Protection Pin — Lx1692

On the LX1692 datasheet, the protection pin is often labeled as Pin 11 (PRO) or simply the protection input. Its function is binary:

Think of it as the emergency stop button for your screen’s backlight. When any internal or external fault condition occurs—such as a broken CCFL tube, a shorted transformer, or an over-voltage event—the chip pulls the protection pin high (or receives a high signal), and the backlight dies.

Internally, the PROT pin is connected to:

Normal Operation: The pin is pulled high (≈5V) by the internal current source. Fault Condition: The IC turns on the internal open-drain transistor, grounding the PROT pin (≈0V). Latch: Once the PROT pin is pulled low (by internal or external means), the IC latches off all PWM outputs. The only way to restart is to cycle the ( V_CC ) power (power-on reset). lx1692 protection pin

In the world of LCD Inverter technology, the Microsemi (now Microchip) LX1692 stands as a legendary workhorse. It is the beating heart of countless CCFL (Cold Cathode Fluorescent Lamp) backlight inverters found in televisions and monitors from the mid-2000s and beyond.

While most technicians focus on the PWM input or the high-voltage transformer outputs, there is a tiny, often overlooked conductor orchestrating the safety of the entire system: The Protection Pin (typically Pin 14 on the DIP package).

It is not merely an "on/off" switch; it is a high-impedance arbitration judge that decides whether your screen lives or dies. Here is why this pin is far more interesting than it looks. On the LX1692 datasheet, the protection pin is

To diagnose an LX1692-based inverter, you need a multimeter with a DC voltage setting. Here is what to look for:

Important: When measuring, you must catch the voltage during the brief moment the backlight tries to turn on. Many technicians use a storage oscilloscope or an analog multimeter with a fast needle response to observe the "ramp-up" before shutdown. Think of it as the emergency stop button

If you are repairing an LCD TV or monitor and suspect the protection circuit is being triggered:

Note: Pin numbers above are referenced from the standard SOIC-20 package versions of the LX1692 (such as LX1692E). Always confirm the pinout against the specific datasheet for your chip version.

In many inverter circuits using the LX1692, the protection pin is tied to a small timing capacitor (typically 0.1µF to 1µF) that filters false triggers. Over time, heat and age cause this capacitor to become leaky. A leaky capacitor acts like a resistor, slowly raising the voltage on the protection pin until it crosses the threshold, even if no real fault exists.