CP3800
Switching Regulator Power Controller
tS = 0.35× CPE(µF)
Short-circuit detection time(or Soft-Start full time) :
tPE = 0.8×CPE(µF)
Setting Oscillating Frequency
The oscillation frequency of a sawtooth waveform is determined by the timing capacitor (CT) and the
timing resistor (RT) which are connected to the OSC pin. When the voltage of the OSC pin falls to
0.1V, the CT is rapidly charged to increase the voltage of the OSC pin instantaneously. When the
voltage of the OSC pin reaches 0.8V, Charging stops, so that CT and RT are discharged to decrease the
voltage of the OSC pin. The waveform of the OSC pin is shown as Figure 2.
Figure 2 Oscillator Waveform
The cycle (T) can be determined as follows:
t
VX
VH
VX = VHe−CT×RT
t = −CT × RT × loge (
)
T = −CT × RT × loge (0.1/ 0.8) = 2.1×CT × RT s
[ ]
Calculating under the conditions of RT=3.0KΩ and CT=270pF , the oscillating frequency is 588kHz,
which is slightly different from the value (600kHz) in the Electrical Characteristics. This difference is
caused by characteristics, such as changes in the maximum voltage amplitude of the sawtooth
waveform with the CT value and the circuit delay causing the maximum amplitude to become large in
the case of a high oscillating frequency even for the same capacitor. It is difficult to incorporate these
non-linear characteristics into the equation. In practical use, therefore, the user should read the RT and
CT values from the characteristic curve or should determine an approximate target value by using the
equation.
* All specs and applications shown above subject to change without prior notice.
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Tel:886-3-3214525
Email: server@ceramate.com.tw
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Rev 1.0 Mar.31,2003
Page 15 of 19
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