Questions on Application of LMP8646EB Current Sensing Op-amp in conjunction with Buck Converter

Harold Taylor

Other Parts Discussed in Thread: LM3102, LM20133, LMP8646, LMP8646EB

1. Two feedback connections are shown in the document -- one in the application information (feedback is connected to Vin- on op-amp) and one in the EVB schematic itself (feedback is connected to the Vin+ on op-amp, which connects to Vout of the DC-DC converter).

2. A super-cap soft-start circuit is shown and implemented on the EVB, however, what is the behavior of the DC-DC converter under reverse voltage (i.e., Vout > Vin) conditions when Vin is removed and Vout gradually decreases as the Super Cap distcharges. Specifically, is there a reverse-voltage protection diode internal to the switcher between Vin and Vout (connected in parallel with the top FET)?

3. The design is shown with LM3102 buck converter. What modifications are required to implement this current-limit op-amp with LM20133?

Best Regards,

Harold Taylor

over 11 years ago

0 Harold Taylor over 11 years ago

Prodigy 190 points

1. Which feedback connection is correct?

2. Do I need a reverse-leakage diode in front of the Buck converter? If so, this reduces Vin and impacts efficiency.

3. LM20133 was selected because LM3102 does not operate with 4.5 to 5.0V input.

0 AmyLe over 11 years ago in reply to Harold Taylor

TI__Intellectual 1635 points

Hi Harold,

I'm the LMP8646 applications engineer.

1) I'm currently trying to find the documents in which there is a discrepancy. I will check and let you know.

2) I think someone from the DC-DC converter group will have to answer this question for you.

3) For the LM20133, I'm looking at its datasheet to verify that FB is 0.8V, and it is.At this moment, I don't see huge difference for the lmp8646 design. You can use this tool http://www.ti.com/tool/lmp8646-sw to design the LMP8646 with any converter with a FB node. Let me know if you have any questions. Thanks.

0 AmyLe over 11 years ago

TI__Intellectual 1635 points

Hi Harold,

For question #1, I've looked at the app section on page 17 of the LMP8646 datasheet (http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=lmp8646&fileType=pdf), and compared this with the schematic shown on page 12 of the LMP8646EB user's guide (http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=snou010&fileType=pdf), and I don't see this discrepancy. Can you please point me to the documents you are referring to? Thanks.

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Questions on Application of LMP8646EB Current Sensing Op-amp in conjunction with Buck Converter