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TD340ID Datasheet(PDF) 8 Page - STMicroelectronics |
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TD340ID Datasheet(HTML) 8 Page - STMicroelectronics |
8 / 21 page TD340 8/21 Figure 2 : PWM Analog and Digital Modes Active (synchronous) rectification for free-wheel current A motor is an inductive load. When driven in PWM mode, motor current is switched on and off at the 25kHz frequency. When the MOS is switched off, current can not instantaneously drop to zero, a so-called ”free-wheel” current arises in the same direction than the power current. A path for this current must be provided, otherwise high voltage could arise and destroy the component. The classical way to handle this situation is to connect a diode in an anti-parallel configuration regarding to the MOS, so that current can continue to flow through this diode, and finally vanishes by the means of ohmic dissipation, mainly in the diode due to its 0.8V direct voltage. For high currents, dissipation can be an important issue (eg: 10A x 0.8V makes 8 W!). Furthermore, high speed diodes have to be used, and are expensive. A more efficient way to handle this problem is to use the high side MOS as a synchronous rectifier. In this mode, the upper MOS is switched ON when the lower one is switched OFF, and carries the free-wheel current with much lower ohmic dissipation. Advantages are : one expensive component less (the fast power diode), and more reliability due to the lower dissipation level. However, we have to take care not to drive the two MOS simultaneously. To avoid transient problems when the MOS are switched, a deadtime is inserted between the opening of one MOS, and the closing of the other one. In the TD340 device, the deadtime is fixed to about 2.5 microseconds. This value is the time between the commands of the gate drivers, not the deadtime between the actual MOS states because of the rising and falling times of the gate voltages (due to capacitance), and the MOS characteristics. The actual value of the deadtime for a typical configuration is about 1.5 microseconds. Figure 3 shows the synchronous rectification principle Table 1 summarizes the status of the Mosfets (and the speed and direction of the motor) according to the Inputs (IN1 and IN2) status in analog and logic modes. M µP TD340 IN1 CF PWM PWM 0V 5V M µP TD340 IN1 PWM PWM 0V 5V ANALOG INPUT + CF (270pF) DIGITAL INPUT + CF GROUNDED PWM OUTPUT PWM OUTPUT CF Vbatt Vbatt |
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