DRV8802 - Stall Current

Hi Anuja

Let me give some explain.

1. OCP and current regulation are indepent functions and circuits inside the chip. Regulation uses Rsense but OCP doesn't use Rsense resistor.
2. OCP deglitch time(or Tblanking time) is about 3.75us for DRV8802, which means if the current monitored is ramping up to Iocp (1.8~5A) within the 3.75us time, the device will report OCP fault.
3. Current regulation is done by monitoring the Rsense voltage. The ramping up rate is depended on the load inductance. Usually the normal current ramping through a motor or solenoid will not that fast to be seen as a OCP event (get to >1.8A within 3.75us). So the current will never have the chance to exceed the setting level (your 300mA) as it was regulated. But some motor may have very small inductance and very small DC resistor, could cause a real OCP event during startup or stall condition. So in this case, serial inductors added to the outputs with L > VM*Tblank/Iocp could help.

Best regards,

Hi,

Thanks for your response.

I was looking at TIDA-00144 Test Report which shows the forward and reverse stall waveforms.I suppose 50mA is the nominal preprogrammed value used in the TIDA-00144 Test Report

Can I conclude that the stall current value is "independent"  of the preprogrammed value, Since it is 12V/56.3ohms=213mA.

Regards,

Anuja

Hi Anuja

In the report, the regulation current level should be at 200mA. In the startup plot, the peak startup current is still a little lower than 200mA, so no regulation kick in. But in stall condition, the motor current was trying to rise above the 200mA level but was regulated at 200mA as a flat line.

Your understanding about a motor with big phase resistor that the VM/Rphase < Iset is correct. In that case, the regulation level will never be reached unless the motor phase is partly shorted. Note that a directly short of the phase usually will cause the OCP protection first before current regulation can kick in.

Best regards,

Hi Wilson
How long does it take for the Rsense circuitry to regulate the stall current to preprogrammed value?
I want to know the "Time for which current remains at the stall current value before it is preprogrammed"
Thanks,
Anuja

Hi Anuja

The timing between current rising from 0 to reach the setting limit is depended on the VM, Iset and the motor phase inductance parameter. A rough estimation of "dt" can be done by L*dI/dt = VM. Giving dt is very small, just use linear opproximation. dt = L*dI(Iset)/VM.

Best regards,

Hi Wilson,
Thanks for your response.
But wouldn't that be during normal conditions?
During Stall if stall current is [ Iset<Stall current<Over current trip],Here is my understanding-
Current waveform would be like
0 ---- Stall current--- Preprogrammed value
I am looking for the time the Device takes to detect stall and set it to preprogrammmed value.
(Similar to OCP blanking time).
I hope I have given a clear explanation.

I am really looking forward to sense this stall current using some external circuit.
I want to trigger an external circuit output as soon as the current reachees the stall current value.
Hence, I am hoping there is atleast some time before the current is set to preprogrammed value before the stall.

Regards,
Anuja

Hi Wilson,

From your response below:-
The timing between current rising from 0 to reach the setting limit is depended on the VM, Iset and the motor phase inductance parameter. A rough estimation of "dt" can be done by L*dI/dt = VM. Giving dt is very small, just use linear opproximation. dt = L*dI(Iset)/VM.


From the datasheet:
After the current is enabled in an H-bridge, the voltage on the xISEN pin is ignored for a fixed period of time
before enabling the current sense circuitry. This blanking time is fixed at 3.75 μs. Note that the blanking time also
sets the minimum on time of the PWM.

Does this mean during every first 3.5us of every cycle the current remains at fault value (1.2A) in my case.

Also, In ideal conditions (no fault ) what would be the value of current in the first 3.5uS ( would it be zero ) ?

Hi Anuja,

If [ Iset<Stall current<Over current trip], the real current will not have the chance to exceed Iset. So you will only see the current start from zero and hold to Iset in stall condition.

For DRV8802, if the current rising up to Iocp within 3.5us, the device will be latched at fault condition. You will not see the second cycle before reset the device.

During the blanking time, the current will not be regulated at Iocp(1.8A min), it is out of control. How high the current is will be depend on the VM and the motor resistor and inductor.

Best regards,