Dell S1909WXf Service Manual page 8

2.1.2) Output Circuit and Protection Circuit :( fig.3)
The transformer (T1/T2) secondary winding leakage inductance and output capacitance
(C31/C32) forms a lower pass filter, which converts the square-wave driving signal into a sinusoidal
output voltage signal for CCFL.
The over-voltage protection feature is implemented by using an external capacitor divider
(C31/C32, C33/C34) or resistor divider (R20/R21, R22/R24) to sense the output voltage. The
divide-down voltage signal is sent to the IC Pin9 (VSEN), thus regulating the output voltage.
The voltage signal on negative pole of ZD1 sensed through R35 comes to Pin4 of U1 (ISEN).
The CCFL current detected at resistor R35 is converted to a voltage level and input to the ISEN
pin.C6 is used to dump noise. Once the CCFLs are ignited and current is sensed through resistor
R35, capacitor C6 performs the loop compensation function. The voltage at IC pin12 (SSTCMP)
controls the drive duty cycle of the power MOSFETs to regulate the CCFL current.
If a CCFL is removed, fails or damaged during normal operation, CCFL current is no longer
sensed and the voltage on ISEN pin drops. Once the voltage at the ISEN pin is less than the lamp"
on" threshold, the shutdown timer is activated. The IC maintains the output voltage for
approximately 2-3seconds and once the timer expires, the IC will shutdown. To restart the IC, either
toggle the SST_CMP pin or recycle the power on the VDDA pin.
fig.3
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Service Manual