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

Crystal oscillator

Quartz Oscillator 3225 20M OSC

The LCD liquid crystal backlight system is typically designed either independently or in a two-in-one configuration. The backlight chip includes a timer or Ctime pin, commonly referred to as CP or SCP. This pin serves as the external capacitor terminal for protection delay. When overvoltage, overcurrent, or lamp open circuit failure occurs, the system does not immediately trigger protection but instead delays it by approximately 2 seconds. During startup, the capacitor charges through the chip’s internal 3 microampere current source within 2 seconds. Since the capacitor voltage remains below the threshold during this time, the protection gate remains closed. As a result, even if overvoltage or overcurrent happens, the chip stops working. After 2 seconds, when the capacitor voltage exceeds 3V, the protection gate opens, and the system can respond to overvoltage or overcurrent immediately. To disable the protection function during maintenance, the pin can be grounded. Some chips also allow disabling the protection via the SS soft start pin. Additionally, there's a 3 microamp current source inside the chip that charges the external capacitor connected to the SS terminal. Once the capacitor reaches 3V, the backlight circuit starts operating. Before reaching this voltage, the backlight remains inactive. It's important to note that because the internal current source is only 3 microamps, when measuring the voltage on the pin with a multimeter, the internal resistance of the meter may pull the voltage down, making it appear low. In reality, the circuit is functioning properly. Inside the backlight chip, there are two oscillators. One is the high-frequency oscillator (CT, TR), which generates a 50KHz square wave to drive the lamp, known as the lamp operating frequency. This oscillator is not controlled. The other is the backlight control oscillator (LCT, LRT or BCT, BRT), which produces a 200KHz low-frequency signal to adjust the lamp brightness. The duty cycle of this oscillator—determined by the width of the square wave—is regulated by the backlight brightness adjustment voltage and feedback from the main board. When the ambient temperature rises, the lamp current increases, and the feedback enters the IS pin of the chip. This causes the output square wave of the low-frequency oscillator to narrow, reducing the lamp’s duty cycle and bringing the current back to a normal level. If the voltage at the IS pin falls below 0.4V, the chip’s comparator detects an open lamp (due to polarity reversal) and triggers open-circuit protection. If the voltage exceeds 2V, the internal comparator identifies excessive lamp current or a short in the high-voltage Transformer winding, activating overcurrent protection. Similarly, if the high-voltage feedback terminal VS exceeds 1.2V, the chip recognizes overvoltage and activates overvoltage protection. For an LCD TV, if the lamp turns off upon power-on or flickers three times before shutting off, possible issues include a faulty lamp, a shorted high-voltage transformer, a malfunctioning overvoltage/overcurrent/open-circuit detection circuit, or a defective oscillation circuit in the backlight chip. First, check the secondary coil of the high-voltage transformer and compare its resistance with others to identify any faulty components. If the transformer is fine, observe whether the lamp lights up after power-on. If it doesn’t light, appears dark red, or shows black ends, the lamp might be damaged. If the lamp is functional, test the overcurrent or overvoltage detection circuit. If everything checks out, inspect the surrounding resistors and transistors for abnormalities.