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3962 Datenblatt(PDF) 6 Page - Allegro MicroSystems |
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3962 Datenblatt(HTML) 6 Page - Allegro MicroSystems |
6 / 8 page 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 3962 DUAL FULL-BRIDGE PWM MOTOR DRIVER 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 hyster- esis 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: where VBB is the motor supply voltage, VBEMF is the back- EMF voltage of the load, RLOAD and LLOAD are the resis- tance 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 aforemen- tioned 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: where tOFF = RTCT, RLOAD is the series resistance of the load, VBB is the motor supply voltage and t ON(min)max is specified in the electrical characteristics table. When the motor is rotating, the back EMF generated will influence the above relationship. For brush dc motor applications, the current regulation is improved. For stepper motor applications when the motor is rotating, the effect is dependent on the polarity and magnitude of the motor’s back EMF. The following procedure can be used to evaluate the worst case internal PWM load current regulation in the system: Set VREF 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 “RC Blanking” above). Enable PWM. Toggling the ENABLE input turns ON and OFF the selected source and sink drivers. The corre- sponding 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 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 (VBB – [(ITRIP • RLOAD) + VBEMF]) tPWM LLOAD IOS ≈ 0.5 ITRIPmax ± 50% RS ≈ |
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Ähnliche Beschreibung - 3962 |
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