STPM098C datasheet(82/110 Pages) STMICROELECTRONICS

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3.14.6.3

PWM spread spectrum modulation

STPM098C PWM modulator implements a frequency modulation (spread spectrum modulation) feature to reduce

the power stages emissions around the main frequency by spreading the power spectrum over a larger frequency

range. PWM SSM can be enabled by a dedicated PMBus™ bit as follows:

Table 66. PWM SSM enable bit

BUCK_CLK_SSM_EN

Description

PWM SSM disabled (default)

PWM SSM enabled

The implemented mechanism makes use of FLL to introduce a linear variation on the ON time to be converted as

a period modulation by the main control loop.

Figure 66. PWM SSM modulation operation

TON

TON_NOMINAL

SS_PWM_STEP

SS_PWM_AMPL

PWM1

PWM2

PWM3

The PWM SSM parameter can be programmed by means of the dedicated PMBus™ commands:

SSPWMAMPL=SSMlsb×SSPWMAMPL15:0

(48)

SSPWMSTEP=SSMlsb×SSPWMSTEP14:0

(49)

Where:

SSM_lsb is the minimum timing step [25 ns/128].

Default values are SS_PWM_AMPL = 0x0000 and SS_PWM_STEP = 0x0000

PWM SSM operation makes use of FLL structure; as a consequence, to enable buck SSM, FFL shall be enabled

as well. PWM SSM is effective in steady state conditions. During transients events (load, VIN, VID etc.) frequency

is determined by the main control loop. PWM SSM varies the steady state frequency periodically, as a

consequence an enhanced voltage ripple on output node shall be expected.

STPM098C

Functional description

DS14440 - Rev 1

page 82/110

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