DRV8870_16 Datasheet, PDF (12/25 Pages) Texas Instruments – Brushed DC Motor Driver

DRV8870
SLVSCY8A – AUGUST 2015 – REVISED JANUARY 2016
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P_D I² R_DS(on)Source R_DS(on)Sink
(2)
The DRV8870 has been measured to be capable of 2-A RMS current at 25°C on standard FR-4 PCBs. The max
RMS current varies based on the PCB design, ambient temperature, and PWM frequency. Typically, switching
the inputs at 200 kHz compared to 20 kHz causes 20% more power loss in heat.
8.2.2.3 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 I_RMS2 × R. For example, if peak motor current is 3 A, RMS
motor current is 1.5 A, and a 0.2-Ω sense resistor is used, the resistor will dissipate 1.5 A² × 0.2 Ω = 0.45 W.
The power quickly increases with higher current levels.
Resistors typically have a rated power within some ambient temperature range, along with a derated power curve
for high ambient temperatures. When a PCB is shared with other components generating heat, the system
designer should add margin. It is always best to measure the actual sense resistor temperature in a final system.
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.
8.2.3 Application Curves
Figure 8. Current Ramp With a 2-Ω, 1 mH,
RL Load and VM = 12 V
Figure 9. Current Ramp With a 2-Ω, 1 mH,
RL Load and VM = 24 V
12
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