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A3964 Datasheet(PDF) 6 Page - Allegro MicroSystems |
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A3964 Datasheet(HTML) 6 Page - Allegro MicroSystems |
6 / 7 page Dual Full-Bridge PWM Motor Driver A3964 6 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com The following procedure can be used to evaluate the worst case internal PWM load current regulation in the system: set VREF(IN) to 0 volts. With the load connected and the PWM current control operating in slow decay mode, use an oscilloscope to measure the time the output is low (sink on) for the output that is chopping. This is the typical minimum on time (tON(min)typ) for the device. The CT then should be increased until the measured value of tON(min) is equal to tON(min)max as specified in the electrical characteristics table. When the new value of CT has been set, the value of RT should be decreased so the value for tOFF = RTCT (with the artificially increased value of CT) is equal to the nominal design value. The worst-case load-current regulation then can be measured in the system under operating conditions. PWM of the Phase and Enable Inputs. The PHASE and ENABLE inputs can be pulse width modulated to regulate load current. Typical propagation delays from the PHASE and ENABLE inputs to transitions of the power outputs are specified in the electrical characteristics table. If the internal PWM current control is used, the comparator blanking function is active during phase and enable transitions. This eliminates false tripping of the over-current comparator caused by switching transients (see the RC Blanking section, above). Enable PWM. Toggling the ENABLE input turns on and off the selected source and sink drivers. The corresponding pair of flyback and ground clamp diodes conduct after the drivers are disabled, resulting in fast current decay. When the device is enabled the internal current control circuitry will be active and can be used to limit the load current in a slow current-decay mode. Phase PWM. Toggling the PHASE terminal selects which sink/ source pair is enabled, producing a load current that varies with the duty cycle and remains continuous at all times. This can have added benefits in bidirectional brush dc servo motor applications as the transfer function between the duty cycle on the PHASE input and the average voltage applied to the motor is more linear than in the case of ENABLE PWM control (which produces a discontinuous current at low current levels). Miscellaneous Information. An internally generated dead time prevents crossover currents that can occur when switching phase. Thermal protection circuitry turns off all drivers should the junction temperature reach +165°C (typical). This is intended only to protect the device from failures due to excessive junction temperatures and should not imply that output short circuits are permitted. The hysteresis of the thermal shutdown circuit is approximately 15°C. APPLICATION NOTES Current Sensing. The actual peak load current (IPEAK) will be above the calculated value of ITRIP due to delays in the turn off of the drivers. The amount of overshoot can be approximated by: (VBB – [(ITRIP x RLOAD) + VBEMF]) x tPWM LLOAD IOS ≈ where VBB is the motor supply voltage, VBEMF is the back-EMF voltage of the load, RLOAD and LLOAD are the resistance and inductance of the load respectively, and t PWM is specified in the electrical characteristics table. To minimize current sensing inaccuracies caused by ground trace IR drops, each current-sensing resistor should have a separate return to the ground terminal of the device. For low-value sense resistors, the IR drops in the PCB can be significant and should be taken into account. The use of sockets should be avoided as their contact resistance can cause variations in the effective value of RS. Generally, larger values of RS reduce the aforementioned effects but can result in excessive heating and power loss in the sense resistor. The selected value of RS should not cause the absolute maximum voltage rating of 1.0 V, for the SENSE terminal, to be exceeded. The recommended value of RS is in the range of: 0.5 ITRIPmax RS ≈ ± 50% If desired, the reference input voltage can be filtered by placing a capacitor from REFIN to ground. The ground return for this capacitor as well as RB should be independent from the high- current power-ground trace to avoid changes in REFIN due to IR drops. Thermal Considerations. For reliable operation, it is recommended that the maximum junction temperature be kept below 110°C to 125°C. The junction temperature can be measured best by attaching a thermocouple to the power tab/ batwing of the device and measuring the tab temperature, TTAB . The junction temperature can then be approximated by using the formula: TJ ≈ TTAB + (2 ILOAD VF RθJT) where VF can be chosen from the electrical specification table for the given level of ILOAD. The value for RθJT is approximately 6°C/W. The power dissipation of the batwing packages can be improved by 20 to 30% by adding a section of printed circuit board copper (typically 6 to 18 square centimeters) connected to the batwing terminals of the device. |
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