Local

Bulk Capacitor

Parasitic Wire

Inductance

+

±

Motor

Driver

Power Supply

Motor Drive System

VM

GND

+

IC Bypass

Capacitor

V B A T + 1 1 .5 V

V M <

2

DRV8701

SLVSCX5B – MARCH 2015 – REVISED JULY 2015

www.ti.com

8.2.3.2

VM Boost Voltage

To determine an effective voltage to boost VM, first determine the minimum VBAT at which the system must

operate. Select VM such that the gate driver clamps do not turn on during normal operation.

(10)

Example: If VBAT minimum is 4.0 V,

VM < 7.75 V

So VM = 7 V is selected to allow for adequate margin.

9 Power Supply Recommendations

The DRV8701 is designed to operate from an input voltage supply (VM) range between 5.9 and 45 V. A 0.1-µF

ceramic capacitor rated for VM must be placed as close to the DRV8701 as possible. In addition, the designer

must include a bulk capacitor with a valued of at least 10 µF on VM.

Bypassing the external H-bridge FETs requires additional bulk capacitance.

9.1 Bulk Capacitance Sizing

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 braking 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 41. 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.

32

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