A Simple Pixel Circuit using LTPS TFTs with Mirror Compensation for AMOLED Displays

A Simple Pixel Circuit using LTPS TFTs with Mirror

Compensation for AMOLED Displays

Po-Syun Chen, Yeng-Ting Liu, Fu-Hsing Chen, and Chih-Lung Lin*

Department of Electrical Engineering, National Cheng Kung University, Tainan 701-01, Taiwan, R. O. C.

Abstract

This work presents a new pixel circuit using low-temperature polycrystalline-silicon (LTPS) thin-film transistors (TFTs) for active-matrix organic light-emitting diode (AMOLED) displays. The proposed circuit can effectively compensate for the threshold voltage variation of TFTs and power line I-R drop by a simple mirror structure.

Fig. 2. Simulated waveform of node A of proposed pixel circuit with TFT V_TH variations.

Proposed Driver Circuit and Its

Timing Diagram

Circuit Operations

Fig. 1. Proposed pixel circuit and corresponding timing diagram.

Fig. 3. OLED currents and relative current error rates versus data voltage.

Circuit Simulations

Fig. 4. OLED currents of different gray levels when VDD drops.

1st. Reset phase 2nd. Compensation phase 3rd. Emission phase

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DD low data TH T TH T DD low data

k I V V k k V V V V V V = − = − + − = − _ DD DD high A REF V V V V = = _ _ 3 _ _ 3 | | | | DD DD low A data TH T Cs DD low data TH T V V V V V V V V V = = − = − + _ DD DD high V = V V_DATA OLED V_REF VSS V_DD T1 T2 T3 SCAN C_S A SCAN V_DD V_DATA V_{DD_high} V_{DD_low} V_data (1) (2) (3) 15 20 25 30 35 40 45 Time (µs) -8 -4 0 4 8 V o lt a g e (V ) VTH = 0 V VTH = 0.5 V VTH = -0.5 V (1) (2) (3) V_DATA OLED V_REF VSS V_DD T1 T2 T3 SCAN C_S A V_REF V_{DD_high} V_DATA OLED V_REF VSS V_DD T1 T2 T3 SCAN C_S A V_data V_{DD_low} V_data-|V_{TH_T3}| V_DATA OLED V_REF VSS V_DD T1 T2 T3 SCAN C_S A I_OLED V_{DD_high} V_{DD_high}-V_{DD_low}+V_data-|V_{TH_T3}|

_ _ _ 3 | | A DD high DD LOW data TH T V V V V V = − + − -1.2 -0.8 -0.4 0 0.4 0.8 1.2 Data Voltage (V) 0 1000 2000 3000 4000 O L E D C u rr en t (n A ) 0 2 4 6 8 10 R el a ti v e C u rr en t E rr o r R a te ( % ) I (VTH = 0 V) I (VTH = 0.5 V) I (VTH = - 0.5 V) Error (VTH = 0.5 V) Error (VTH = -0.5 V) 0 0.2 0.4 0.6 V_DD Drop (V) 0 1000 2000 3000 4000 O L E D C u rr en t (n A ) V_data = 0.5 V, I_OLED = 175 nA V_data = -0.25 V, I_OLED = 1272 nA V_data = -0.75 V, I_OLED = 2693 nA