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L6280 Scheda tecnica(PDF) 9 Page - STMicroelectronics |
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L6280 Scheda tecnica(HTML) 9 Page - STMicroelectronics |
9 / 26 page Pwm Current Control Loop The current control is achieved big a cycle of charge (TON) and discharge (TOFF) of the energy stored in each couple of windings of the driven motor (MA and MB). Fig. 8 shows the windings MA of an unipolar stepper motor during TON. FF1 is setted by the clock pulse and the transistor QA is ON. At the moment Q1is ON the current expo- nentially increases until RS x IP equals VREF. A reset pulse is produced, QA is switched OFF and Q2 is switched ON (Fig. 9). Since the mag- netic flux 0MA =NA IP cannot suddenly change and since the coil tourus number in the discharge loop is doubled, the peak current IP modifies it self into IP/2. The OFF time is characteryzed by a slow recirculation of the current IP/2 that de- creases until a new clock pulse sets a new TON configuration. To control the current in two sepa- rate windings MA and MB with just one sense re- sistor RS and one comparator, a special PWM control loop based on a ”time sharing” technique (Patented) is used (Fig. 10). In this configuration the chopping frequency, that defines the TON +TOFF period of each phase, is halved by FF3 that drives ON G1 and G2 alter- nately. During TOFF of one winding, for instance MA (and QA is OFF), its current does not flow throught the sensing resistor that can be used to monitor the current that flows through the second winding MB, allowed by the ON-status of QB. Fig. 11 shows a simplified timing before and dur- ing the phase change from AB to AB (CCW, full step). It can be seen that before the time t1,IA and IB are alternately controlled in a chopping pe- riod Tch1 of 4 oscillator periods or two clock peri- ods. The time sharing is 50% - 50% and the chopping frequency is typically of 20KHz (fosc = 80KHz). Afther the time t1, as soon as IA is sensed, a dif- ferent time sharing is generated. In fact since a Reset pulse is last after one clock pulse, FF2 can drive FF3 to change for IB chopping only at the next clock pulse (Fig 10; Fig 11). This means that the chopping time becomes Tch2 = 6 oscillator pulses, the frequency decreases to 16.6KHz (fosc = 80 KHz) and the time sharing be- comes of 67% - 33%. At the end of the phase change period tphc the time sharing comes back to 50% - 50% again. It can be noted that this behaviour allows a faster phase change and then a higher speed of the motor. The cost of that, is the increase of the TOFF of the unchanged phase B and then a small increase of the ripple of the current IB (see ∆IB1 < ∆IB2 in Fig. 11). This time sharing current control method is also used when two indipendent load are driven by one single channel. when only one load is pre- sent, such as a DC motor could be, the time shar- ing is automatically switched OFF and the PWM frequency becomes fosc/4 = 20KHz. Table 1 shows how the reference voltage can be modified with a three bits DAC to allow microstepping op- erations (see below). Figure 8 - TON Configuration: Motor Windings MA (A; A). Figure 9 - TOFF Configuration: Motor Windings MA (A: A). L6280 9/26 |
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