Measurement of RF current waveform of a source driver chip used in a liquid crystal-TV display panel

Abstract

Instantaneous RF current waveform of the power line for a source driver chip used in a typical colour active matrix type liquid crystal-TV display panel has been evaluated for the first time based on magnetic near field measurement. A 16-inch panel set was opened to bare the 10 parallelly-connected source driver chips. Each chip is with 2.7 mm × 12.0 mm in size, and drives 207 liquid crystal cells with nominally 100 pF for each at 16 Vmax with common voltage level of 8 V, which corresponds to the maximum charge of 0.17 µC. A shielded-loop probe (CP-2S, window size: 0.2 mm × 1.0 mm) was used to measure the induced voltage waveform. The analogue power line was identified as the major source of RF magnetic field noise. Then RF current waveform was calculated based on the Faraday's low, using the size and geometry of measurement system. It is known the cells are fully charged when all the panel cells display “white.” The measured corresponding peak current was 130 mA with rise time of less than 10 ns at the turned-off of clock pulse, followed by a triangular-like decrease for 1.7 µs. The corresponding electric charge was 0.14 µC, which is closely to the maximum possible charge of 0.17 µC. The spectrum of RF current has significant intensity even around 1 GHz.