What is max current allowed at feedback pin of LM3478 through Feedback resistors.

shrikant bhairavkar

Other Parts Discussed in Thread: LM3478

Hi I am using LM3478 as boost converter. I want to know what is the max current that can flow through LM3478 feedback pin.

Thanks & Regards,

Shrikant Bhairavkar

over 10 years ago

0 Doyel Ghosh over 10 years ago

TI__Intellectual 770 points

Hi Srikant,

For the FB Pin, for an Abs Max of 1.3V, the max current should be typ. 3.5nA ; no greater than 15nA.

Under stressed conditions of Abs Max of 2.5V, the max current should be typ 0.7mA and no greater than 0.95mA.

Thanks,

Doyel

0 shrikant bhairavkar over 10 years ago in reply to Doyel Ghosh

Prodigy 200 points

Doyel Ghosh said:

Hi Srikant,

For the FB Pin, for an Abs Max of 1.3V, the max current should be typ. 3.5nA ; no greater than 15nA.

Under stressed conditions of Abs Max of 2.5V, the max current should be typ 0.7mA and no greater than 0.95mA.

Thanks,

Doyel

Hi Doyel,

Thanks for reply. In my case I am using LM3478 as Boost converter. I have 8V output & 0.75 AMps load current. My feedback resistor values as per workbench design is 5.36K & 1K will that be right value since my feedback current in that case is 1.9mA. Please help.

Thanks,

Shrikant

0 JohnTucker over 10 years ago in reply to shrikant bhairavkar

TI__Guru**** 191445 points

That is the current flowing in the FB resistors (actually 8/(5.36k + 1k = 1.3 mA not 1.9 mA) The error amplifier input is very high impedance, so most of the current flows directly to GND from Vout, not into the FB pin (just a few nA).

0 shrikant bhairavkar over 10 years ago in reply to JohnTucker

Prodigy 200 points

Hi John,Thanks for your reply.
Also I want help like as per LM3478 datasheet the diode selection is said to be based on the peak current which in my case comes to 2.7Amps. But my load current is 0.75 Amps. So as per that I have selected 2Amps diode as per average current it is schottky will that work. Schottky part no. MBR230S1F.

0 JohnTucker over 10 years ago in reply to shrikant bhairavkar

TI__Guru**** 191445 points

I was just making a general comment about feed back resistors and the very high input impedance of the FB pin that is common to many dc/dc converter ICs. I do not have any particular experience with LM3478. Perhaps Doyel will give a recommendation on that.

0 Doyel Ghosh over 10 years ago in reply to shrikant bhairavkar

TI__Intellectual 770 points

Hi Shrikant,

Yes, the Schottky you have selected will work okay in this case.

Thanks,
Doyel

0 shrikant bhairavkar over 10 years ago in reply to Doyel Ghosh

Prodigy 200 points

Thanks Doyel. Only last query from calculations I got the peak current for the Inductor as 2.7Amps. I have selected 3.3Amps rating Inductor with max saturation current of 2.7Amps. Pls suggest if the part will be ok. Part no. is SRU1028-8R2Y.

0 Doyel Ghosh over 10 years ago in reply to shrikant bhairavkar

TI__Intellectual 770 points

Hi Shrikant,

I recommend an inductor with a higher Isat; for instance SRR1240-8R2M ; which belongs to the same family but with a slightly higher footprint, with Isat of 4.3A and Irms of 4.5A.

Thanks,
Doyel

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What is max current allowed at feedback pin of LM3478 through Feedback resistors.