LP8555 datasheet(21/64 Pages) TI1

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Brightness (PWM)

Time

tSlope Timet

Advanced Slope

Brightness (PWM)

Sloper Input

PWM Output

Time

Normal Slope

LP8555

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SNVS857 – FEBRUARY 2014

Figure 32. Sloping Principle

8.3.4 Start-up and Shutdown Sequences

Depending on brightness control mode the LP8555 can be started up or shut down differently. Below are

explained typical start-up/shutdown sequences with corresponding timings for operation states. Diagrams have

more details and illustrated waveforms for typical usage cases.

8.3.4.1 Start-up With PWM Input Brightness Control Mode (BRTMODE = 00b)

When VDD and EN/VDDIO are above min operational value the LP8555 enters start-up mode. During start-up

mode the LDO is started, and EPROM values are read. I2C is available after the start-up sequence has ended.

In standby mode the device waits for the ON bit to go high to start the boost start-up sequence. In standby mode

PWM input duty cycle measurement is active.

Once the ON bit is set to 1 (it can be also programmed to 1 by default in EPROM, and no I2C write is then

needed for entering active mode), boost is started, and device enters active mode with brightness set by PWM

input duty cycle. If no brightness is set, the backlight stays off until two PWM pulses are received in the PWM

input, or if PWM input is set high for more than 1/75 Hz time. Boost starts initially to the level programmed in

EPROM, and after backlight is turned on the adaptive control adjusts the voltage to get to the minimal headroom

voltage.

8.3.4.2 Shutdown With PWM Input Brightness Control Mode (BRTMODE = 00b)

The backlight can be turned off by setting PWM input low or by writing the ON bit low. After a 1/75Hz timeout

period in the PWM input, the backlight slopes down (if slope is enabled), and boost is returned to the initial

voltage level programmed to the EPROM. If the backlight is shut down with the ON bit, it shuts down immediately

even if slopes are enabled and boost turns off as well. To enter standby mode where boost is disabled and the

power consumption is minimal, the ON bit must be written to 0. If PWMSB bit has been programmed to 1, then

the LP8555 enters standby mode when PWM input has been low for more than 50 ms even if the ON bit is high.

The device shuts down completely by setting EN/VDDIO and/or VDD to low state.

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