STPM098C datasheet(80/110 Pages) STMICROELECTRONICS

80 / 110 page

3.14.6

PWM modulator

PWM modulator macrosection is a halfway analog/digital section responsible for generating the PWMn signal

aligned to the compensated voltage according to the COT scheme.

3.14.6.1

Digitally-controlled oscillator (DCO) and interleaver

The PWM modulator core is based on a digitally controlled oscillator (DCO). The oscillator output is used as a

time-base to trigger the PWM ON-pulses of each phase. This triggering pulse is then multiplexed to each phase

by means of a shift register thus accomplishing the phase interleaving.

The carrier frequency can be programmed through a dedicated PMBus™ register as follows:

Table 63. PWM period selection bits

TSWITCH

PWM period [μs]

PWM frequency [MHz]

TSWITCH

PWM period [μs]

PWM frequency [MHz]

0.200

1.675

0.597

4.45

0.225

1.6

0.625

0.250

1.55

0.645

3.625

0.276

1.475

0.678

3.325

0.301

1.425

0.702

3.075

0.325

1.375

0.727

2.85

0.351

1.325

0.755

2.675

0.374

1.3

0.769

2.5

0.400

1.25

0.800

2.35

0.426

1.1

0.909

2.225

0.449

1.000

2.1

0.476

0.9

1.111

2 (default)

0.500 (default)

0.825

1.212

1.9

0.526

0.775

1.290

1.825

0.548

0.725

1.379

1.75

0.571

0.675

1.481

3.14.6.2

ON time generator

PWM signals to be delivered to each phase are composed starting from the DCO signal on which the computed

ON time is applied.

To safeguard the external power stage from wrong operation due to PWM marginality, STPM098C allows to

introduce a limitation over the minimum ON and OFF time applied on PWM pins.

The minimum ON/OFF time limitation can be programmed by means of the dedicated PMBus™ command

MFR_T_MIN [7:0].

TOFFMIN=TMINlsb*MFRTMIN7:4

(43)

TONMIN=TMINlsb*MFRTMIN3:0

(44)

Where:

T_MIN _lsb is the minimum timing step [25 ns].

Default values are:

T_OFF_MIN = 0x02 (50 ns)

T_ON_MIN = 0x03 (75 ns)

No internal limitation is applied to programmed MFR_T_MIN: this means that if the configured value is zero, then

no MIN TON/TOFF is applied.

STPM098C allows two possible methods for the controller to compute the nominal PWM ON time:

•

Feedforward;

STPM098C

Functional description

DS14440 - Rev 1

page 80/110

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