A3984: DMOS Microstepping Driver with Translator

The A3984 is a complete microstepping motor driver with built-in translator for easy operation. It is designed to operate bipolar stepper motors in full-, half-, quarter-, and sixteenth-step modes, with an output drive capacity of up to 35 V and ±2 A. The A3984 includes a fixed off-time current regulator which has the ability to operate in slow or mixed decay modes.

The translator is the key to the easy implementation of the A3984. Simply inputting one pulse on the STEP input drives the motor one microstep. There are no phase sequence tables, high frequency control lines, or complex interfaces to program. The A3984 interface is an ideal fit for applications where a complex microprocessor is unavailable or is overburdened.

The chopping control in the A3984 automatically selects the current decay mode (Slow or Mixed). When a signal occurs at the STEP input pin, the A3984 determines if that step results in a higher or lower current in each of the motor phases. If the change is to a higher current, then the decay mode is set to Slow decay. If the change is to a lower current, then the current decay is set to Mixed (set initially to a fast decay for a period amounting to 31.25% of the fixed off-time, then to a slow decay for the remainder of the off-time). This current decay control scheme results in reduced audible motor noise, increased step accuracy, and reduced power dissipation.

Internal synchronous rectification control circuitry is provided to improve power dissipation during PWM operation.

Internal circuit protection includes: thermal shutdown with hysteresis, undervoltage lockout (UVLO), and crossover-current protection. Special power-on sequencing is not required.

The A3984 is supplied in a low-profile (1.2 mm maximum), 24-pin TSSOP with exposed thermal pad (package LP). It is also available in a lead (Pb) free version (suffix –T), with 100% matte tin plated leadframes.

DATASHEETS
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Astrorep Inc. (Allegro and Sanken Semiconductors, Sanken Power Supplies)
141 John Street, Suite 300
Babylon, NY 11702
Phone: (631)422-2500
Fax: (631)422-2504
Web: www.astrorepinc.com
Email: donna.tweedy@astrorepinc.com

If this is not your local representative, find your local sales rep here.

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  • Small 24-pin eTSSOP package (7.8 x 4.4 x 1.2 mm)
    Less than one-third the PC board space of our closest competitor. In many applications PC board space is tight, so smaller is better. Thermal resistance: RθJA 28C/W.
  • STEP and DIRECTION interface
    Two control lines vs. eight, 10 or more control lines for competing devices. Additional I/O lines can add to system cost. Commutation tables are not required. Customer can switch to other Allegro Step/Direction interface devices without changing software. Common interface for all applications.
  • Synchronous rectification
    Reduces power dissipation and can eliminate need for external Schottky diodes. Typically a 20% power savings.
  • Low Ron DMOS outputs
    Reduces power dissipation. Can achieve higher average output current due to the lower die temperature rise.
  • Automatic slow, fast and mixed decay
    Improved microstepping accuracy and reduced audible noise, an Allegro-patented feature.
  • 2 A peak output current
    The 2 A rating is 0.5 A above most competitors. Additional safety margin for high-reliability designs.
  • Less than 10 mA sleep mode current
    New designs are requiring low current draw when disabled. Most bipolar competitors to the A3983/84 have 10 to 1000 times the current draw when disabled.
  • Competitive price
    The A3983/84 can be priced to compete with any stepper motor driver with similar output ratings.

The A3984 is supplied in a low-profile (1.2 mm maximum), 24-pin TSSOP with exposed thermal pad (package LP). It is also available in a lead (Pb) free version (suffix –T), with 100% matte tin plated leadframes.

LP TSSOP 24 pin

Frequently Asked Questions


Q1: Is there an application note for the A3982/83/84?

Q2: What type of microstepping can be performed with the A3982/83/84?

Q3: Are there drop-in, or pin-for-pin replacements for the A3982/83/84?

Q4: What is the advantage of having the translator and driver in one package?

Q5: Do I need pull-up/pull-down resistors on the input pins?

Q6: What type of protection features do the A3982/83/84 offer?

Q7: What is the maximum allowable motor supply voltage?

Q8: The datasheet states an output current of ±2.0 A. Is this total current for the device, or current per phase?

Q9: What type of external components are required?

Q10: What is the best way to determine a value for RS, so as not to exceed 0.5 V on the SENSE pin?

Q11: Are there any layout concerns I should be aware of?

Q12: Are the A3982/83/84 constant-current or constant-voltage controlled devices?

Q13: What is the recommended minimum copper ground plane area for reducing power dissipation at high currents?

Q14: Are there special techniques to reduce the package power dissipation when running at high currents?

Q15: Are the A3982/83/84 capable of being used in portable applications?

Q16: Can you change the step resolution while running?

Q17: What does automatic current decay mode detection/selection mean?



Q1: Is there an application note for the A3982/83/84?

At this time, no. However, there is one for the A3977, which is a similar device in the Step and Direction family. The difference would be in the output current (A3977 = 2.5 A and the 3982/83/84 = 2.0 A peaks). See 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 A3982/83/84?

The A3982 offers full and half step modes, the A3983 offers full, half, quarter, and eighth step modes, and the 3984 offers full, half, quarter, and sixteenth step modes.


Q3: Are there drop-in, or pin-for-pin replacements for the A3982/83/84 ?

