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STPM098C データシート(PDF) 81 Page - STMicroelectronics |
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STPM098C データシート(HTML) 81 Page - STMicroelectronics |
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81 / 110 page  • 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 0 Closed-loop feedforward disabled (default) 1 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 0 0 Maximum ON time correction = ±100% TON (default) 0 1 Maximum ON time correction = ±0% TON 1 0 Maximum ON time correction = ±25% TON 1 1 NO maximum ON time correction STPM098C Functional description DS14440 - Rev 1 page 81/110 |
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