DRV8834 – About accuracy of ITRIP

Kanemaru,

Let me confirm the accuracy figure with designer, but I would suggest to increase the Vref above 1 volt as per data-sheet suggestion.  Current trip ccuracy is greatly sacrified below this level.

Best Regards

Milan-Motor Application Team

Hi Milan-san,

Thank you for your prompt reply.

I’m looking forward to hearing from you.

And, I’m FAE at Distributor in Japan.

I have shared the customer information with Nishimura-san.

Regards,

Kanemaru

Hi Milan-san,

Thank you for your support.

We would be grateful if you could reply as soon as possible.

Regards,

Kanemaru

Kanemaru-San,

Below is response from designer about Itrip accuracy at Vref=0.7:

The main error comes from the offset of the internal comparator that can be as high as ~10mv.

So when Vref is high, the error is Vref+/-10mV and this 10mv is a small %of this high Vref. If Vref is small, this 10mV becomes a higher % of Vref. This problem becomes worse when you micro-step since this 10mv error will be larger % of smaller steps. Eg. In 1/32 step  , the 1^st step value is equal to Vref/20*1/5=Vref/100. If Vref is 2v, 1^st step = 20mv and error of 10mv = 50% of this 1^st step value. Now if Vref = 0.7v, 1^st step is 0.7/100 = 7mV and this 10mV is greater than 100% of 1^st step value.

In summary: ITRIP error is = (V_stepRef +/- 10mv)/ (5*RISENSE) .

Best Regards

Milan-Motor Application Team

Hi Milan-san,

Thank you for your kind support!

I will report this information to our customer.

Regards,

Kanemaru

Hi Milan-san,

I have one more question.

Does the offset voltage of current sense amp (gain=5V/V) affect the full scale ITRIP error?

So, I think that the full scale ITRIP error is the following calculating formula.

Is my understanding correct?

And, please let me know it if you have this value.

<ITRIP error>

Full scale ITRIP error is = VREF +/- 10mV / (5 +/- ??mV * RISENSE)

Regards,

Kanemaru

Hi Milan-san,

Could you please let us know the current status regarding this question?

I’d greatly appreciate yourverification.

Regards,

Kanemaru

Kanemaru-San,

Sorry for delayed response, I was on travel and limited access to emails.

Yes yout undertstading is right, it also affects Itrip.

Data-sheet specifiy minimum value of Itrip as 2A, refer to shetet6, Protection circuit row.

Best Regards

Milan-Motor Application Team

Hi Milan-san,

Thank you for your replay.

Could you please let me know about min and max value of the current sense amplifier gain?

Regards,

Kanemaru

Hi Milan-san,

Thank you always for your kind support!

Could you please let us know the current status regarding this question?

I’m looking forward to hearing from you.

Regards,

Kanemaru

Kanemaru San,

The Aisense (current sense amplifier gain) is ideally 5v/v. it normally varies +/-3% with temperature and across different devices. The variation is guaranteed to be within +/- 10%. Please note this 10% is a conservative 6-sigma number.

Thanks and Regards,
Sudhir

Hi Sudhir-san,

Thank you for the information!

I understood that the full scale ITRIP error is the following calculating formula.

<Full scale ITRIP error>

Full scale ITRIP error is = VREF +/- 10mV / (5 +/- 10% * RISENSE)

Full scale ITRIP error is = VREF +/- 10mV / (5 +/- 3% * RISENSE)

<Result1 at 5V/V +/-10%>

ITRIP ≒ 0.297A at VREF=0.7V , 5V/V

ITRIP ≒ 0.335A at VREF=0.71V(0.7V + 10mV) , 4.5V/V (5 * 90%)

ITRIP ≒ 0.267A at VREF=0.69V(0.7V – 10mV) , 5.5V/V (5 * 110%)

So, the worst full scale ITRIP error is about +/- 12%.

<Result2 at 5V/V +/-3%>

ITRIP ≒ 0.297A at VREF=0.7V , 5V/V

ITRIP ≒ 0.311A at VREF=0.71V(0.7V + 10mV) , 4.85V/V (5 * 97%)

ITRIP ≒ 0.285A at VREF=0.69V(0.7V – 10mV) , 5.15V/V (5 * 103%)

So, the worst full scale ITRIP error is about +/- 5%

Is my understanding correct?

Regards,

Kanemaru

Kanemaru-San,

Can you help me understand how you arrived ITRIP = 0.297Amap at Ideal conditions, Vref=0.7V and gain of 5V/V? What is Rsense vlaue?

Input on analysis: you must understand that you have done extreme value worst case analysis. The parameters are varied for extreme cases, in direction that give worst case values. In real world, simultaneous occurrence of such extreme values is rare. Therefore, there are some statistical approaches such as RSS (Root sum square) or monte-carlo which provides the tolerances numbers different than above calculation.

My input: If worst case figures are acceptable for end application requirement, one can skip the details analysis, however if figures are beyond the application requirements, than it becomes absolute need to calculate the real tolerance as per statistical approaches.

Best Regards

Milan-Motor Application Team

Hi Milan-san,

Thank you for your response.

The Rsense value is 0.47ohm.

Currently, our customer is evaluating this device.

Therefore, they would like to know the value of ITIRP.

Regards,

Kanemaru