5V
V
CM
=3.0V
25
V
CM
3
GNDREF
VDD
VS2
VS1
NEGDC
VCP
OUTDC
+
–
V
CM
4
REF
AMPIN
AMPOUT
XTAL2
XTAL1
MAG
RAMP
CLOCK
GENERATOR
24
RT
23
CT
1
RGOOD
27
ENBL
+
–
V
CM
AMP
V
CM
DETAILED BLOCK DIAGRAM
UDG-99074
Note: All pin numbers are for N or DW Packages.
Mode Reference
Polarity
Power
Flow
E.A.
Output
Source
(PWM)
Switch
Rectifier
Switch
1 + + – Q1 Q2
2 + – + Q3 (D1)
3 – + + Q1 Q3
4 – – – Q2 (D1)
Table II. Operating modes.