4

UCC1807-1/-2/-3

UCC2807-1/-2/-3

UCC3807-1/-2/-3

TRIG/DISCH: Oscillator control pins. Trig is the oscillator

timing input, which has an RC-type charge/discharge sig-

nal controlling the chip’s internal oscillator. DISCH is the

pin which provides the low impedance discharge path for

the external RC network during normal operation. Oscil-

lator frequency and maximum duty cycle are computed

as follows:

fre que ncy

RR

C

duty cycle

RR

RR

AB

T

AB

AB

14

2

2

.

as shown in Figure 1.

is 47pF.

VDD: The power input connection for this device. Total

VDD current is the sum of quiescent current and the av-

erage OUT current. Knowing the operating frequency

and the MOSFET gate charge (Qg), average OUT cur-

rent can be calculated from

F, where F is frequency.

To prevent noise problems, bypass VDD to GND with a

ceramic capacitor as close to the chip as possible in par-

allel with an electrolytic capacitor.

PIN DESCRIPTIONS (cont.)

Figure 1. Oscillator Block Diagram

UDG-95002-1

T1:

Core

Primary:

Secondary:

Magnetics Inc. #P-42625-UG (ungapped)

28 turns of 2x #26AWG

6 turns of 50x0.2mm Litz wire

L1:

Core:

Main Winding:

Second Winding:

Magnetics Inc. #P-42625-SG-37 (0.020”

gap)

13 turns of 2x #18AWG

11 turns of #26AWG

Magnetics Inc.

900 E. Butler Road

P.O. Box 391

Butler, PA 16003

Tel: (412) 282-8282

Fax: (412) 282-6955

The circuit shown in Fig. 2 illustrates the use of the

UCC3807 in a typical off-line application. The 100W,

200kHz, universal input forward converter produces a

regulated 12VDC at 8 Amps. The programmable maxi-

mum duty cycle of the UCC3807 allows operation down

to 80VRMS and up to 265VRMS with a simple RCD

clamp to limit the MOSFET voltage and provide core re-

set. In this application the maximum duty cycle is set to

about 65%. Another feature of the design is the use of a

flyback winding on the output filter choke for both boot-

strapping and voltage regulation. This method of loop clo-

sure eliminates the optocoupler and secondary side

regulator, common to most off-line designs, while provid-

ing good line and load regulation.

APPLICATIONS INFORMATION