UCC1807 Datasheet, PDF (4/11 Pages) Texas Instruments – Programmable Maximum Duty Cycle PWM Controller

PIN DESCRIPTIONS (cont.)
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
duty cycle
1.4
R _A 2 R _B C_T
R_A R_B
R_A 2R_B
as shown in Figure 1.
UCC1807-1/-2/-3
UCC2807-1/-2/-3
UCC3807-1/-2/-3
For best performance, keep the lead from C_T to GND as
short as possible. A separate ground connection for C_T is
desirable. The minimum value of R_A is 10kW , the mini-
mum value of R_B is 2.2kW , and the minimum value of C_T
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
I_OUT = Qg 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.
UDG-95002-1
Figure 1. Oscillator Block Diagram
APPLICATIONS INFORMATION
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.
T1:
Core
Magnetics Inc. #P-42625-UG (ungapped)
Primary:
28 turns of 2x #26AWG
Secondary:
6 turns of 50x0.2mm Litz wire
L1:
Core:
Magnetics Inc. #P-42625-SG-37 (0.020”
gap)
Main Winding: 13 turns of 2x #18AWG
Second Winding: 11 turns of #26AWG
Magnetics Inc.
900 E. Butler Road
P.O. Box 391
Butler, PA 16003
Tel: [phone]
Fax: [phone]
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