9.2.2.1 Power Dissipation

Average power dissipation in the DRV8842-EP when running a DC motor can be roughly estimated by Equation 2.

Equation 2.

where

• P is the power dissipation of one H-bridge
• R_DS(ON) is the resistance of each FET
• I_OUT is the RMS output current being applied to each winding

I_OUT is equal to the average current drawn by the DC motor. Note that at start-up and fault conditions, this current is much higher than normal running current; also consider the peak currents and their duration. The factor of 2 is due to two FETs conducting winding current (one high side and one low side) at any instant.

The maximum amount of power that can be dissipated in the device depends on ambient temperature and heatsinking.

Note that R_DS(ON) increases with temperature, so as the device heats, the power dissipation increases. Take this into consideration when sizing the heatsink.

9.2.2.2 Current Regulation Considerations

For the DRV8842-EP, the set full-scale current (I_FS) is the maximum current that can be driven. This quantity depends on the VREF analog voltage and the sense resistor value (R_SENSE). The gain of DRV8842-EP is set for 5 V/V. This value can be adjusted from 0% to 100% through the use of the relative current bits I[4:0].

Equation 3.

To achieve I_FS = 3.5 A with R_SENSE of 0.1 Ω, V_(VREF) should be 1.75 V, and I[4:0] should be 0x1F.

9.2.2.3 Slow, Fast, and Mixed Decay Modes

The DRV8842-EP supports three different decay modes: slow decay, fast decay, and mixed decay. The current through the motor winding is regulated using a fixed-frequency PWM scheme. This means that after any drive phase, when a motor winding current has hit the current chopping threshold (I_TRIP), the DRV8842-EP places the winding in one of the three decay modes until the PWM cycle has expired. After, a new drive phase starts.

The blanking time, T_BLANK, defines the minimum drive time for the current chopping. I_TRIP is ignored during T_BLANK, so the winding current may overshoot the trip level.