LP8555 datasheet(13/64 Pages) TI1

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Duty Cycle D = 1 - VIN / VOUT

tSW = 1 / fSW

fSW = 500 or 1000 kHz

Slew Rate Control,

Programmable

Spread Spectrum Scheme,

Programmable Pseudo Random SW

Frequency Changes to Reduce EMI

Time

VBOOST

VOUT_A

Driver

Headroom

LP8555

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SNVS857 – FEBRUARY 2014

8.3.1.4 Adaptive Boost Output Voltage Control

The boost converters operate in adaptive voltage control mode in typical application. The voltages at the LED

terminals is monitored by the control loop. It raises the boost voltage when the measured voltage of ANY of the

LED strings in a bank falls below the voltage threshold of its corresponding LOW comparator. If the headrooms

of ALL of the LED strings in a bank are above the voltage threshold of their corresponding MID comparator, then

the boost voltage is lowered. Both banks have independent boost voltage control to save power in case of Vf

mismatch between LED strings.

The initial boost voltage is configured with the VINIT field. The VMAX field sets the maximum boost voltage.

When an LED terminal is open, the monitored voltage will never have enough headroom and the adaptive mode

control loop will keep raising the boost voltage. The VMAX field allows the boost voltage to be limited to stay

under the voltage rating of the external components.

Figure 21. Boost Adaptive Control Principle for Bank A Boost Converter With Phase Shifted Outputs

8.3.1.5 EMI Reduction

The LP8555 features three EMI reduction schemes.

First scheme, Programmable Slew Rate Control, uses a combination of four drivers for boost switch. Enabling all

four 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 node. It should also be noted that the shortest transition times bring the best

efficiency as the switching losses are the lowest. Same controls effect both boost converters.

The second EMI reduction scheme is the Spread Spectrum Scheme which deliberately spreads the frequency

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

waveform's noise spectrum bandwidth wider and ultimately reduces its EMI spectral density.

The third feature for reducing EMI is Phase Shifted Clocking mode, where boost converters’ clocks are operating

180° phase shifted. This prevents boost switches switching on at the same time when operating in PWM mode.

This reduces input rail load transient spikes caused by boost inductor current and gate driver currents.

Figure 22. Boost Converter EMI Reduction Schemes

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