TPS65400 Max Current accuracy

Mark Nohr

I've been using the TPS65400EVM and am finding the outputs switch off much lower than the maximum current I program the device for.

I am using an Agilent programmable load to ramp up the load current in 100mA steps.

setting the IMAX to 3A results in the supply shutting down at 2.4A. I'm finding this same behavior at all IMAX settings.

Can you help me understand this?

over 9 years ago

0 JohnTucker over 9 years ago

TI__Guru**** 191445 points

The current limit is the peak switch current not the load current. The load current will be lower than the peak.

0 Mark Nohr over 9 years ago in reply to JohnTucker

Intellectual 570 points

Fair enough.

Is there a formula to calculate this?

0 Eric Guo1 over 9 years ago in reply to Mark Nohr

TI__Intellectual 2590 points

Hi Nohr,

you can find the current limit accuracy in the EC table. please see below.

The limit is used for peak current of inductor, not the DC output current.

you can use DS's formula to calculate the inductor current.

DC current plus half inductor current ripple should equal to peak current of inductor.

thanks!

0 Mark Nohr over 9 years ago in reply to Eric Guo1

Intellectual 570 points

Hi Eric,

EC = Electrical Characteristics... Got it :). My oversight not finding it in the specification.

Answer: +-25%

So if I want to prevent my design from delivering more than 3.5 Amps of load current I use 3.5A+ 1/2 of the inductor ripple current of 0.5A (see attached spreadsheet image) = 4A

I have to set the limit to 5A since setting the limit to 4A means the supply could trip as low at 3A (-25%).

Setting it at 5A means the supply could trip at loads as high as 5.75A (+25%) or 2.25A above my target maximum load.

Do I have this correct?

-Mark

Supply Design Calcs.pdf

0 Eric Guo1 over 9 years ago in reply to Mark Nohr

TI__Intellectual 2590 points

Yes, the max DC output current could come to 5.75A if you send current limit to 5A.

thanks!

0 Mark Nohr over 9 years ago in reply to Eric Guo1

Intellectual 570 points

Very well then.

Thank you for your help in this matter.

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TPS65400 Max Current accuracy