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A3958 Datasheet(PDF) 8 Page - Allegro MicroSystems |
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A3958 Datasheet(HTML) 8 Page - Allegro MicroSystems |
8 / 12 page 3958 DMOS FULL-BRIDGE PWM MOTOR DRIVER 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 8 APPLICATIONS INFORMATION Current Sensing. To minimize inaccuracies in sensing the ITRIP current level, which may be caused by ground trace IR drops, the sense resistor should have an independent ground return to the ground terminal of the device. For low-value sense resistors the IR drops in the PCB sense resistor’s traces can be significant and should be taken into account. The use of sockets should be avoided as they can introduce variation in RS due to their contact resistance. The maximum value of RS is given as RS ≤ 0.5/ITRIP. Braking. The braking function is implemented by driving the device in slow-decay mode via serial port bit D13, enabling synchronous rectification via bit D12, and chopping with the combination of D14 and the ENABLE input terminal. Because it is possible to drive current in either direction through the DMOS drivers, this configuration effectively shorts out the motor-generated BEMF as long as the ENABLE chop mode is asserted. It is important to note that the internal PWM current-control circuit will not limit the current when braking, because the current does not flow through the sense resistor. The maximum brake current can be approximated by VBEMF/ RL. Care should be taken to ensure that the maximum ratings of the device are not exceeded in worst-case braking situations of high speed and high inertial loads. Thermal Protection. Circuitry turns off all drivers when the junction temperature reaches 165 °C typically. It is intended only to protect the device from failures due to excessive junction temperatures and should not imply that output short circuits are permitted. Thermal shutdown has a hysteresis of approximately 15 °C. Layout. The printed wiring board should use a heavy ground plane. For optimum electrical and thermal perfor- mance, the driver should be soldered directly onto the board. The ground side of RS should have an individual path to the ground terminals of the device. This path should be as short as is possible physically and should not have any other components connected to it. It is recom- mended that a 0.1 µF capacitor be placed between SENSE and ground as close to the device as possible; the load supply terminal, VBB, should be decoupled with an electrolytic capacitor (> 47 µF is recommended) placed as close to the device as is possible. |
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