UCC1809-1/-2
UCC2809-1/-2
UCC3809-1/-2
APPLICATION INFORMATION (cont.)
CT CT
CHARGING DIS CHARGING
3.33V
1.67V
CT
S (OS C)
R(OS C)
Q(OS C)=CLK
=S (P WM)
1V
FB
R(P WM)
Q(P WM)
70%
ON
FB S IGNAL DOMINANT
30%
OFF
OUT
MAX. DUTY CYCLE DOMINANT
UDG-99037
Figure 3. Waveforms associated with the oscillator latch and the PWM latch.
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.
1000
FREQUENCY [kHz]
100
10
100
1000
CT [pF]
10000
Figure 4. Oscillator frequency vs. C
T
(RT1 = 10k,
RT2 = 4.32k)
6
TI [ TEXAS INSTRUMENTS ]
