DRV8821

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SLVS912J – JANUARY 2009 – REVISED JANUARY 2016

9 Power Supply Recommendations

9.1 Bulk Capacitance

Having appropriate local bulk capacitance is an important factor in motor drive system design. It is generally

beneficial to have more bulk capacitance, while the disadvantages are increased cost and physical size.

The amount of local capacitance needed depends on a variety of factors, including:

•

The highest current required by the motor system.

•

The power supply's capacitance and ability to source current.

•

The amount of parasitic inductance between the power supply and motor system.

•

The acceptable voltage ripple.

•

The type of motor used (Brushed DC, Brushless DC, Stepper).

•

The motor breaking method.

The inductance between the power supply and motor drive system will limit the rate current can change from the

power supply. If the local bulk capacitance is too small, the system will respond to excessive current demands or

dumps from the motor with a change in voltage. When adequate bulk capacitance is used, the motor voltage

remains stable and high current can be quickly supplied.

The datasheet generally provides a recommended value, but system-level testing is required to determine the

appropriate sized bulk capacitor.

Figure 17. Example Setup of Motor Drive System with External Power Supply

The voltage rating for bulk capacitors should be higher than the operating voltage, to provide margin for cases

when the motor transfers energy to the supply.

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