DRV8840_15 Datasheet, PDF (15/28 Pages) Texas Instruments – DC Motor Driver IC

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DRV8840
SLVSAB7D – MAY 2010 – REVISED DECEMBER 2015
Typical Application (continued)
8.2.1 Design Requirements
Table 3 shows the design parameters for this application.
DESIGN PARAMETER
Supply Voltage
Motor Winding Resistance
Motor Winding Inductance
Target Chopping Current
Sense Resistor
VREF Voltage
8.2.2 Detailed Design Procedure
Table 3. Design Parameters
REFERENCE
VM
RM
LM
ITRIP
RSENSE
VREF
EXAMPLE VALUE
24 V
3.9 V
2.9 mH
1.5 A
100 mΩ
0.75 V
8.2.2.1 Current Regulation
The maximum current (I_TRIP) is set by the Ix pins, the VREF analog voltage, and the sense resistor value
(R_SENSE). 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.
I_TRIP / (5 × R_SENSE)
(2)
Example: If the desired chopping current is 1.5 A:
• Set RSENSE = 100 mΩ
• VREF would have to be 0.75 V
• Create a resistor divider network from V3P3OUT (3.3 V) to set VREF = 0.75 V
• Set R2 = 10 kΩ and set R1 = 3 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 × R. For example, if the RMS motor current is 1.5 A and
a 200-mΩ 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.
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