3.5Contrast Adjustment and Power Supply Example
The LC panel viewing angle and display screen contrast are greatly affected by the ambient temperature. The recommended LC drive voltage (Vopr) at each temperature is given below. Vopr is a value at which the best display is visually obtained. This value does not always correspond to the value at which the best contrast (Cmax.) is obtained. A contrast adjustment circuit example is shown below:
Temperature (°C) | −20 | 0 | 25 | 50 | 70 | Vopr= VDD − VLC |
Voltage (Vopr) | 13.5 | 13.0 | 12.5 | 11.5 | 10.5 |
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| VDD | RA=10 kΩ | |
5V |
| C1 | ||
| RB=1.2 kΩ | |||
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| VSS | ||
| RA | RV (variable resistor) | ||
C2 |
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| =10 kΩ | ||
12 V |
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RV | VLC | C1, C2=10 μF | ||
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| RB |
| G1216 |
Figure 10 Contrast Adjustment
3.6 LED Backlight Driver Examples
(1) Example 1
A basic LED backlight driver example is shown in Figure 11.
LEDA |
R=30Ω, (≥1/4W) |
VIN = +5V |
LEDC |
G1216 |
Figure 11 LED Backlight Driver Example 1
Where resistance “R” is the limit resistance of the LED forward current. The forward current depends upon the temperature. Especially, it must be decreased at high temperature. For temperature dependence, refer to forward current reduction characteristics described in 1.8 (1). The operating temperature for the G1216 ranges from −20°C to +70°C. It is necessary to determine limit resistance “R” so that the forward current becomes 50 mA or lower at +70°C.
Forward voltage VF at each temperature of the LED backlight is shown in Table 3.
Table 3 Forward Voltage At Temperatures
Temperature (Ta) | Conditions | VF min. | VF typ. | VF max. |
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−20°C | IF=90 mA | 3.9 V | 4.3 V | 4.6 V |
+25°C | IF=90 mA | 3.8 V | 4.1 V | 4.4 V |
+70°C | IF=50 mA | 3.5 V | 3.7 V | 3.9 V |
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