ꢀꢁ
ꢀ ꢁ
ꢀ ꢁ
ꢂ ꢃꢄ ꢅ ꢆ ꢇ
ꢅꢃ ꢄꢅ ꢆ ꢇ
ꢄꢃ ꢄꢉ ꢆ ꢇ
ꢀ
ꢀ
ꢀ
ꢁ
ꢁ
ꢁ
ꢄ
ꢅ
ꢄ
ꢃ
ꢃ
ꢃ
ꢄ
ꢄ
ꢄ
ꢅ
ꢅ
ꢉ
ꢆ
ꢈ
ꢈ
ꢇ
ꢇ
ꢇ
ꢀ
ꢀ
ꢀ
ꢁ
ꢁ
ꢁ
ꢄ
ꢂ
ꢅ
ꢃ
ꢃ
ꢃ
ꢄ
ꢄ
ꢄ
ꢅ
ꢉ
ꢉ
ꢈ
ꢆ
ꢆ
ꢇ
ꢇ
ꢇ
ꢀ
ꢁ
ꢅ
ꢃꢄ
ꢉ
ꢈ
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SLUS334C − AUGUST 1995 − REVISED AUGUST 2004
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 ). The falling edge of the sawtooth sets dead time for
CT
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 . The design equations are:
MAX
ǒ1.6 DMAXǓ
3 V
R +
C +
T
T
10 mA ǒ1 * DMAXǓ
ǒR fǓ
T
(
)
(1)
Recommended values for R range from 1 kΩ to 100 kΩ. Control of D
less than 70% is not recommended.
T
MAX
UDG−95102
Figure 1. Oscillator
OSCILLATOR FREQUENCY
vs
MAXIMUM DUTY CYCLE
vs
TIMING RESISTANCE
TIMING RESISTANCE
100
95
10 M
1 M
90
85
80
100 k
10 k
75
70
10 k
1 k
10 k
100 k
1 k
100 k
R
T
− Timing Resistance − W
R − Timing Resistance − W
T
Figure 3
Figure 2
6
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