Datasheet

UC1823A, UC2823A, UC2823B,
UC3823A, UC3823B, UC1825A,
UC2825A, UC2825B, UC3825A, UC3825B
SLUS334E − AUGUST 1995 − REVISED SEPTEMBER 2010
www.ti.com
6
APPLICATION INFORMATION
The oscillator of the UC3823A, UC3823B, UC3825A, and UC3825B is a saw tooth. The rising edge is governed by a current
controlled by the RT pin and value of capacitance at the CT pin (C
CT
). The falling edge of the sawtooth sets dead time for
the outputs. Selection of RT should be done first, based on desired maximum duty cycle. CT can then be chosen based
on the desired frequency (RT) and D
MAX
. The design equations are:
R
T
+
3V
(
10 mA
)
ǒ
1 * D
MAX
Ǔ
C
T
+
ǒ
1.6 D
MAX
Ǔ
ǒ
R
T
f
Ǔ
Recommended values for R
T
range from 1 kΩ to 100 kΩ. Control of D
MAX
less than 70% is not recommended.
UDG−95102
Figure 1. Oscillator
10 M
1 M
100 k
10 k
1 k
10 k
100 k
Figure 2
R
T
− Timing Resistance − W
OSCILLATOR FREQUENCY
vs
TIMING RESISTANCE
f − Frequency − Hz
Figure 3
1 k 10 k
70
75
80
85
90
95
100
100 k
D
MAX
− Maximum Duty Cycle − %
R
T
− Timing Resistance − W
MAXIMUM DUTY CYCLE
vs
TIMING RESISTANCE
(1)