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ML4428CP Datasheet(PDF) 10 Page - Micro Linear Corporation |
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ML4428CP Datasheet(HTML) 10 Page - Micro Linear Corporation |
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10 / 16 page  10 ML4428 CSNS A capacitor to ground at this pin sets the ON time of the 6 current sense pulses used for position detection at start-up and at low speeds. The ON time is set by: TON = CSNS (35.7k) Referring to Figure 5, each of the 6 current sense pulses is governed by a rise time with a time constant of L/R where L is the inductance of the motor network with 2 windings shorted and R is the total resistance in series with the motor between the supply rails. R includes the ON-resistance of the power-FETs and RSNS. The RDSON of the high side FET should match that of the low side FET. L is a function of rotor position. Each pulse will have a peak value VSENSEPEAK of VR V R SENSEPEAK SNS MOTOR T e ON LR =− − 1 / where RR R R LL L L SDON SENSE LL =× + × ()+ =× − − 075 2 075 . . What is important for sensing rotor position is the amplitude difference between each of the three pairs of current sense pulses. This can be seen by triggering on ISNS on an oscilloscope with the RCVCO pin shorted to ground. One should see the current waveform of Figure 5. Allowing the peak current sense pulse to reach an amplitude of 0.5V (by adjusting CSNS, and hence TON) or, allowing the difference between the maximum and minimum of the 6 pulses to be >50mV, should suffice for adequate rotor position sensing. A good starting value for TON is 200µs, requiring CSNS = 5.6nF. RINIT The initial time interval between sample pulses during start-up is set by RINIT. This time interval (tINIT) occurs while the RCVCO pin is less than 0.25 volts. R t C INIT INIT VCO = 343 . AB C D E F A 3.75V CVCO 2.0V VCO OUT Figure 6. Commutation Timing and Sequencing. DIRECTION OUTPUTS INPUT SAMPLES STATE REVERSE N3 N2 N1 P3 P2 P1 FORWARD N1 N2 N3 P1 P2 P3 FORWARD REVERSE A OFF OFF ON ON OFF OFF PH2 PH2 B OFF OFF ON OFF ON OFF PH1 PH3 C ON OFF OFF OFF ON OFF PH3 PH1 D ON OFF OFF OFF OFF ON PH2 PH2 E OFF ON OFF OFF OFF ON PH1 PH3 F OFF ON OFF ON OFF OFF PH3 PH1 Table 1. Commutation States. |
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