CP3800
Switching Regulator Power Controller
Functional Description
Soft Start SCP
Figure 1 Soft-Start SCP Equivalent circuit
At power-on, the CP3800 operates in the soft-start mode. Since no current is supplied, comp1, comp2,
and SCP latch are inactive. Immediately after power-on, the CPE capacitor is charged at a constant
current of 1µA to increase from 0V. In the soft-start mode, the VSCP (capacitor CPE voltage) is input
to the PWM comparator, so that the soft-start circuit is controlled at the on-duty ratio of 0% with the
increase of the VSCP.
When the VSCP reaches 0.8V, the soft-start supplies current to comp1, comp2, and SCP latch and at
the same time discharges the CPE capacitor (VSCP=0V). This changes the soft-start mode to the
normal mode (output short circuit detection wait state) and the SCP becomes active (stays low). In the
normal mode, the input from the soft-start circuit to the PWM comparator is kept at 0.8V.
If the switching regulator output suddenly drops due to load effect, the error amplifier output (FB pin)
is fixed at VOM+ and capacitor CPE starts charging. When the voltage at the SCP pin reaches
approximately 0.8V, the output pin is set low and the SCP pin stays low.
Set time by the following expressions (see Timing Chart):
Soft-Start time (The time until the output ON duty reaches approximately 50%)
* All specs and applications shown above subject to change without prior notice.
1F-5 NO.66 SEC.2 NAN-KAN RD ., LUCHU , TAOYUAN, TAIWAN
Tel:886-3-3214525
Email: server@ceramate.com.tw
Http: www.ceramate.com.tw
Rev 1.0 Mar.31,2003
Page 14 of 19
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CERAMATE [ CERAMATE TECHNICAL ]
