Limit resistance “R” is calculated using the following equation. The resistance becomes the value shown in Figure 11.
| VIN − VF |
| VIN | =Input voltage (power voltage)(V) | |
R= | (Ω) | VF | =LED forward voltage (V) | ||
IF | |||||
|
| IF | =Allowable LED forward current (A) | ||
|
|
|
For this resistance value, the forward current becomes lower than 50 mA at 25°C. The surface brightness of the LED backlight varies with the forward current. See Figure 12. Compared with 1.8 (4) brightness (IF=90 mA), the brightness is about 40%.
1.2
1.0
0.8
Relative
brightness 0.6 0.5 0.4
0.2
Forward current:
3040 50 60 70 80 90 100 110 IF(mA) Figure 12 Forward
(2) Example 2
When you want to keep the brightness (2 nit) at 25°C, use a thermosensitive element, like a thermistor, and a transistor as shown in Figure 13. Set the thermosensitive element to about IF=90 mA at 25°C and configure it so that “IF” will be reduced as the temperature rises.
VIN
R2
R3
R4
LEDA
LEDC
G1216
Tr
Thermistor
R1
Figure 13 LED Backlight Driver Example 2
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