Seiko Instruments G1216B1N000-3D0E - Manual (Page 25)

AN.No.G1216B1N000-3D0E
- 22 -
Limit resistance “R” is calculated using the following equation. The resistance becomes the value
shown in Figure 11.
R=
V
IN
−
V
F
I
F
V
IN
=Input voltage (power voltage)(V)
V
F
=LED forward voltage (V)
I
F
=Allowable LED forward current (A)
(
Ω
)
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%.
Forward current:
I
F
(mA)
0.2
0.4
0.6
0.8
1.0
30 40 50 60 70 80 90 100 110
Relative
brightness
0.5
1.2
(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.
G1216
V
IN
LEDA
LEDC
Tr
R1
Thermistor
R4
R3
R2
Figure 12 Forward Current-Brightness Characteristics (Ta=25
°
C)
Figure 13 LED Backlight Driver Example 2