LM5117 as a constant current source

Natallia Holubeva1

Other Parts Discussed in Thread: LM5117

Hello dear e2e,

I was wondering if LEM (current sensor) could be placed in series with the inductor L1 instead of the synchronous rectifier Q6. I was thinking that maybe the ramp would need to be disabled if we did this.

Could you please provide some feedback?

Thanks a lot,

Natallia

over 11 years ago

0 EricLee over 11 years ago

TI__Guru 58851 points

Will the output of the current sensor be connected to CSG and CS ? or will it be used for an external constant current source circuit ?

What's the purpose of using LEM instead of sense resistor ?

Regards,

0 Natallia Holubeva1 over 11 years ago in reply to EricLee

TI__Expert 6550 points

Hello Eric,

It will be an external constant current sense ckt. The reason for it is that it will also drovide the data to the host. Basically, what will happen is that for the first half cycle, CSG and CS will see the exact waveform as with Rs, but for the second half cycle, it will be different. Regading the datasheet (sample and hold), in the second half cycle it will hold and disregard the data. Are there any concerns of doin it?

Thank you very much for your help!

0 EricLee over 11 years ago in reply to Natallia Holubeva1

TI__Guru 58851 points

My recommendation is to use a sense resistor at the source side of the synchronous switch, together with your LEM.

The sense resistor will be used for LM5117 and the LEM will provide the data to the host.

Regards,

0 Natallia Holubeva1 over 11 years ago in reply to EricLee

TI__Expert 6550 points

Thank you, Eric.

But if only LEM is used, can you think of any potential problems?

Natallia

0 EricLee over 11 years ago in reply to Natallia Holubeva1

TI__Guru 58851 points

I can think of something unclear.

During normal operation, at the time when LM5117 samples inductor current, CSG voltage is ground, CS voltage is below ground level.

Regards,

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LM5117 as a constant current source