STPM098C datasheet(81/110 Pages) STMICROELECTRONICS

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Fixed ON time

Feedforward

The feedforward method can be enabled through a dedicated PMBus™ register as follows:

Table 64. Closed-loop feedforward enable bit

FEED_FORWARD_EN

Description

Closed-loop feedforward disabled (default)

Closed-loop feedforward enabled

With this method, the control loop automatically computes the PWM nominal ON time based on direct

measurement of input voltage through the VIN ADC conversion.

TON=VOUTVINTPWM

(45)

Where:

Vout is the programmed output voltage.

VIN is the input voltage measured on POWERIN_P.

TPWM is the programmed PWM period.

Fixed ON time

The fixed method is active if feedforward is disabled. With this method the nominal ON time is fixed and defined

by means of a dedicated PMBus™ register as follows:

FEEDFORWARDCONST9:0=1.22mV×219

VINTYP

(46)

Where:

FEED_FORWARD_CONST [9:0] are the dedicated PMBus™ configuration bits [0 ~ 1024].

The default value is FEED_FORWARD_CONST = 0x000 (corresponds to nominal on time of 3Tck).

As a COT-based architecture the PWM frequency variation is used to compensate for the load current variation so

that it is not inherently a constant frequency scheme.

To overcome this drawback the nominal ON time defined by the main control loop is adjusted by a second loop in

order to lock the steady state frequency of the PWM signal to the programmed value (frequency locked loop).

The FLL compares the difference between DCO period and the period programmed in TSWITCH divided by N

(number of active phases), and proportionally changes the delay line control thus adjusting ON time to lock the

PWM frequency in steady conditions.

The FLL control gain can be configured by means of a dedicate PMBus™ command as follows:

FLLCOEFF=ΔLSB×FLLCOEF9:0

(47)

Where:

ΔLSB is the minimum slew rate step [1].

FLL_COEFF [9:0] is the dedicated PMBus™ configuration bits [0 ~ 1024].

In case FLL_COEFF = 0x000, the frequency locked loop function is disabled and, as a consequence, the PWM

frequency may deviate from the programmed one.

The default value is FLL_COEFF = 0x002.

The FF maximum correction can be configured as well by means of a dedicated PMBus™ command as follows:

Table 65. FLL limit configuration bits

FLL_LIMIT_STRAT1

FLL_LIMIT_STRAT0

Description

Maximum ON time correction = ±100% TON (default)

Maximum ON time correction = ±0% TON

Maximum ON time correction = ±25% TON

NO maximum ON time correction

STPM098C

Functional description

DS14440 - Rev 1

page 81/110

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