(

)

TRIP

SENSE

VREF

I

5

R

=

´

DRV8828

SLVSA11G – OCTOBER 2009 – REVISED NOVEMBER 2015

www.ti.com

Typical Application (continued)

8.2.1 Design Requirements

For this design example, use the parameters listed in Table 3

Table 3. Design Parameters

PARAMETER

REFERENCE

VALUE

Supply Voltage

VM

24 V

Motor Winding Resistance

RM

3.9 Ohm

Motor Winding Inductance

LM

2.9 mH

Target Chopping Current

ITRIP

1.5 A

Sense Resistor

RSENSE

200 m

Ω

VREF Voltage

VREF

1.5 V

8.2.2 Detailed Design Procedure

8.2.2.1 Current Regulation

The maximum current (ITRIP) is set by the Ix pins, the VREF analog voltage, and the sense resistor value

(RSENSE). When starting a brushed DC motor, a large inrush current may occur because there is no back-EMF

and high detent torque. Current regulation will act to limit this inrush current and prevent high current on start-up.

(2)

Example - If the desired chopping current is 1.5 A:

•

Set RSENSE = 200 mΩ

•

VREF would have to be 1.5 V

•

Create a resistor divider network from V3P3OUT (3.3 V) to set VREF = 1.5 V

•

Set R2 = 10 k

Ω and set R1 = 12 kΩ

8.2.2.2 Sense Resistor

For optimal performance, it is important for the sense resistor to be:

•

Surface-mount

•

Low inductance

•

Rated for high enough power

•

Placed closely to the motor driver

The power dissipated by the sense resistor equals Irms² x R. For example, if the RMS motor current is 1.5 A and

a 200 m

Ω sense resistor is used, the resistor will dissipate 1.5 A² × 0.2 Ω = 0.3 W. The power quickly increases

with greater current levels.

Resistors typically have a rated power within some ambient temperature range, along with a de-rated power

curve for high ambient temperatures. When a PCB is shared with other components generating heat, margin

should be added. It is always best to measure the actual sense resistor temperature in a final system, along with

the power MOSFETs, as those are often the hottest components.

Because power resistors are larger and more expensive than standard resistors, it is common practice to use

multiple standard resistors in parallel, between the sense node and ground. This distributes the current and heat

dissipation.

14

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