4

UCC2752

UCC3752

Power Stage Operation

The power stage used for the UCC3752 application has

two distinct switching circuits which together produce the

required low frequency signal on the output. The primary

side switching circuit consists of a current fed push-pull

resonant circuit that generates the high frequency sinu-

soidal waveform across the transformer winding. The op-

eration of this type of circuit is extensively covered in

Unitrode Application notes U-141 and U-148. Resonant

the

primary

and

secondary

resonances

are

well

matched. Also, for the UCC3752 operation, switching fre-

quency is fixed by crystal selection. So, the resonant

components must be selected to yield a resonant fre-

quency close enough to the switching frequency to get a

low distortion sine-wave. Practically, since it is impossi-

ble to get an exact match between the two frequencies,

the switching frequency should always be higher than

the resonant frequency to ensure low distortion and take

advantage of ZVT operation. Switches Q1 and Q2 are

pulsed at 50% duty cycle at the switching frequency

(89.489 kHz) determined by a crystal (3.579545 MHz)

connected to the UCC3752. The input voltage for the

resonant stage (typically 12V) determines the voltage

stress of Q1 and Q2. Transformer turns ratio is

determined by the output voltage requirements. On the

secondary side, the high frequency waveform is sampled

at a predetermined frequency (e.g. 89.469 kHz) which dif-

fers from the primary switching frequency by the desired

output frequency (e.g. 20 Hz). The sampling is accom-

plished using a bi-directional switching circuit as shown in

Figure 1 and Figure 2. Figure 1 shows the sampling

mechanism consisting of two back-to-back FET switches

allowing current flow in both directions. The sampling can

also be done with a single active switch and a full-bridge

rectifier as shown in Fig. 2. The DRVS pin of the

UCC3752 provides the drive signal for the sampling

switch(es) and this signal is coupled through a pulse

transformer. Typical pulsewidth of the sampling signal is

280nS. As a result of sampling, the resultant output sig-

nal matches the secondary voltage in amplitude and has

a low output frequency desired for ring generation.

The secondary winding of the power transformer also has

a tap (or a separate winding) to generate a loosely regu-

lated DC voltage. This DC voltage can be used to offset

the ring generator output. It an also be used as a power

supply for supplying talk battery voltage in some applica-

tions.

APPLICATION INFORMATION

11

4

13

OHD

DRV1

DCLIM

14

9

3

12

OFFHOOK

FS0

FS1

5

6

1

DRVS

VDD

GND

VS12

7

8

DRV2

PGND

5VOLT

REFERENCE

ONE-SHOT

4.5V

MODULO

3,560

COUNTER

MODULO

1,800

COUNTER

MODULO

4,480

COUNTER

ONE-SHOT

300mV

XTAL2

15

10

MODULO

2

COUNTER

MODULO

40

COUNTER

XTAL1

16

ENABLE

N/C

2

300mV

MODULO

20

COUNTER

1-SHOT

BLOCK DIAGRAM

UDG-98023