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NCP5021
Figure 11. Typical Gradual Dimming DOWNWARD
Backlight Dimming
The built ? in I2C interface provides a simple way to
accurately control the output peak current flowing in the
LED. The internal register ILEDREG[7:0] is set up by the
content of the SDA byte send by the external MCU. For
Table 12. ILEDREG[0..7]
typical application, the 100 m A reference current forced by
the external resistor yields a 25 mA maximum in the output
LED. The waveforms given on illustrate a normal
programming sequence.
B7
B6
B5
B4
B3
B2
B1
B0
step
RESET
0
0
0
0
0
0
ILED16
0
ILED8
0
ILED4
0
ILED2
0
ILED1
0
[B7:B6] = Reserved for future use
[B4..B0] = Output LED mA/step current. The content of these bits is latched to the current reference on the 8 th SCL clock pulse.
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 activated or de ? activated 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:
1. upper I ? LED as defined by the en user : the photo
2. lower I ? LED as defined by the en 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 drive the
counter, yielding a smooth and slow brightness variation.
The 100 Hz noise, coming from the 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 15.
sense cannot increase the I ? LED above such a
limit
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