LED string

1

LED string

2

LED string

3

LED string

4

LED string

5

LED string

6

Time

New PWM

value

LP8543

SNVS604D – AUGUST 2009 – REVISED MARCH 2013

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Ambient Light Compensation

LP8543 supports an external ambient light sensor to control the backlight brightness (Display1) and its usage is

controlled with two bits in the Config2 register, namely <ALSO_EN> and <ALSO_CALC_EN>. <ALSO_EN> bit

controls enabling/disabling of the sensor itself, and <ALSO_CALC_EN> bit determines whether the ALS

measurement data will be used by an external processor (Host) or by LP8543’s internal control logic to control

the brightness.

If <ALSO_EN> bit is 1 the ALSO output pin is set high and the input frequency measuring is enabled. Frequency

is measured for 500 ms, and the result is divided with 10-bit prescaler (defined in EEPROM), resulting in a 10-bit

value. This 10-bit result can be read from ALS MSB and ALS LSB registers. ALS MSB register must be read

first followed by ALS LSB register. If ALS_CALC_EN bit is set to 0, then the measurement data is not used by

LP8543’s internal PWM logic but left for the host to adjust the brightness.

On the other hand if the ALS_CALC_EN bit is set to 1, ALS measurement result will control backlight brightness

in all but direct external PWM control mode. The measured ALS value is converted to PWM value using a three

segment linear curve. The calculated PWM value is used as a multiplier for the LED PWM value obtained from

brightness register, PWM input pin or combination of both depending which mode is selected. The conversion

curve parameters are stored in EEPROM memory. Conversion curve is similar as in PWM control.

Smoothing filter is used to prevent rapid changes. Smoothing filter has EEPROM programmable slopes from 0 to

2s. The slope defines the time it takes to change brightness from one value to next. Slope control can be also

used to smooth changes to backlight brightness caused by other PWM controls (brightness register or external

PWM input).

Table 3. Slope Selections

SLOPE_SEL[1:0]

Slope

00

130 ms

01

0.5s

10

1.0s

11

2s

ALSO output can be used as GPO if not used for ALS control. ALSO pin state is then controlled with

<ALSO_EN> register bit.

Phase Shift PWM (PSPWM)

PSPWM improves the system efficiency by optimizing the boost converter voltage on a cycle by cycle basis

Display1 group. Phase shift PWM control principle is illustrated in the picture below using an example of 6 string

implementation and 41.7% brightness setting. In a 6-string implementation, each of the string supports a

maximum of 16.67% (1/6) of the total backlight brightness. The brightness set value in this example is 41.7%.

Hence two strings are fully on (2 x 16.67% = 33.33%) and one string is 50% on (0.5 x 16.67% = 8.34%). This

pattern of two 100% and one 50% strings is then cycled through all 6 output strings. After 6 cycles the brightness

value is changed to 83.33%, resulting in 5 LEDs fully on (5 x 16.67%).

Figure 13. Principle of the PSPWM Operation

18

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