LM5170-Q1: photo coupler bandwidth in Feedback comparison

Chun Hung Chen

Intellectual 590 points

Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

Hi TI Team

We have a project that will design in LM5170-Q1.

We have a problem we want a high precision/accurate outer current sensing.

We set the outer current sensing circuit at the voltage output positive port(as a ground of current sensing circuit).

And the ground of LM5170 is setting at the voltage output negative port.

So, two parts have different ground, how can I connect them?

If we use the optical coupler/photocoupler, the system bandwidth will be limited by it.

Could I have the other way to connect two circuits but different ground?

Or how could I solve the bandwidth of coupler/photocoupler?

By the way, the outer current is for the whole system and the signal will deliver to LM5170-Q1(inner current loop).

Thanks

over 6 years ago

0 Youhao Xi over 6 years ago

TI__Guru* 78240 points

Hi Chun Hong,

Thank you for considering the LM5170. It is not very clear what you really want to do. You don't have to use an optocoupler. A level shift can do the job. Before we can propose something, can you show your system block diagram, and what exactly do you want the LM5170 to do in this system?

If you don't want to disclose in e2e, could you please contact a TI FAE in your region, so we can work together with the FAE?

Thanks,

Youhao Xi, Applications Engineering

0 Chun Hung Chen over 6 years ago in reply to Youhao Xi

Intellectual 590 points

0 Chun Hung Chen over 6 years ago in reply to Chun Hung Chen

Intellectual 590 points

We have two reference ground, the control and current sensing is 12VA(Vo+).

The LM5170 reference ground is 12VS(Vo-).

0 Youhao Xi over 6 years ago in reply to Chun Hung Chen

TI__Guru* 78240 points

I don't think the optocoupler is necessary here. You can use a level shift circuit as in the file attached below.Levelshift Sensing.docx

0 Chun Hung Chen over 6 years ago in reply to Youhao Xi

Intellectual 590 points

Thank you.

Do you have any article or paper about the Levelshift Sensing?

Thanks

0 Youhao Xi over 6 years ago in reply to Chun Hung Chen

TI__Guru* 78240 points

A correction: I missed a connection in the level shift circuit. Please discard the previous one and use the attached.

I don't have any article but the idea is straight forward. The two Vbes canceled, and the all differential voltage V1 applies to the top resistor, producing Ic =V1/R

Assuming the transistor HFE is high (usually >100), then Ie =~Ic, and V2 =Ie x R =~ V1

Vout Level Shift Sensing.vsd

Thanks,

Youhao