ZHCSE25 August 2015 DRV8870
PRODUCTION DATA.
8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
8.1 Application Information
The DRV8870 is typically used to drive one brushed DC motor.
8.2 Typical Application
Figure 7. Typical Connections
8.2.1 Design Requirements
Table 3 lists the design parameters.
Table 3. Design Parameters
| DESIGN PARAMETER | REFERENCE | EXAMPLE VALUE |
|---|---|---|
| Motor voltage | VM | 24 V |
| Motor RMS current | IRMS | 0.8 A |
| Motor startup current | ISTART | 2 A |
| Motor current trip point | ITRIP | 2.2 A |
| VREF voltage | VREF | 3.3 V |
| Sense resistance | RISEN | 0.15 Ω |
| PWM frequency | fPWM | 5 kHz |
8.2.2 Detailed Design Procedure
8.2.2.1 Motor Voltage
The motor voltage to use will depend on the ratings of the motor selected and the desired RPM. A higher voltage spins a brushed DC motor faster with the same PWM duty cycle applied to the power FETs. A higher voltage also increases the rate of current change through the inductive motor windings.
8.2.2.2 Drive Current
The current path is through the high-side sourcing DMOS power driver, motor winding, and low-side sinking DMOS power driver. Power dissipation losses in one source and sink DMOS power driver are shown in the following equation.
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 will vary based on PCB design and the ambient temperature.
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 IRMS2 × 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 10. Current Ramp With a 2-Ω, 1 mH, RL Load and VM = 45 V
Figure 12. Current Regulation With VREF = 2 V and 150 mΩ
Figure 9. Current Ramp With a 2-Ω, 1 mH, RL Load and VM = 24 V
Figure 11. tPD
Figure 13. OCP With 45 V and the Outputs Shorted Together