UCC1809-1/-2
UCC2809-1/-2
UCC3809-1/-2
APPLICATION INFORMATION (cont.)
The Typical Application Diagram shows an isolated reference) sensed through RT1. The R input to the oscil-
flyback converter utilizing the UCC3809. Note that the lator latch, R(OSC), is also level sensitive and resets the
capacitors CREF and CVDD are local decoupling capaci- CLK signal low when CT crosses the 1.67V threshold,
tors for the reference and IC input voltage, respectively. turning off Q2 and turning on Q1, initiating another charg-
Both capacitors should be low ESR and ESL ceramic, ing cycle.
placed as close to the IC pins as possible, and returned
Figure 3 shows the waveforms associated with the oscil-
directly to the ground pin of the chip for best stability.
lator latch and the PWM latch (shown in the Typical Ap-
REF provides the internal bias to many of the IC func-
plication Diagram). A high CLK signal not only initiates a
discharge cycle for CT, it also turns on the internal NMOS
FET on the FB pin causing any external capacitance
tions and CREF should be at least 0.47µF to prevent REF
from drooping.
used for leading edge blanking connected to this pin to
be discharged to ground. By discharging any external
capacitor completely to ground during the external
switch’s off-time, the noise immunity of the converter is
enhanced allowing the user to design in smaller RC com-
ponents for leading edge blanking. A high CLK signal
also sets the level sensitive S input of the PWM latch,
S(PWM), high, resulting in a high output, Q(PWM), as
shown in Figure 3. This Q(PWM) signal will remain high
until a reset signal, R(PWM) is received. A high R(PWM)
signal results from the FB signal crossing the 1V thresh-
old, or during soft start or if the SS pin is disabled.
FB Pin
The basic premise of the UCC3809 is that the voltage
sense feedback signal originates from an optocoupler
that is modulated by an external error amplifier located
on the secondary side. This signal is summed with the
current sense signal and any slope compensation at the
FB pin and compared to a 1V threshold, as shown in the
Typical Application Diagram. Crossing this 1V threshold
resets the PWM latch and modulates the output driver
on-time much like the current sense comparator used in
the UC3842. In the absence of a FB signal, the output
will follow the programmed maximum on-time of the os-
cillator.
Assuming the UVLO threshold is satisfied, the OUT sig-
nal of the IC will be high as long as Q(PWM) is high and
S(PWM), also referred to as CLK, is low. The OUT sig-
nal will be dominated by the FB signal as long as the FB
signal trips the 1V threshold while CLK is low. If the FB
signal does not cross the 1V threshold while CLK is low,
the OUT signal will be dominated by the maximum duty
cycle programmed by the user. Figure 3 illustrates the
various waveforms for a design set up for a maximum
duty cycle of 70%.
When adding slope compensation, it is important to use
a small capacitor to AC couple the oscillator waveform
before summing this signal into the FB pin. By correctly
selecting the emitter resistor of the optocoupler, the volt-
age sense signal can force the FB node to exceed the
1V threshold when the output that is being compared ex-
ceeds a desired level. Doing so drives the UCC3809 to
zero percent duty cycle.
Oscillator
The following equation sets the oscillator frequency:
V
−1
REF
(
)
(
)
FOS C =[0.74 • CT + 27pF • RT1+RT 2 ]
(
)
DMAX = 0.74 • RT1• CT + 27pF • FOS C
Q1
Referring to Figure 2 and the waveforms in Figure 3,
when Q1is on, CT charges via the RDS(on) of Q1 and
RT1. During this charging process, the voltage of CT is
sensed through RT2. The S input of the oscillator latch,
S(OSC), is level sensitive, so crossing the upper thresh-
old (set at 2/3 VREF or 3.33V for a typical 5.0V refer-
ence) sets the Q output (CLK signal) of the oscillator
latch high. A high CLK signal results in turning off Q1 and
turning on Q2. CT now discharges through RT2 and the
RDS(on) of Q2. CT discharges from 3.33V to the lower
threshold (set at 1/3 VREF or 1.67V for a typical 5.0V
3
+
CLK
S
Q
–
3.33V
1.67V
RT1
+
–
4
R
RT2
OS CILLATOR
LATCH
Q2
CT
OSC
UDG-97195
Figure 2. UCC3809 oscillator.
5