# Why refresh rate is needed in a passive-matrix LCD

In a passive-matrix LCD we have horizontal and vertical conductors. The intersection points are pixels which are cells of liquid crystal molecules.

For drawing a frame, we provide the necessary voltages to specific conductors. But once we have provided the voltages and the picture has been drawn on the screen, why do we need to refresh?

As long as the voltage is on the picture is still there; it's not like a CRT where the emission from phosphorus atoms decays. If the reason is video or animation, then we are changing the frame buffer and then drawing the new frame buffer. But do we need to refresh the same frame buffer at some rate? If so, why?

• Because you have chosen a display technology that needs refresh. If you don't want to refresh the display, choose a display technology that doesn't need refresh.
– user16324
Dec 28, 2020 at 13:18

## 1 Answer

LCD crystals want to align themselves along the electric field, set by the voltage between electrodes. Each LCD pixel is like a small capacitor.

When potential is set during scanning of the matrix, the applied voltage does not stay charged forever in the tiny capacitor, it slowly leaks and must be refreshed like in the case of DRAM cells.

And the electrodes will degrade when DC potential is applied, so the electrodes must be driven with an AC voltage that has average DC voltage of 0V.

So constantly refreshing the LCD is needed to alternate the polarity and to keep the pixels on and off by keeping the electrodes charged or discharged.

• So you're saying that if I have a single LCD cell , I cannot apply a continuous electric field across it to keep it in the off state for as long as I want ? Dec 28, 2020 at 15:19
• No, that is not what I said. To keep it off, you must constantly or periodically remove any electric field or charge that might leak into it, i.e. keep the element/cell/segment/pixel discharged, so that it stays off. In real life, a single LCD cell is driven by two anti-phase square wave to turn it on, and driven with in-phase square wave to turn if off (due to multiplexing of several segments with single common) Dec 28, 2020 at 17:12