How to protect and repair LCD backlight (Part 1)

by:Kimeery     2021-09-04
When repairing the mobile phone LCD backlight, in order to remove the protection function, this pin can be grounded. Some chips ground the SS soft-start terminal to remove the protection function. There is also a 3 microampere current source in the chip to charge the external capacitor on the SS terminal. When the charging reaches 3V, the backlight circuit enters the working state. Before reaching 3V, the backlight circuit does not work. Note that: because the SS terminal and time The current source current inside the terminal is extremely small (3 microamperes), so when measuring the voltage of this pin with a meter, the pin level will be pulled low through the internal resistance of the meter, so that the high level cannot be measured. Repeat the test The voltages are all very low voltages, in fact, the circuit works normally at this time. There are two oscillators inside the backlight chip: one is the high-frequency oscillator CT and TR, which generate a 50K high-frequency drive square wave and drive the lamp to emit light, which is called the lamp operating frequency (this high-frequency oscillator is not controlled). The other is backlight control: the low frequency oscillator LCT, LRT or BCT, BRT produces a low frequency oscillation of 200K, which is used to adjust the brightness of the lamp. The duty cycle of this oscillator, that is, the width of the square wave, is controlled by the backlight brightness adjustment voltage and lamp current feedback control from the motherboard. When the temperature of the ring mirror increases, the lamp current will increase, and the current will be fed back into the IS terminal of the chip, which narrows the square wave of the output of the backlight control low-frequency oscillator, narrows the duty cycle of the lamp light-emitting time, and decreases the lamp current To normal value.
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