Full, half, quarter, one-eighth, one-sixteenth, and one-thirty-second step modes are possible. In addition, the onboard linear DAC allows current to be programmed in 1.56% increments of the maximum value 100%/64.

Q2: What is the function of the MUX pin? (pin 14)

It is used by Allegro for testing during production. It should NOT be used in actual applications.

Q3: Is the A3972 available in a surface-mount package?

This device is currently available only in a 24-lead plastic "batwing" DIP package. However, the A3992, the next generation version of the A3972, will be released in January 2006. That will be available in a 24-pin TSSOP package with exposed thermal pad. In addition, the A3973, a pin-compatible 30 V version of the A3972, is available in an SOIC package.

Q4: What is the difference between the "Sleep" and "Idle" modes?

Sleep mode disables all circuits, including UVLO, and resets the serial port to all zeros. Idle mode disables all circuits except UVLO.

Q5: Are there applications tools available for this device?

Yes. We now have an applications board that will help simplify a new design using the A3972. Click here to visit the Allegro On-Line store, where this board can be purchased.

Q6: What is the maximum allowable motor supply voltage?

50 V. This must not be exceeded.

Q7: Do I need any external components?

Yes. The following components are required:

Sense₁ & Sense₂ - the external sense resistors. These should be
non-inductive. There also needs to be a 0.1 µF ceramic capacitor in
parallel with each of these resistors.
A 0.22 µF ceramic capacitor must be placed between the CP1 and CP2 pins.
V_REG - This should be decoupled with a 0.22 µF capacitor to ground.
Decoupling capacitor - logic supply. A 0.01 µF ceramic capacitor is recommended.
Decoupling capacitor - load supply. A 47 µF electrolytic capacitor is recommended. In addition, a 0.1 µF ceramic capacitor should be placed in parallel with this decoupling cap.

Q8: What protection features come with the A3972?

Thermal shutdown (TSD)
Crossover current protection
Under-voltage lockout (UVLO)
Charge pump undervoltage monitor

Q9: Are there layout concerns that I should be aware of?

Yes. Power ground for the sense resistor, Rs, should be connected as close to the device as possible. The ground side of the sense resistor should return on a separate trace to the ground pin(s) of thes device. Also, Rs should be non-inductive, and the circuit board ground run should be as large as physically possible. Finally, a 47 µF electrolytic decoupling capacitor should be placed between the load supply pins and ground. For further layout considerations, please refer to application note 29501.5A, "Improving Batwing Power Dissipation."

Q10: Is this a constant-current or constant-voltage controlled device?

Constant-current. Motor winding current is controlled by internal fixed-offtime, PWM current-control circuitry.

Q11: Are there special techniques to reduce the package power dissipation when running at high currents?

Use of external Schottky diodes with low V_forward to clamp the outputs to VBB and ground will help to reduce the power dissipation in the A3972. Heat sinks are also a possibility, but not as efficient. Note: When using external diodes, synchronous rectification should be disabled to achieve maximum results. For additional information, please refer to the following on the Allegro website: "Power Drive Circuits" at http://www.allegromicro.com/en/Products/Design/an/AN295048.pdf.

Q11.1: Is there an application note on the use of external diodes?

There is no application note about using external diodes for the A3972. In most applications, each of the outputs should have one Schottky diode connected to VBB (cathode to VBB), and also one Schottky diode connected to ground (anode to ground, not to the sense pins).

Q11.2: Do you have a recommended Schottky diode?

We typically don't recommend a specific diode due to the range of voltages and currents that can be used. The V/I rating of the diode should be greater than or equal to the application requirements.

Q12: What is the recommended minimum copper ground plane area for taking heat away from the device?

A ground plane area at least two times larger than the package outline is a good place to start. For further layout considerations, please refer to application note 29501.5A, "Improving Batwing Power Dissipation."

Q13: Is there an applications note for the A3972?

Not at this time, but one is planned for the future.

Q14: If more than 19 bits are clocked into the serial port, what happens to the "excess" bits?

Like typical shift registers, the most recent bits are the ones that are retained. As an example, if 24 bits are clocked in , the last 19 are the ones that will be used.

Q15: Can Word 0 bit assignments overwrite Word 1 bit assignments?

No. Since Word 0 and Word 1 are separate, they cannot write to each other or over each other. The appropriate word has to be selected to be overwritten.

Q16: If I use the internal oscillator what do I do with the external oscillator pin?

Pull it up to V_DD through a 51 kW resistor or ground it. It should be noted that accuracy will be improved if the 51 kW resistor to V_DD is used.

Q17: If the internal V_REF is used, what do I do with the external V_REF?

Let it float.

Q18: Is the A3972 capable of being used in portable applications?

Yes. The A3972 has Sleep mode, which minimizes power consumption when the device is not in use. During Sleep mode, the device only draws a maximum of 100 µA. The logic supply voltage range of 3.3 to 5.5 V makes it compatible with some battery operated equipment.

Q19: Are there recommended setup functions for Word 0 and Word 1?

Each application is different. For this reason, there isn’t a recommended setup. However, when evaluating the A3972, care should be taken to ensure that the motor will start up and run. This can be achieved by choosing (addressing) the Word 0 and Word 1 bit assigments for a mid-range setting. Specifically, set the following bits initially:

Word 0	Word 1
D1 through D6 D7 through D12	D1 and D2 D3 through D7 D8 through D12

This configuration allows the motor to start and run. After it is running, adjust the bit assignments slightly for optimum performance.

High-Performance Power ICs and Hall-Effect Sensors

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FAQs - A3972 Dual DMOS Full-Bridge Microstepping PWM Motor Driver