DRV8874-Q1: Current overcurrent protection issue

Hi Cherry,

I will be happy to help you out today:

Q: how long does Iout fall when the output current reaches Itrip?

A: This well depend entirely on which current regulation decay mode is being used, whether Fixed Off Time current chopping or cycle by cycle current chopping. If it is fixed off time, then the current will be disabled for Toff which is 25us. However if it is cycle by cycle it will remain off until the next PWM cycle comes in from the input (note that because of this, cycle by cycle regulation does not support 100% duty cycle as it will need a rising edge to work properly. 

The current chopping mode and therefore timing can be set by the IMODE pin and more information can be found on section 7.3.3 of the datasheet:

Q: Is there only one overcurrent protection? Or also have a next level of protection? If this overcurrent protection is failed, what will happen?

A: There are two levels of overcurrent protection, the first one is with current regulation described in section 7.3.3, the second one is overcurrent protection that is not configurable: An analog current limit circuit on each MOSFET limits the peak current out of the device even in hard short circuit events. If the output current exceeds the overcurrent threshold, IOCP, for longer than tOCP, all MOSFETs in the H-bridge will be disabled and the nFAULT pin driven low. This will again depend on the IMODE pin setting as shown in the table above. 

In automatic retry mode, the MOSFETs will be disabled and nFAULT pin driven low for a duration of tRETRY. After tRETRY, the MOSFETs are re-enabled according to the state of the EN/IN1 and PH/IN2 pins. If the overcurrent condition is still present, the cycle repeats; otherwise normal device operation resumes.

In latched off mode, the MOSFETs will remain disabled and nFAULT pin driven low until the device is reset through either the nSLEEP pin or by removing the VM power supply.

In Half Bridge Control mode, the OCP behavior is slightly modified. If an overcurrent event is detected, only the corresponding half-bridge will be disabled and the nFAULT pin driven low. The other half-bridge will continue normal operation. This allows for the device to manage independent fault events when driving independent loads. If an overcurrent event is detected in both half-bridges, both half-bridges will be disabled and the nFAULT pin driven low. In automatic retry mode, both half-bridges share the same overcurrent retry timer. If an overcurrent event occurs first in one half-bridge and then later in the secondary half-bridge, but before tRETRY has expired, the retry timer for the first half-bridge will be reset to tRETRY and both half-bridges will enable again after the retry timer expires.

Q: The motor 100%duty starts with a 16-A Inrush current for 0.3 S, will it have any effect on DRV8874-Q1? 

A: The most likely scenario is that if the current rises faster than current chopping can come into effect, then overcurrent protection will trigger. It is important to note that Ichop does not equal Iocp. How fast the current will rise will be dependent on input voltage and the inductance of the motor so it would be hard to say what will trigger first. However I don't see why retry mode overcurrent protection wouldn't be able to handle it. Just be sure that you set the IMODE pin to a setting that has automatic retry instead .

Please let me know if you have any other questions, I will be happy to assist you.

Best,

Pedro Arango Ramirez