LP8556 datasheet(17/52 Pages) TI

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LP8556

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SNVS871B – JULY 2012 – REVISED SEPTEMBER 2012

Brightness Control

LP8556 enables various methods of brightness control. The brightness can be controlled using an external PWM

signal or the Brightness register accessible by users via an I2C interface or both. How these two input sources

are selected and combined is set by the BRT_MODE EPROM bits and described in the following sections,

Figure 20, and Table 2. The LP8556 can also be preconfigured via EPROM memory to allow direct and

unaltered brightness control by an external PWM signal. This mode of operation is obtained by setting

PWM_DIRECT EPROM bit to '1' (CFG5[7] = 1).

BRT_MODE = 00

With BRT_MODE = 00, the LED output is controlled by the PWM input duty cycle. The PWM detector block

measures the duty cycle at the PWM pin and uses this 16-bit value to generate an internal to the device PWM

data. Before the output is generated, the PWM data goes through the PWM Curve Shaper block. Then, the data

goes into the Adaptive Dimming function which determines the range of the PWM and Current control as

described in OUTPUT DIMMING SCHEMES. The outcome of the Adaptive Dimming function is 12-bit Current

and / or up to 6 PWM output signals. The current is then passed through the non-linear compensation block

while the output PWM signals are channeled through the Dither block.

BRT_MODE = 01

With BRT_MODE = 01, the PWM output is controlled by the PWM input duty cycle and the Brightness register.

The PWM detector block measures the duty cycle at the PWM pin and uses this 16-bit value to generate the

PWM data. Before the output is generated, the PWM data is first multiplied with BRT[7:0] register, then it goes

through the PWM Curve Shaper block. Then, the data goes into the Adaptive Dimming function which

determines the range of the PWM and Current control as described in OUTPUT DIMMING SCHEMES . The

outcome of the Adaptive Dimming function is 12-bit Current and / or up to 6 PWM output signals. The current is

then passed through the non-linear compensation block while the output PWM signals are channeled through the

Dither block.

BRT_MODE = 10

With BRT_MODE = 10, the PWM output is controlled only by the Brightness register. From BRT[7:0] register, the

data goes through the PWM Curve Shaper block. Then, the data goes into the Adaptive Dimming function which

determines the range of the PWM and Current control as described in OUTPUT DIMMING SCHEMES . The

outcome of the Adaptive Dimming function is 12-bit Current and / or up to 6 PWM output signals. The current is

then passed through the non-linear compensation block while the output PWM signals are channeled through the

Dither block.

BRT_MODE = 11

With BRT_MODE = 11, the PWM control signal path is similar to the path when BRT_MODE = 01 except that the

PWM input signal is multiplied with BRT[7:0] data after the Curve Shaper block.

Table 2. Brightness Control Methods Truth Table

PWM_DIRECT

BRT_MODE [1:0]

Brightness Control Source

Output ILED Form

External PWM Signal

External PWM Signal and Brightness Register

(multiplied before Curve Shaper)

Adaptive. See OUTPUT

DIMMING SCHEMES

Brightness Register

External PWM Signal and Brightness Register

(multiplied after Curve Shaper)

Same as the external

don't care

External PWM Signal

PWM input

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