DRV8844 Short circuit protection problem

Hi Ritesh,

It will be related to the shortage condition at 100kHz 50% inputs condition. Here are some basic points of short protection.

1. First of all, we need big bulk caps such as >100uF + 0.1uF together and placed close to the VM pins. This is very very important to make the short protection function normally.

2. The protection is working as this way: During about 5us blanking time, the output current is not controlled, the shortage will cause 10A ~ 30A current depend on the shortage condition. There will be a slew rate of the current rising. After the blanking time, if the current seen higher than OCP level, output was shutdown.

So in 100k (10us) switching condition, just about 5us for current rising. it is possible that the current is much higher than OCP level but stopped as the duty cycle finished. And the cycles repeat. So the device is working at an average high current but not get protected or shut down. This is a dangerous condition.

Also it is possible that in this condition, although the output is short but the current rising rate (slew rate) is not so high because of the inductance of the shortage wire, the average current is not to much (< OCP level) at this 100K condition. So the device is just working normally in this short condition (we can think it as just a heavy load condition).

Best regards,

Hi Wilson,

Thank you for your reply

1. About point one I have used 100uF+0.1uF capacitors for VM placed very near to pin, still no impact on short circuit at 100KHz.

2. Considering blanking time of 5us that might be OCP deglitch time, but as you have mentioned in application note www.ti.com/.../slva505 that fast acting analog current limit responds in tens of nanoseconds by operating FET in linear region after which timer starts for deglitch time, so I think it should work at 100khz.

3. As this is important requirement of driver IC to protect output from short circuit which may cause while motor running at higher frequencies, so there should not be case which should be depend upon frequency.

4. So I think your short circuit protection is depend upon operating frequency.

Hi Ritesh

1. Good!

2. That is two level thing. the 400ns response is an analog gate driver function to protect the device from suddenly burning down by the more than 10A current. But after the 400ns, the current level reduced but still will be at a risky level such as 5A~9A if the shortage is not removed. Our device can bear for that current level only for several micro seconds. That is why the second level protection, the OCP after Tblanking time 5us will assert and shutdown the device and latched. Then, the device is truely safe now.

But the 100kHz may have the effect to over taking the Tblanking function and recycling all the time.

3. Yes. It is idea the OCP can work at high PWM frequency, but now it is dependent.

4. The high freq PWM will have effect to the normal protect function. I think for motor control application, 10k to 50k will be a good range for the outputs. Too high will cause too much switching loss and high temperature of the device caused by more recirculation consumption of the body diodes.

Best regards,

Hi Wilson,

1. So, Can you suggest me anything so that I can get protection up to 200Khz?

2. do you have any other driver IC?

As per my observation i think you need a large capacitors greater than the 100uF together and placed close to the VM pins.
This must be considered to make the short protection function normally.
I also want to say same as "The protection is working as this way: During about 5us blanking time, the output current is not controlled, the shortage will cause 10A ~ 30A current depend on the shortage condition. There will be a slew rate of the current rising. After the blanking time, if the current seen higher than OCP level, output was shutdown. "

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