LCD liquid crystal backlight to protect the maintenance of the idea - News - Global IC Trade Starts Here.

Crystal oscillator

Quartz Oscillator 3225 20M OSC

LCD liquid crystal backlights are designed independently or in two-in-one mode. The backlight chip has a timer or Ctime pin that is familiar to everyone. Some are labeled CP or SCP, and this pin is the protection delay external capacitor terminal. When the lamp overvoltage, overcurrent, and lamp open circuit failure, it is not immediately protected, but it is delayed by about 2 seconds. After the power is turned on, the capacitor is charged through the chip's 3 microampere current source, within 2 seconds of booting. Because the voltage on the capacitor is lower than the threshold, the pass gate of the protection signal is turned off. At this time, even if overvoltage or overcurrent occurs, the protection signal is cut off and the chip stops working. After 2 seconds, the capacitor is charged more than 3V, and the protection door is opened. At this time, overvoltage and overcurrent occur, and protection occurs immediately. Therefore, in order to remove the protection function during maintenance, the foot can be grounded. Some chips are grounded to remove the protection function from the SS soft start. There is also a 3 microamp current source in the chip to charge the external capacitor of the SS terminal. When the charging reaches 3V, the backlight circuit enters the working state. Before the 3V is reached, the backlight circuit does not work. It should be noted that because the current source current inside the SS terminal and the timer terminal is only (3 microamperes), when the voltage is measured by the meter, the level of the pin is pulled low by the internal resistance of the watch. As a result, the high level is not detected, and the repeated voltage measurement is a very low voltage. In fact, the circuit works normally.
There are two oscillators inside the backlight chip: one is the high-frequency oscillator CT, TR, which produces a 50K high-frequency driven square wave, which drives the lamp to emit light, which is called the lamp operating frequency (this high-frequency oscillator is not controlled). The other is backlight control: low frequency oscillator LCT, LRT or BCT, BRT, generating 200K low frequency oscillation for adjusting 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 from the main board. When the ring mirror temperature rises, the lamp current will increase, and the current feedback will enter the IS end of the chip, so that the square wave of the output of the backlight control low frequency oscillator will be narrowed, the duty cycle of the lamp illumination time will be narrowed, and the lamp current will decrease. To normal value.
When the voltage of the lamp current feedback terminal IS is lower than 0.4V, the comparator in the chip will consider that the lamp is open (positive and negative gate reversal) and enter the lamp open circuit protection. When the pin is higher than 2V, the internal comparator of the chip will think that the lamp current is too large, the high voltage transformer winding has a short circuit, and enter the overcurrent protection. When the high voltage feedback terminal VS of the lamp is higher than 1.2V, it is considered that the high voltage is too high to enter the overvoltage protection.
For the LCD LCD TV, the lamp will turn off when it is turned on, or it will be turned off after 3 flashes. The maintenance idea--there may be a bad lamp; the backlight high-voltage transformer is short-circuited; the lamp over-voltage, over-current, and lamp The open circuit detection circuit is defective; the backlight chip oscillation circuit is defective. At this point, first check the high-voltage transformer secondary line package, compared with other line packages (resistance value), find a bad high-voltage transformer. If it is normal, when the lamp is turned on after the power is turned on, whether the lamp is not lit, the lamp is dark red, the two ends of the lamp are black, and the lamp holder is connected. If the lamp is normal, the detection lamp Whether the tube overcurrent or overvoltage detection circuit is abnormal. If it is normal, it is necessary to detect the resistance of the periphery of the backlight chip, and the transistor is abnormal.