LP8556 datasheet(16/52 Pages) TI

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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

SNVS871B – JULY 2012 – REVISED SEPTEMBER 2012

www.ti.com

EMI REDUCTION

The LP8556 features two EMI reduction schemes.

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 terminal. It should also be noted 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 Boost Converter Electrical Characteristics(2) for the slew rate options.

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 bandwidth wider and ultimately reduces its EMI spectral density.

Figure 19. Principles of EMI Reduction Schemes

(2)

Min and Max limits are guaranteed by design, test, or statistical analysis. Typical numbers are not guaranteed, but do represent the

most likely norm.

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LP8556 Datasheet(PDF) 16 Page - Texas Instruments