Datasheet

6
UCC1809-1/-2
UCC2809-1/-2
UCC3809-1/-2
The recommended value for CT is 1nF for frequencies in
the 100 kHz or less range and smaller CT for higher fre
-
quencies. The minimum recommended values of RT1
and RT2 are 10kΩ and 4.32kΩ, respectively. Using these
values maintains a ratio of at least 20:1 between the
R
DS(on)
of the internal FETs and the external timing resis
-
tors, resulting in minimal change in frequency over tem
-
perature. Because of the oscillator's susceptibility to
capacitive coupling, examine the oscillator frequency by
looking at the common RT1-RT2-CT node on the circuit
board as opposed to looking at pins 3 and 4 directly. For
good noise immunity, RT1 and RT2 should be placed as
close to pins 3 and 4 of the IC as possible. CT should be
returned directly to the ground pin of the IC with minimal
stray inductance and capacitance.
10
100
1000
100 1000 10000
CT [pF]
FREQUENCY [kHz]
Figure 4. Oscillator frequency vs. C
T
(RT1 = 10k,
RT2 = 4.32k)
CT
S(OSC)
R(OS C)
Q(OS C)=CLK
=S(PWM)
FB
R(P WM)
Q(P WM)
OUT
CT
CHARGING
CT
DIS C HARGING
3.33V
1.67V
1V
70%
ON
30%
OFF
FB S IG NAL DO MINANT MAX. DUTY C YC LE DO MINANT
Figure 3. Waveforms associated with the oscillator latch and the PWM latch.
APPLICATION INFORMATION (cont.)
UDG-99037