NCP5890
Ambient Light Control
The ambient light can be monitored, by an extra photo
diode, to automatically adjust the I ? LED peak current as a
function of the ambient light. A dedicated I2C command is
used to activate or de ? activate this function. On the other
hand, the end user shall set up the maximum I ? LED current
by means of an I2C command. The photo sense is
automatically de ? activated when an I2C command is send
through the port, and resume to the pre ? programmed status
when the I2C command is completed.
The concept is based on analog monitoring of the I ? LED
current and the photo sense feedback, associated to a
Up/Down counter to properly setup the contrast at display
level. The photo sense action is bounded by two limits:
? upper I ? LED as defined by the end user: the photo
sense cannot increase the I ? LED above such a limit
? lower I ? LED as defined by the end user: the photo
sense cannot reduce the back light current to zero.
When the photo sense activates the counter (in either
direction), a selectable low frequency clock drives the
counter, yielding a smooth and slow brightness variation.
The 100 Hz noise, coming from standard fluorescent
tubes, is filter out by means of an external network built
between the photo sensor and the AMBS pin. Generally
speaking, such a filter is built with a RC network designed
to cope with the electrical performances of the selected
photo sense. As a consequence, the AMBS pin is biased by
a low level voltage signal and processed accordingly the
ambient light control structure: see Figure 16.
SFH5711
U3
VSB
4
+VBat
VSB
GAIN
Log
4
R2
AMBS
R
R/2
220k
5
1 2
GND
R1 22k
C1
2.2uF
U1
GND
ANALOG
MONITORING
R/4
R
GND
I2C
R/2
R
2R
4R
Figure 13. Basic Photo Sense Input Circuit
Since several types of photo sensor can be used in the final application, provisions have been taken into account to dynam ?
ically adjust the gain of the photo current. Such capability is carried out by the internal register loaded through the I2C port.
$08 PHGAIN[0..4][7]
B7
VSD
B6
?
B5
?
B4
PHG5
B3
PHG4
B2
PHG3
B1
PHG2
B0
PHG1
RESET
0
0
0
0
0
0
0
0
Gain
4
2
1
0.50
0.25
Bit [B7] : Photo sense VSD bias control
Bit [B7] : B7 = 0 3 VSD disconnected, photo sense function de ? activated
Bit [B7] : B7 = 1 3 VSD connected, photo sense function activated
Bits [B6:B5] : RFU
Bits [B4:B0] : Photo sensor gain adjust
The photo sensor no longer influences the I ? LED when the gain is setup to $00. On the other hand, the five bits can be
digitally combined to get different gain in the range 0 to 7.75.
http://onsemi.com
13
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