LP8556 datasheet(15/63 Pages) TI1

15 / 63 page

Duty cycle D = 1 - VIN / VOUT

tSW = 1/fSW

Slew rate control,

programmable

Spread spectrum scheme,

programmable pseudo

random duty cycle changes

minimize EMI

LP8556

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SNVS871I – JULY 2012 – REVISED MARCH 2016

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8.3.1.4 EMI Reduction

The LP8556 device features two EMI reduction schemes.

The first scheme, Programmable Slew Rate Control, uses a combination of three drivers for boost switch.

Enabling all three drivers allows boost switch on/off transition times to be the shortest. On the other hand,

enabling just one driver allows boost switch on/off transition times to be the longest. The longer the transition

times, the lower the switching noise on the SW pin. Note that the shortest transition times bring the best

efficiency as the switching losses are the lowest.

EN_DRV2 and EN_DRV3 bits in the EPROM determine the boost switch driver configuration. Refer to the SW

pin slew rate parameter listed under Electrical Characteristics — Boost Converter for the slew rate options.

The second EMI reduction scheme is the spread spectrum. This scheme deliberately spreads the frequency

content of the boost switching waveform, which inherently has a narrow bandwidth, makes the bandwidth of the

switching waveform wider, and ultimately reduces its EMI spectral density.

Figure 6. Principles of EMI Reduction Scheme

8.3.2 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 BRT_MODE = 00 through

BRT_MODE = 11, Figure 7, 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).

8.3.2.1 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.

8.3.2.2 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.

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