DFE252012F-R47M datasheet(48/122 Pages) AD

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Table 13. Phase Sequence (continued)

3+1

PH1(M): 0

⁰

PH2(S): 240

⁰

PH3(S): 120

⁰

PH4(M): 270

⁰

PH1(M): Rising Edge

PH2(S): 240

⁰ from M1

PH3(S): 120

⁰ from M1

PH4(M): Falling Edge

PH1(M): 0

⁰

PH2(S): 180

⁰

PH3(S): 90

⁰

PH4(S): 270

⁰

PH1(M): Rising Edge

PH2(S): 180

⁰ from M1

PH3(S): 90

⁰ from M1

PH4(S): 270

⁰ from M1

Spread-Spectrum Modulation

The bucks are capable of dithering its switching frequency for noise-sensitive applications. The spread-spectrum

function of each buck is individually enabled by setting the Mx_SS_ENV[1:0] bits. The spread-spectrum function

is activated only in continuous conduction mode (CCM) and it is automatically deactivated when the bucks enter

discontinuous conduction mode (DCM). The spread-spectrum modulation pattern is programmable either in pseudo-

random or triangular patterns by the Mx_SSM_PAT[1:0] bits. The spread-spectrum modulation is characterized by

modulation envelope and modulation frequency:

● The modulation envelope (∆FSS) determines the maximum difference between the modulated switching frequency

and the nominal switching frequency. The modulation envelope is programmable (±8%, ±12%, or ±16%) with the

Mx_SS_ENV[1:0] bits and it controls how wide the switching frequency dithers.

● The modulation frequency (FSS_MOD) determines how often the switching frequency changes from one value to

another. The modulation frequency is also programmable (1kHz, 3kHz, 5kHz, or 7kHz) with the Mx_SS_FREQ[1:0]

bits and it controls how fast the switching frequency dithers.

Pseudo-Random Pattern

The pseudo-random engine uses a 4-bit linear feedback shift register (LFSR) to create a pseudo-random value as shown

in Figure 8. The LFSR value is converted to an analog signal and then amplified before being added to the output of the

on-time generator circuit. The pseudo-random value shortens or lengthens the on-time. This causes the buck controller

to increase or decrease the switching frequency to maintain voltage regulation. Each buck has its own pseudo-random

pattern generator.

CLOCK

LINEAR FEEDBACK

SHIFT REGISTER

GAIN

DAC

REF

PSEUDO-RANDOM

MODULATION SIGNAL

Mx_SSM_PAT[1:0]

DECODE

ENVELOPE

RATE

Mx_SS_ENV[1:0]

Mx_SS_FREQ[1:0]

Figure 8. Pseudo-Random Modulator Engine

The modulation envelope and frequency are programmable with the Mx_SS_ENV[1:0] and the Mx_FREQ[1:0] bits. The

FSS_MOD sets the frequency at which the LFSR wraps back to the seed value. The clock rate of the LFSR is the FLFSR.

This is the frequency at which one pseudo-random value changes to another. An example is shown in Figure 9.

MAX77542

16VIN/16A, Quad-Phase High-Efficiency Buck

Converter

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