Panel_Vid_Measurement_and_Setting

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LCD Panel VID Measurement & Setting
Alex Liu 2009/11/25
What is VID – View of Circuit
LVDS outputs consist of a current source (nominal 3.5 mA) that drives the differential pair lines. The basic receiver has a high DC input impedance, so the majority of driver current flows across the 100W termination resistor generating about 350 mV across the receiver inputs. When the driver switches, it changes the direction of current flow across the resistor, thereby creating a valid “one” or “zero” logic state.
(a)
(b)
(c) (d)
VID,min ~ VID,max 2 * VID,min ~ 2 * VID,max 1 1 V ID, min ~ 2 2 VID,max (VID,min – VCM) ~ (VID,max – VCM)
Answer: (b)
Single-End VA and VB
350mV + VCM
VA VID = 350mV
VCM
0V Differential VAB = VA - VB +350mV
VB
0V
-350mV
2*VID = 700mV
Difference between Signal-End and Differential Signal
Quiz

If we use differential probe to measure the waveform between LVDS+ and LVDS-, the peakto-peak voltage must meet which range that defined in the panel spec
LVDS+ - LVDS- = -200mV

The Range of Differential Signal is 400mV
(200mV - -200mV) = 400mV |VID| = |LVDS+ - LVDS-| = 200mV VID = ± 200mV |VID| = |LVDS+ - LVDS-| = 200mV

LGD LM230WF4-TLB1 Test Report – (1)
LGD LM230WF4-TLB1 Test Report – (2)
We should make sure that the VID Tolerance is enough.
How to Set LCD Panel VID

If Signal-End VID = 200mV Differential Signal (LVDS+ - LVDS-)

Case 1: LVDS+ = 200mV, LVDS- = 0V
LVDS+ - LVDS- = 200mV

Case 2: LVDS+ = 0V, LVDS- = 200mV
What is VID – View of Signals (1)
wk.baidu.com
LGD Spec – Single-End
What is VID – View of Signals (2)
Samsung Spec
What is VID – View of Signals (3)
Analysis of VID



We can change the LVDS Driving Current via Scaler to control VID. Measure VID and adjust Scaler’s registers to make VID in the panel spec (it is usually between 200mV to 600mV). Implement proper LVDS Driving Current settings which are got from above.
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