Table of Contents
Overview#
The Voltage Holding Ratio (VHR) is a critical parameter in LCD displays, measuring how well a pixel maintains its voltage between refresh cycles. Higher VHR means better image quality and reduced flicker.
Definition#
Voltage Holding Ratio#
$$ \text{VHR} = \frac{V_{end}}{V_{initial}} \times 100\% $$Where:
- \(V_{initial}\): Voltage at start of frame
- \(V_{end}\): Voltage at end of frame
Ideal vs Reality#
| Condition | VHR |
|---|---|
| Ideal (no leakage) | 100% |
| Typical TFT-LCD | 95-99% |
| Minimum acceptable | ~90% |
Voltage Decay Mechanism#
During Frame Period#
V(t)
│▓▓▓▓▓▓▓▓▓
│ ╲
│ ╲
│ ╲▓▓▓▓▓
└─────────────────→ t
Write Frame periodDecay Equation#
$$ V(t) = V_0 \cdot e^{-t/\tau} $$Where:
$$ \tau = R_{off} \cdot C_{total} $$- \(R_{off}\): TFT off-resistance
- \(C_{total}\): Pixel capacitance (Clc + Cst)
Leakage Sources#
1. TFT Leakage#
$$ I_{TFT} = I_0 \cdot e^{(V_{gs} - V_{th})/nV_T} $$Even in “off” state, small current flows.
2. Liquid Crystal Leakage#
$$ I_{LC} = \frac{V_{pixel}}{R_{LC}} $$LC has finite resistivity.
3. Gate Dielectric Leakage#
Through gate insulator.
4. Parasitic Paths#
Surface and bulk leakage currents.
Impact of Low VHR#
Image Quality Issues#
| VHR | Effect |
|---|---|
| >98% | Excellent |
| 95-98% | Good |
| 90-95% | Visible gray level shift |
| <90% | Flicker, poor image |
Gray Level Accuracy#
If voltage drops during frame:
- Brightness changes
- Wrong gray level displayed
- Worse at low gray levels
Factors Affecting VHR#
Temperature#
$$ I_{leak} \propto e^{-E_a/kT} $$Higher temperature → more leakage → lower VHR.
| Temperature | VHR Change |
|---|---|
| 25°C | Reference |
| 50°C | -5% typical |
| 70°C | -10% typical |
Frame Rate#
Longer frame time → more decay:
$$ \text{VHR} = e^{-t_{frame}/\tau} $$| Refresh Rate | Frame Time | VHR Impact |
|---|---|---|
| 120 Hz | 8.3 ms | Highest |
| 60 Hz | 16.7 ms | Standard |
| 30 Hz | 33.3 ms | Lowest |
Pixel Capacitance#
$$ \Delta V = \frac{I_{leak} \cdot t}{C_{total}} $$Larger capacitance → less voltage drop → better VHR.
Improving VHR#
Design Strategies#
| Strategy | Effect |
|---|---|
| Larger Cst | More charge storage |
| Better TFT | Lower off-current |
| Higher refresh | Less time for decay |
| Low-ion LC | Reduces LC leakage |
Material Selection#
- High-resistivity LC materials
- Low-leakage TFT technology (IGZO vs a-Si)
- Quality dielectrics
Measurement Method#
Test Setup#
- Apply known voltage to pixel
- Wait one frame period
- Measure remaining voltage
Typical Results#
Applied: 5.0V
After 16.7ms: 4.9V
VHR = 4.9/5.0 = 98%VHR vs TFT Technology#
| TFT Type | Typical Off-Current | VHR |
|---|---|---|
| a-Si | ~1 pA | 95-98% |
| LTPS | ~0.1 pA | 97-99% |
| IGZO | ~0.01 pA | 99%+ |
IGZO’s extremely low leakage enables:
- Lower refresh rates (power saving)
- Higher resolution (more time per line)
Design Trade-offs#
Capacitor Size#
| Larger Cst | Smaller Cst |
|---|---|
| Better VHR | Lower VHR |
| Lower aperture | Higher aperture |
| Slower charging | Faster charging |
Refresh Rate#
| Higher Rate | Lower Rate |
|---|---|
| Better VHR | Lower VHR |
| More power | Less power |
| Less motion blur | More motion blur |