No. The A3982/83/84 are generally a more cost-effective solution than most typical motor driver ICs. These devices, as well as the entire Step and Direction family, can replace two or more devices in many designs, thereby reducing overall 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 A3983/83/84 can be controlled using 2 control lines: step and direction. 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?

If the logic inputs are pulled up to VDD, it is good practice to use a 1 to 5 kΩ pull-up resistor in order to limit current to the logic inputs, should an overvoltage event occur.


Q6: What type of protection features do the A3982/83/84 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 ±2.0 A. Is this total current for the device, or current per phase?

The value ±2 A defines the maximum current that each phase of the driver can support continuously. This is independent of temperature rise.

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 A3982/83/84:

  • RSENSE1 and RSENSE2, the external sense resistors required for the PWM current control circuit. These should be noninductive type resistors. The value for RS can be calculated using the formula: ITRIP(max) = VREF / (8 x RS). When selecting a value for RS it is very important not to exceed the 0.5 V limit on the SENSE pin over the full expected current range. For very short time durations during switching, transient voltages larger than 0.5 V may be observed. Using a reasonably smaller value for RS will dissipate less power in RS and provide headroom.
  • A 0.1 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.
  • A logic supply decoupling capacitor; a 10 µF ceramic capacitor is recommended.
  • A load supply decoupling capacitor; a value of > 47 µF electrolytic capacitor is recommended. In addition, a 0.1 µF ceramic capacitor should be placed in parallel.
  • A 0.1 µF capacitor is required on the VREF pin.

Q10: What is the best way to determine a value for Rs, so as not to exceed 0.5 V on the SENSE pin?

By using the formula:

RS = 0.5 / ITRIP(max),

where:

  • RS is the sense resistor,
  • 0.5 is the absolute maximum allowable voltage on the SENSE pin, and
  • ITRIP(max) is the maximum expected current.

This will ensure that the 0.5 V limit on the SENSE pin is never exceeded.


Q11: Are there any layout concerns I should be aware of?

Yes. The printed circuit board should use a heavy groundplane. For optimum electrical and thermal performance, the exposed pad on the underside of the device provides a path for enhanced thermal dissipation. The thermal pad should be soldered directly to an exposed surface on the PCB. Thermal vias are used to transfer heat to other layers of the PCB.

In order to minimize the effects of ground bounce and offset issues, it is important to have a low impedance singlepoint ground, known as a star ground, located very close to the device. By making the connection between the exposed thermal pad and the groundplane directly under the device, that area becomes an ideal location for a star ground point.

The two input capacitors (electrolytic and ceramic) should be placed in parallel, and as close to the device supply pins as possible. The ceramic capacitor should be closer to the pins than the bulk capacitor. This is necessary because the ceramic capacitor will be responsible for delivering the high frequency current components.

The sense resistors, RSx, should have a very low impedance path to ground, because they must carry a large current while supporting very accurate voltage measurements by the current sense comparators. Long ground traces will cause additional voltage drops, adversely affecting the ability of the comparators to accurately measure the current in the windings.


Q12: Are the A3982/83/84 constant-current or constant-voltage controlled devices?

The A3982/83/84 provide constant-current control. Motor winding current is controlled by an internal PWM current-control circuit. Off-time is set by a resistor from the ROSC pin to ground and is defined by the formula: tOFF  = ROSC / 825. If the ROSC pin is tied directly to VDD, the off-time defaults to 30 µs.


Q13: 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.

NOTE: The datasheet also defines RθJA with various copper areas. See page 4 of the datasheet.


Q14: Are there special techniques to reduce the package power dissipation when running at high currents?

In a typical stepper-motor application, the motor driver IC is in current-decay (recirculation) mode for a higher percentage of the PWM cycle compared to the on-time. This means that most of the power dissipation is a result of the forward-voltage drop of the internal body diode of the power DMOS. However, the A3982/83/84 offers synchronous rectification (SR). This feature turns on the appropriate DMOS devices during current decay and effectively shorts out the body diodes with the low RDS(on) of the driver. The power dissipation reduction in the SR feature can eliminate the need for external Schottky diodes in most stepper-motor applications, thereby saving the cost and board space for these components. Heat sinks are also a possibility. For additional information, please refer to application note AN29504.8, Power Drive Circuits.


Q15: Are the A3982/83/84 capable of being used in portable applications?

Absolutely. The A3983/84 has Sleep mode, which minimizes power consumption when not in use. During Sleep mode, the device will only draw a maximum of 10 µA. The logic supply voltage range of 3.0 to 5.5 V makes it compatible with typical battery operated equipment.


Q16: 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 must 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.). This is only important if position must be maintained or if there is no position control loop.

It should be mentioned that changing the MSx pins at any time will not cause damage to the device. To keep the motor moving at a constant speed while changing sequencing modes, the step frequency must 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 by 11 eighth-steps. This would make keeping the motor at constant speed very tricky.


Q17: What does automatic current decay mode detection/selection mean?

The A3982/83/84 automatically selects 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. 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 sets both bridges to mixed-decay.

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Part Number Specifications and Availability

Part Number Package Type Temperature RoHS
Compliant
Part Composition /
RoHS Data
Comments Samples Check
Distributor Stock
A3984SLPTR-T 24-lead TSSOP -20°C to 85°C Yes View Data Contact your
local sales rep
Check Stock
APEK3984SLP-01-T DEMO BOARD -20°C to 85°C No -- Contact your
local sales rep
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