Datasheet
SLUS456D − APRIL 1999 - REVISED AUGUST 2002
5
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pin assignments (continued)
SQ
R
4
OSCILLATOR
OUTPUT
0.2 V
VDD
2
RC
(APPROXIMATE
FREQUENCY)
FREQUENCY =
1.41
RC
UDG-00095
Figure 1. Block Diagram for Oscillator
NOTE A: The oscillator generates a sawtooth waveform on RC. During the RC rise time, the output stages alternate on time, but both stages are
off during the RC fall time. The output stages switch a 1/2 the oscillator frequency, with ensured duty cycle of < 50% for both outputs.
VDD: The power input connection for this device. Although quiescent VDD current is very low, total supply current
will be higher, depending on OUTA and OUTB current, and the programmed oscillator frequency. Total VDD current
is the sum of quiescent VDD current and the average OUT current. Knowing the operating frequency and the
MOSFET gate charge (Qg), average OUT current can be calculated from:
I
OUT
+ Q
g
F, where F is frequency
To prevent noise problems, bypass VDD to GND with a ceramic capacitor as close to the chip as possible along with
an electrolytic capacitor. A 1-µF decoupling capacitor is recommended.
APPLICATION INFORMATION
A 200-kHz push-pull application circuit with a full-wave rectifier is shown in Figure 2. The output, V
O
, provides 5 V
at 50 W maximum and is electrically isolated from the input. Since the UCC3808A is a peak-current-mode controller
the 2N2907 emitter following amplifier (buffers the CT waveform) provides slope compensation which is necessary
for duty ratios greater than 50%. Capacitor decoupling is very important with a single ground IC controller, and a 1
µF is suggested as close to the IC as possible. The controller supply is a series RC for start-up, paralleled with a bias
winding on the output inductor used in steady state operation.
Isolation is provided by an optocoupler with regulation done on the secondary side using the TL431 adjustable
precision shunt regulator. Small signal compensation with tight voltage regulation is achieved using this part on the
secondary side. Many choices exist for the output inductor depending on cost, volume, and mechanicall strength.
Several design options are iron powder, molypermalloy (MPP), or a ferrite core with an air gap as shown here. The
main power transformer has a Magnetics Inc. ER28 size core made of P material for efficient operation at this
frequency and temperature. The input voltage may range from 36 V dc to 72 V dc.