The IPROPI pin outputs an analog current proportional to the current flowing through the low-side power MOSFETs in the H-bridge scaled by A_IPROPI. The IPROPI output current can be calculated by Equation 1. The I_LSx in Equation 1 is only valid when the current flows from drain to source in the low-side MOSFET. If current flows from source to drain or through the body diode, the value of I_LSx for that channel is zero. For instance, if the bridge is in the brake, slow-decay state, then the current out of IPROPI is only proportional to the current in one of the low-side MOSFETs.

The A_ERR parameter in the Electrical Characteristics table is the error associated with the A_IPROPI gain. It indicates the combined effect of offset error added to the I_OUT current and gain error.

The motor current is measured by an internal current mirror architecture on the low-side FETs which removes the need for an external power sense resistor as shown in Figure 8-4. The current mirror architecture allows for the motor winding current to be sensed in both the drive and brake low-side slow-decay periods allowing for continuous current monitoring in typical bidirectional brushed DC motor applications. In coast mode, the current is freewheeling and cannot be sensed because it flows from source to drain. However, the current can be sampled by briefly reenabling the driver in either drive or slow-decay modes and measuring the current before switching back to coast mode again.

Figure 8-4 Integrated Current Sensing

The IPROPI pin should be connected to an external resistor (R_IPROPI) to ground in order to generate a proportional voltage (V_IPROPI) on the IPROPI pin with the I_IPROPI analog current output. This allows for the load current to be measured as the voltage drop across the R_IPROPI resistor with a standard analog to digital converter (ADC). The R_IPROPI resistor can be sized based on the expected load current in the application so that the full range of the controller ADC is utilized. Additionally, the DRV8251A device implements an internal IPROPI voltage clamp circuit to limit V_IPROPI with respect to V_VREF on the VREF pin and protect the external ADC in case of output overcurrent or unexpected high current events.

The corresponding IPROPI voltage to the output current can be calculated by Equation 2.

The IPROPI output bandwidth is limited by the sense delay time (t_DELAY) of the internal current sensing circuit. This time is the delay from the low-side MOSFET enable command (from the INx pins) to the IPROPI output being ready.

If the device is alternating between drive and slow-decay (brake) in an H-bridge PWM pattern then the low-side MOSFET sensing the current is continuously on and the sense delay time has no impact to the IPROPI output. If a command on the INx pins disables the low-side MOSFETs (according to the logic tables in Section 8.4.1), the IPROPI output will disable with the input logic signal. Although the low-side MOSFETs may still conduct current as they disable according to the device slew rate (noted in the Electrical Characteristics table by t_RISE time), IPROPI will not represent the current in the low-side MOSFETs during this turnoff time.