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NCP5021

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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

typical application, the 100

mA 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.

Table 12. ILEDREG[0..7]

step

ILED16

ILED8

ILED4

ILED2

ILED1

RESET

[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 8th 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

sense cannot increase the I−LED above such a

limit

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.

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