10.1 Layout Guidelines

The bulk capacitor should be placed to minimize the distance of the high-current path through the motor driver device. The connecting metal trace widths should be as wide as possible, and numerous vias should be used when connecting PCB layers. These practices minimize inductance and allow the bulk capacitor to deliver high current.

Small-value capacitors should be ceramic, and placed closely to device pins.

The high-current device outputs should use wide metal traces.

The device thermal pad should be soldered to the PCB top-layer ground plane. Multiple vias should be used to connect to a large bottom-layer ground plane. The use of large metal planes and multiple vias help dissipate the I² × RDS(on) heat that is generated in the device.

10.2 Layout Example

Figure 20. Typical Layout of DRV8823-Q1

10.3 Thermal Considerations

The DRV8823-Q1 device has thermal shutdown (TSD) as described above. If the die temperature exceeds approximately 150°C, the device is disabled until the temperature drops to a safe level.

Any tendency of the device to enter thermal shutdown is an indication of either excessive power dissipation, insufficient heatsinking, or too high an ambient temperature.

10.3.1 Power Dissipation

Power dissipation in the DRV8823-Q1 device is dominated by the power dissipated in the output FET resistance, or R_DS(ON). Average power dissipation when running a stepper motor can be roughly estimated by Equation 3.

Equation 3.

Where: P_TOT is the total power dissipation, R_DS(ON) is the resistance of each FET, and I_OUT(RMS) is the RMS output current applied to each winding. I_OUT(RMS) is equal to approximately 0.7x the full-scale output current setting. The factor of 4 is derived from the two motor windings, and at any instant two FETs are conducting winding current for each winding (one high-side and one low-side). The DRV8823-Q1 device has two stepper motor drivers, so the power dissipation of each must be added together to determine the total device power dissipation.

The maximum amount of power that can be dissipated in the DRV8823-Q1 device is dependent on ambient temperature and heatsinking. The thermal dissipation ratings table in the datasheet can be used to estimate the temperature rise for typical PCB constructions.

Note that R_DS(ON) increases with temperature, so as the device heats, the power dissipation increases. This must be taken into consideration when sizing the heatsink.

10.3.2 Heatsinking

The PowerPAD integrated circuit package uses an exposed pad to remove heat from the device. For proper operation, this pad must be thermally connected to copper on the PCB to dissipate heat. On a multi-layer PCB with a ground plane, this can be accomplished by adding a number of vias to connect the thermal pad to the ground plane. On PCBs without internal planes, copper area can be added on either side of the PCB to dissipate heat. If the copper area is on the opposite side of the PCB from the device, thermal vias are used to transfer the heat between top and bottom layers.

For details about how to design the PCB, refer to TI application report SLMA002, PowerPAD™ Thermally Enhanced Package and TI application brief SLMA004, PowerPAD™ Made Easy, available at www.ti.com.

In general, the more copper area that can be provided, the more power can be dissipated. Figure 21 shows thermal resistance versus copper plane area for both a single-sided PCB with 2-oz copper heatsink area, and a 4-layer PCB with 1-oz copper and a solid ground plane. Both PCBs are 76 mm x 114 mm, and 1.6 mm thick. The heatsink effectiveness increases rapidly to about 20 cm², then levels off somewhat for larger areas.

Six pins on the center of each side of the package are also connected to the device ground. A copper area can be used on the PCB that connects to the PowerPAD integrated circuit package as well as to all the ground pins on each side of the device, which is especially useful for single-layer PCB designs.

Figure 21. Thermal Resistance vs Copper Plane Area