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RAW10V
RAW12V
RAW-5V
U32 provides an accurate, regulated voltage that is used as the reference voltage for the A/D inside the
CPU. Filtering is provided by C6, C12, and R124. The voltage output VREF is 5 V.
9.10.17 Ambient Light
The ambient light circuit is illustrated in Figure 9-27.
Diode D8 is a photodiode that is used to measure ambient light. Q8, R68, and R136 provide current
gain for D8 photocurrent. The amplified photocurrent flowing through R136 creates a voltage drop,
which is measured by the CPU. The CPU continually monitors the light source output at
AMB_LIGHT (TP72). Under low ambient light conditions, the CPU automatically switches on the
display backlight.
9.10.18 Ambient Temperature
The ambient temperature circuit is illustrated in Figure 9-28.
U32
R123
2
VIN
4
182
GND
C95
22UF
LT1021
R161
10K
VREF
+12V
R121
221
C91
C80
22UF
0.1UF
-5V
R122
221
C93
C94
22UF
0.1UF
Figure 9-26: Analog Reference Voltage Circuit
LIGHT SENSOR
VCC
VCC
D8
VTB8442B
Q8
MMBTA13L
R68
2.2M
R136
33.2K
Figure 9-27: Ambient Light Circuit
VREF
TP60
6
VOUT
5
TRIM
C6
0.1UF
Q23
2N3904
+5.7
Q24
2N3904
TP61
TP62
TP72
C63
.01UF
Ambient
Light
Technical Supplement
R124
1
C12
22UF
TP70
GND
9-25
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