LP3971 datasheet(21/57 Pages) TI1

21 / 57 page

SCL

SDA

data

change

allowed

data valid

data

change

allowed

data valid

data

change

allowed

2040

2060

2080

2100

2120

2140

-60

-50

-40

-30

-20

-10

FREQUENCY (kHz)

LP3971

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SNVS432U – JANUARY 2006 – REVISED OCTOBER 2008

SPREAD SPECTRUM FEATURE

Periodic switching in the buck regulator is inherently a noisier function block compared to an LDO. It can be

challenging in some critical applications to comply with stringent regulatory standards or simply to minimize

interference to sensitive circuits in space limited portable systems. The regulator’s switching frequency and

harmonics can cause “noise” in the signal spectrum. The magnitude of this noise is measured by its power

spectral density. The power spectral density of the switching frequency, FC, is one parameter that system

designers want to be as low as practical to reduce interference to the environment and subsystems within their

products. The LP3971 has a user selectable function on chip, wherein a noise reduction technique known as

“spread spectrum” can be employed to ease customer’s design and production issues.

The principle behind spread spectrum is to modulate the switching frequency slightly and slowly, and spread the

signal frequency over a broader bandwidth. Thus, its power spectral density becomes attenuated, and the

associated interference electro-magnetic energy is reduced. The clock used to modulate the LP3971 buck

regulator can be used as a spread spectrum clock via 2 I2C control register (System Control Register 1 (SCR1)

8h’80) bits bk_ssen, and slomod. With this feature enabled, the intense energy of the clock frequency can be

spread across a small band of frequencies in the neighborhood of the center frequency. This results in a

reduction of the peak energy!

The LP3971 spread spectrum clock uses a triangular modulation profile with equal rise and fall slopes. The

modulation has the following characteristics:

•

The center frequency: FC = 2 MHz, and

•

The modulating frequency, fM = 6.8 kHz or 12 kHz.

•

Peak frequency deviation:

Δ_f = ±100 kHz (or ±5%)

•

Modulation index

β = Δ_f/fM = 14.7 or 8.3

Figure 6. Switching Energy RBW = 300 Hz

2C Compatible Interface

2C DATA VALIDITY

The data on SDA line must be stable during the HIGH period of the clock signal (SCL). In other words, state of

the data line can only be changed when CLK is LOW.

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