LP8543

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SNVS604D – AUGUST 2009 – REVISED MARCH 2013

B. PWM Input Pin Control (Duty Cycle-based)

An external PWM signal can be used to set the brightness of the Display1 group. LP8543 measures the duty

cycle of this input signal to calculate the output PWM value. Input PWM frequency can vary from 100 Hz to 25

kHz. Output PWM frequency is set by EEPROM registers.

Note: In this mode, Ambient Light compensation and PSPWM scheme can be also used.

C. PWM Control Using Brightness Register

Generation of PWM for LED strings can be based on Brightness register value. For Display1 group, this scheme

is enabled when <PWM_SEL> bit is set to 0 and <PWM_MD> is set to 1. Display2 group has the brightness

register control enabled by default. Two separate 8-bit registers Displ1 brightness and Displ2 brightness store

the brightness values for Display1 and Display2 respectively. For Display1, this 8-bit brightness value from the

register is converted to 10-bit LED PWM value using a three-part piecewise linear transfer curve as shown

below. This makes it possible to calibrate the 100% brightness and the dimming behavior. The curve coefficients

are stored in the EEPROM and are user programmable if needed. The LED PWM frequency is set by EEPROM

register.

Note: In this mode, Ambient Light compensation and PSPWM scheme can be also used.

Figure 12. Three-Segment Transfer Curve Example

D. PWM Pin and Register Control

In this mode, PWM control pin can linearly reduce the brightness of Display1 from the value set by the

Brightness Register and Ambient Light sensor. Same controls can be used as in brightness register based PWM

control. Output PWM frequency is set by EEPROM registers. This mode is compatible with Intel DPST (Display

Power Saving Technology).

Stand Alone Mode

PWM input pins are the only controls for the device. To enable stand-alone mode, EEPROM bit

<EN_STANDALONE> must be set to 1 in register B4h. In this mode PWM pin sets the brightness and with EN

pin the backlight can be turned on. When PWM or EN input pin is permanently low, the chip is turned off. Turn off

time is typically 50 ms. When there is signal in PWM input and EN is high, the chip turns on and adjusts

brightness according to PWM signal duty cycle. All settings needed for operation like LED current, number of

LEDs etc. are obtained from EEPROM. If only one signal control is needed, the EN and PWM pin can be tied

SMBus control is not possible or available to use.

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