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DRV880x device is used in medium voltage brushed DC motor control applications.
For this design example, use the parameters listed in Table 2 as the input parameters.
|DESIGN PARAMETER||REFERENCE||EXAMPLE VALUE|
|Motor Voltage||VBB||24 V|
|Motor RMS Current||IRMS||0.8 A|
|Motor Startup Current||ISTART||2 A|
|Motor Current Trip Point||ITRIP||2.5 A|
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.
The power dissipation of the DRV880x is a function of the RMS motor current and the each output’s FET resistance (RDS(ON)).
For this example, the ambient temperature is 35°C, and the junction temperature reaches 65°C. At 65°C, the sum of RDS(ON) is about 1Ω. With an example motor current of 0.8A, the dissipated power in the form of heat will be 0.8 A2x 1 Ω = 0.64 W.
The temperature that the DRV880x reaches will depend on the thermal resistance to the air and PCB. It is important to solder the device PowerPAD to the PCB ground plane, with vias to the top and bottom board layers, to dissipate heat into the PCB and reduce the device temperature. In the example used here, the DRV880x had an effective thermal resistance RθJA of 47°C/W, and:
When the voltage on pin SENSE exceeds VTRIP (0.5 V), overcurrent is detected. The RSENSE resistor should be sized to set the desired ITRIP level.
To set ITRIP to 2.5 A, RSENSE = 0.5 V / 2.5 A = 0.2 Ω.
To prevent false trips, ITRIP must be higher than regular operating current. Motor current during startup is typically much higher than steady-state spinning, because the initial load torque is higher, and the absence of back-EMF causes a higher voltage and extra current across the motor windings.
It can be beneficial to limit startup current by using series inductors on the DRV880x output, as that allows ITRIP to be lower, and it may decrease the system’s required bulk capacitance. Startup current can also be limited by ramping the forward drive duty cycle.
For optimal performance, it is important for the sense resistor to be:
This current path is through the high-side sourcing DMOS driver, motor winding, and low-side sinking DMOS driver. Power dissipation I2R loses in one source and one sink DMOS driver, as shown in Equation 5.