LP8556 datasheet(19/52 Pages) TI

19 / 52 page

25%

100%

Max current is set with CURRENT and CURRENT_MAX EPROM bits

or CURRENT and CURRENT_MAX EPROM bits and RISET resistor

25%

50%

100%

PURE CURRENT CONTROL

(PWM_TO_I_THRESHOLD = 0000b)

BRIGHTNESS

50%

25%

50%

100%

PWM CONTROL

(PWM_TO_I_THRESHOLD = 1111b)

BRIGHTNESS

Max current is set with CURRENT and CURRENT_MAX EPROM bits

or CURRENT and CURRENT_MAX EPROM bits and RISET resistor

100%

LP8556

www.ti.com

SNVS871B – JULY 2012 – REVISED SEPTEMBER 2012

OUTPUT DIMMING SCHEMES

The LP8556 supports three types of output dimming control methods: PWM Control, Pure Current Control and

Adaptive Dimming (Hybrid PWM & Current) Control.

PWM Control

PWM control is the traditional way of controlling the brightness using PWM of the outputs with a same LED

current across the entire brightness range. Brightness control is achieved by varying the duty cycle proportional

to the input PWM. PWM frequency is set either using an external set Resistor (RFSET) or using the PWM_FREQ

EPROM field. The maximum LED current is set either using an external set Resistor ( RISET) and CURRENT and

CURRENT_MAX EPROM bits or just using the CURRENT and CURRENT_MAX EPROM bits. Note that the

output PWM signal is de-coupled and generated independent of the input PWM signal eliminating display flicker

issues and allowing better noise immunity

Figure 21. PWM Only Output Dimming Scheme

Pure Current Control

In Pure Current Control mode, brightness control is achieved by changing the LED current proportionately from

maximum value to a minimum value across the entire brightness range. Like in PWM Control mode, the

maximum LED current is set either using an external set Resistor ( RISET) and CURRENT and CURRENT_MAX

EPROM bits or just using the CURRENT and CURRENT_MAX EPROM bits. Current resolution in this mode is

12-bits.

Figure 22. Pure Current / Analog Output Dimming Scheme

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