Pin Working Faulty 1 5 5 VREF Reference Output +5V 2 11.7 11.7 VIN Power supply +12V 3 5 (then drops) 0.4 NDR2 push-pull driver output #2 4 0 0 PGND Power GND 5 5 (then drops) 0.2 NDR1 push-pull driver output #1 6 0 0 AGND 7 1 1 RT_CT Sawtooth generator R and C 8 0.58 0.59 LRT_LCT (Triangular wave for burst-mode dimming) 9 2.3 (then drops) 1.24 DIM Dimmer control input 10 2.3 (then drops) 0 VSEN Voltage feedback input 11 2.8 2.8 OVT (Over Voltage Threshold?) 12 0 0 TIME Switch-On input/output 13 2 (then drops) 0 SST-CMP seems to be a power-good output, H = lamps are bright 14 0 0 ISEN on/off control input, H = ON, L = OFF 15 5 5 ENA Enable 16 5 (then drops) 0.74 (then drops) VLS Voltage Low Sense Pin 2 (VIN/VCC) of N901 (OZ9925) receives a 12V supply. When the CPU sends a high-level (greater than 2V) start-up signal to the enable control pin (Pin 15/ENA) of N901 (OZ9925), the IC initiates operation. After the circuit starts, the internal high-frequency oscillator (HF OSC) generates the high-frequency oscillation signal required for backlight lamp operation. This signal is modulated by the brightness signal received at Pin 9 (DIM) and then output as drive signals from Pin 3 (NDR2) and Pin 5 (NDR1) to the subsequent power amplification stage. The functions and operating processes of other relevant pins on N901 (OZ9925), as well as the roles of peripheral components, are described below: 1) Pin 1 (VREF) of N901 is the reference voltage output pin. The 12V supply is converted into a +5V reference voltage by the internal reference voltage regulator of the OZ9925 and output via Pin 1. This reference voltage plays a crucial role in generating internal reference currents and setting reference thresholds for protection control circuits. The +5V output from Pin 1 also serves as the reference voltage for the charging and discharging of the time-constant circuits associated with the high-frequency oscillator (Pin 7) and the PWM triangle-wave oscillator (Pin 8); if this voltage is abnormal, the entire IC—and indeed the entire backlight board—will malfunction. 2) Pin 2 (VIN) of N901 is the VCC operating voltage input pin. The OZ9925 supports a wide supply voltage range, operating normally with a VCC voltage between 10V and 19V. Pins 3 (NDR2) and 5 (NDR1) are drive signal output pins that provide low-impedance, inverted drive signals. Pin 4 (PGND) is the ground pin for the internal drive section. Pin 6 (AGND) is the ground pin for the internal small-signal section. ? Pin (RT_CT) serves as the oscillation frequency control terminal for the internal high-frequency oscillator. An external RC network acts as the timing component to control the oscillation frequency; the RC value (time constant) determines whether the frequency is high or low. The oscillation frequency is determined by the parallel capacitance value of C906 and C907 and the equivalent resistance of R914, R940, and RP901, as shown in Figure (a). The 5V reference voltage from Pin ? is used to power the RC charging and discharging circuits, ensuring frequency stability; RP901 allows for fine-tuning of the oscillation frequency as needed (see the equivalent circuit in Figure (b)). The relationship between the RC value and the oscillation frequency is as follows: a larger RC value results in a lower frequency, while a smaller RC value results in a higher frequency. The oscillation frequency of the OZ9925 can be set within the 30–70 kHz range by adjusting the RC value. 3) Pin ? (DIM) of the OZ9925 is the brightness control input terminal. A DC brightness control signal (varying between 1.5V and 3.5V) from the CPU enters the OZ9925 and is converted into PWM brightness control pulses. These pulses modulate the oscillator's output signal, transforming the continuous high-frequency oscillation wave into an intermittent high-frequency wave that varies according to the PWM duty cycle, thereby controlling the brightness of the backlight lamps. An auxiliary triangle wave signal is required for the conversion of the DC brightness control voltage into PWM signals. 4) Pin ? (LRT LCT) of the OZ9925 is the connection terminal for the frequency-control timing components of the triangle wave oscillator; this oscillator facilitates the conversion of the DC brightness control voltage (input at Pin ?) into PWM pulses. The value of the external RC network (time constant) determines both the triangle wave oscillation frequency and the PWM signal frequency, as shown in the figure below. To prevent screen flickering, the PWM frequency is set to approximately 200 Hz. V902 serves as the control circuit for the PWM external synchronization signal input, allowing the oscillation frequency of the triangular wave to be determined externally. Similarly, the power supply for the RC charging and discharging process utilizes the 5V reference voltage from pin ? to ensure the stability of the triangular wave oscillation frequency. 5) Pin ? (VSEN) of the OZ9925 is the input terminal for monitoring the backlight lamp voltage, used to verify whether the output voltage to the backlight lamps is normal. If the voltage at this pin exceeds the voltage at pin 11?, the entire chip ceases operation, and the backlight lamps turn off. 6) Pin ? (OVT) of the OZ9925 is used to set the over-voltage threshold; it works in conjunction with pin ? to implement functions such as output over-voltage protection, as shown in Figure 9. When the voltage at pin ? exceeds the voltage at pin ?—triggering the over-voltage protection mechanism—the integrated circuit stops operating. In this specific circuit, pin ? also functions to provide protection against backlight lamp open-circuit conditions. 7) Pin ? (TIMR) of the OZ9925 is the terminal for setting the activation delay time for the protection circuit. The backlight circuit is essentially a type of switching power supply, and its protection circuitry needs to respond quickly and sensitively. However, the load on the backlight board consists of Cold Cathode Fluorescent Lamps (CCFLs)—nonlinear gas-discharge devices characterized by various uncertainties. Unlike standard light bulbs that illuminate immediately upon power-up, CCFLs require a startup period of 0.5 to 1 second for the mercury to vaporize and participate in the discharge process. Furthermore, startup times vary between individual lamps and are prolonged in cold winter temperatures. During this startup phase, the backlight lamps do not draw normal operating current, and the sampled data regarding voltage, current, and open-circuit conditions appear abnormal. If the protection circuit were to activate during this interval, it would misinterpret the situation as a backlight circuit fault and trigger the protection mode. To prevent this issue, a delay protection circuit is incorporated into the backlight unit's protection control circuitry. Specifically, if the protection control circuit receives an abnormal sampling signal at the moment of power-up, it does not trigger protection immediately; instead, it waits for a set delay (such as 1 second or 1.5 seconds) before doing so. Capacitor C911, connected externally to pin 029925(12), is used to set this protection delay; the delay duration can be adjusted by changing the capacitor's value—a larger capacitance results in a longer delay.