Rick,
Thank you! Both answers are very informative.
I also have a question regarding overcurrent protection of the external FETs. To be more specific let's take a look at the part used on DRV8711EVM board - CSD18531Q5A. According to its datasheet, the Rdson = 3.5 mOhm typ and default settings for EVM board are OCPTH = 00 (that is the OCP threshold is 250mV) and OCPDEG = 00 (that is the OCP deglitch time is 1 us).
Now let's consider situation when output of the half-bridge is dead-shorted to the power rail Vm and the bottom MOSFET of that same half-bridge is commanded ON. The current floating in the bottom MOSFET under such conditions is similar to the shoot-through current and its value is limited by the Vm source's output impedance, circuit parasitics, FET's Rdson, and sense resistor's value. So this current could be quite high. And for the settings mentioned above it would be about 250mV / 3.5 mOhm = 71.4 Amp spike. Sounds a little extreme for the small device rated for 19 A with typical copper footprint.
So the questions are below.
1. Is such a spike actually safe for CSD18531Q5A device? Or is that too much and OCP won't be able to protect FET under such conditions?
2. If such a spike is OK, what parameter should I look for in the FET's datasheet to ensure its safety? Is that Idm, Pulsed Drain Current, under Absolute Maximum Ratings table, page 1?
3. And more generic question. What's an anticipated response time of the OCP circuitry once OCP deglitch timer has expired?
Thanks, Michael
Thank you! Both answers are very informative.
I also have a question regarding overcurrent protection of the external FETs. To be more specific let's take a look at the part used on DRV8711EVM board - CSD18531Q5A. According to its datasheet, the Rdson = 3.5 mOhm typ and default settings for EVM board are OCPTH = 00 (that is the OCP threshold is 250mV) and OCPDEG = 00 (that is the OCP deglitch time is 1 us).
Now let's consider situation when output of the half-bridge is dead-shorted to the power rail Vm and the bottom MOSFET of that same half-bridge is commanded ON. The current floating in the bottom MOSFET under such conditions is similar to the shoot-through current and its value is limited by the Vm source's output impedance, circuit parasitics, FET's Rdson, and sense resistor's value. So this current could be quite high. And for the settings mentioned above it would be about 250mV / 3.5 mOhm = 71.4 Amp spike. Sounds a little extreme for the small device rated for 19 A with typical copper footprint.
So the questions are below.
1. Is such a spike actually safe for CSD18531Q5A device? Or is that too much and OCP won't be able to protect FET under such conditions?
2. If such a spike is OK, what parameter should I look for in the FET's datasheet to ensure its safety? Is that Idm, Pulsed Drain Current, under Absolute Maximum Ratings table, page 1?
3. And more generic question. What's an anticipated response time of the OCP circuitry once OCP deglitch timer has expired?
Thanks, Michael