A3977 Frequently Asked Questions
Q1 - Is there an application note for the A3977?
Yes, Application Note STP01-2, "A New Microstepping Motor-Driver IC with Integrated Step & Direction Translator Interface".
Q2: What type of microstepping can be performed with the A3977?
The A3977 offers full, half, quarter, and eighth step mode operations.
Q3 - Is the A3977 a drop-in, or pin-for-pin replacement, for any other device?
No. The A3977 is generally a more cost-effective solution than most typical motor driver ICs. This single device can replace two or more devices in many designs, thereby reducing system cost.
Q4 - What is the advantage of having the translator and driver in one package?
The advantage of having the translator and driver in one package is minimizing the number of control lines. The A3977 can be controlled using two control lines, where typical motor drivers can have as many as 6 to 8 control lines to achieve the same functionality.
Q5 - Do I need pull-up/pull-down resistors on the input pins?
Not necessarily. The inputs can be tied directly to Vdd or ground, depending on the logic level you desire. If pull-up/pull-down resistors are required for your particular design, 1k ohm resistors are recommended.
Q6 - What type of protection features does the A3977 offer?
Thermal shutdown (TSD)
Undervoltage lockout (UVLO)
Crossover current protection
Vreg and charge pump monitors
Q7 - What is the maximum allowable motor supply voltage?
35 V. This must not be exceeded under any circumstances.
Q8 - The datasheet states an output current of ±2.5 A. Is this total current for the device, or current per phase?
The output current rating is current per phase. Caution should be taken to never exceed a junction temperature of 150°C when running the device.
Q9 - What type of external components are required?
The following components are required for correct operation of the A3977:
Rsense1 and Rsense2 - The external sense resistors required for the PWM current control circuit. These should be noninductive type resistors. Recommended maximum Rs value can be calculated using Rs = 0.5 / Itripmax. Using a reasonably smaller value for Rs will dissipate less power in Rs and provide headroom. There also needs to be a 0.1 µF mono/ceramic capacitor in parallel with each of these resistors.
A 0.22 µF mono/ceramic capacitor must be placed between the CP1 and CP2 pins.
The VREG pin should be decoupled with a 0.22 µF capacitor to ground.
Logic supply decoupling capacitor. A 0.1 µF ceramic capacitor is recommended.
Decoupling capacitor - load supply. A value of > 47 µF electrolytic capacitor is recommended. In addition, a 0.1 µF ceramic capacitor should be placed in parallel, if high frequency issues are a concern.
A 0.1 µF capacitor is required on the PFD (Percent Fast Decay) pin.
Q10 - Are there any layout concerns I should be aware of?
Yes. The sense resistor, Rs, should be connected as close as possible to the device. The ground side of Rs should return on a separate trace to the ground pin(s) of the device. The ground traces at the device should be as large as physically possible. A 47 µF or larger electrolytic decoupling capacitor should be placed between the load supply pins and ground and be placed as close as physically possible to the device.
Q11 - Is the A3977 a constant-current or constant-voltage controlled device?
The A3977 provides constant-current control. Motor winding current is controlled by an internal PWM current-control circuit, which incorporates an external RC circuit to set the fixed off-time.
Q12 - What is the recommended minimum copper ground plane area for reducing power dissipation at high currents?
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 "Package Thermal Characteristics".
Q13 - Are there special techniques to reduce the package power dissipation when running at high currents?
Use of external Schottky diodes with low Vforward, to clamp the outputs to VBB and ground, will help to reduce the power dissipation in the A3977. Heat sinks are also a possibility, but not as efficient. Note: When using external diodes, synchronous rectification should be disabled to achieve maximum results.
Q14 - Is there an application note on the use of external diodes?
There is no application note about using external diodes for the A3977. Each of the outputs should have one Schottky diode connected to VBB (cathode to VBB). Each output should also have one Schottky diode connected to ground (anode to ground, not to the sense pins). When full-step mode is used, or if PFD is set to slow decay only, then use only the four Schottky diodes between the outputs and ground. The four Schottky diodes from the outputs to VBB will not help improve thermal performance in slow decay mode.
Q15 - 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.
Q16 - Is the A3977 capable of being used in portable applications?
Absolutely. The A3977 has Sleep mode, which minimizes power consumption when not in use. During Sleep mode, the device will only draw a maximum of 20 µA. The logic supply voltage range of 3.0 V to 5.5 V makes it compatible with typical battery operated equipment.
Q17 - Can you change the step resolution while running?
Yes, as long as the timing requirements are met. The easiest way to change sequencing modes, to higher or lower resolution, is to do it at the HOME position (HOME is low). Otherwise, when going from a lower resolution to a higher resolution mode (half-step to quarter-step, etc.) both sequences have identical output currents. (Both sequences fall on the same row of table 2 in the datasheet.) The translator will keep the output current levels unchanged until the next step, at which time it will begin the smaller steps.
To keep the motor moving at a constant speed while changing sequencing modes, the step frequency will need to be multiplied by 2, 4, or 8, depending on the modes you jump from and to. Going from a higher-resolution mode to a lower-resolution mode should only be done when both modes appear on the same row of table 2 from the datasheet (Eighth Step #5 and Full Step #1, etc.).
To keep the motor moving at a constant speed while changing sequencing modes, the step frequency will need to be divided by 2, 4, or 8, depending on the modes you jump from and to. If you go from a higher resolution mode to a lower resolution mode and do it at a position that is not a valid possibility for the lower resolution mode in Table 2 of the datasheet, then the sequencer will advance to first possibility without actually changing the output currents. When the next step arrives, the device will go from the position the translator was at before the sequencing mode was changed, to the next position of the new sequencing mode. For example, with direction low, if you changed from Eighth Step mode to Full Step mode when you were at Eighth Step #2, the translator will advance to Full Step #2 (but not change the output currents). When the next step occurs, the position will go to Full Step #3. The effect would be that the motor would move 11 eighth steps. This would make keeping the motor at constant speed very tricky.
Q18 - What does automatic current decay mode detection/selection mean?
The A3977 will automatically select the decay modes suitable for optimum performance. If the output current at the previous step was higher than the output current for the present step, then the PFD pin controls the decay mode (falling current, moving towards zero). If the output current at the previous step was lower than the output current for the present step, then the decay mode is fixed to slow decay (rising current, away from zero). When first powering-up the device, coming out of reset, or coming out of Sleep mode, the device will set both bridges to mixed-decay (PFD controls the decay mode).