TPS63900: Average input current limit minimums/maximums

Hi Karl,

Let me check and reply to you later.

Regards

Lei

Hi Karl,

Sorry for this device, I can't find an appropriate estimation for the min/max value of the input current limit.

You may refer the fig 8-2 for max Iout.

Can a boost converter work in your application? If yes then the TPS61299 may be a good choice, of which the input current limit has the min and max values.

Regards

Lei

Thank you for the response, Lei! 

Unfortunately, no, a boost converter does not work for us.  We are trying to do this:

www.ti.com/.../slvaev9.pdf

Hi Karl,

If so, is it possible you select a current limit that is one step bigger than the one you actually need?

For example, in your application, if the min current limit to support the max Iout is 10mA, then you set the current limit at 25mA.

Regards

Lei

We are using the current limit to estimate battery life, essentially a very rough version of coulomb counting.  So any deviation in that current limit value means inaccuracy in our battery life estimate

Hi Karl,

I see. Let me try to dig out some information about the variation estimation.

I'll let you know once I have the results.

Regards

Lei

Hi Karl,

I just found the sigma data for your reference. When the current limit is 2.5mA, the sigma is 0.235mA.

Regards

Lei

Sigma meaning standard deviation?  So it could still be outside of 2.5+/-0.235?

Do you have data for 1mA as well?

Hi Karl,

KLL said:

Sigma meaning standard deviation?

Yes.

Sorry 0.235mA is the 3 x sigma, not one sigma.

The actual value of one unit may outside of 2.5+/-0.235.

KLL said:

Do you have data for 1mA as well?

Sorry I can't find the sigma of 1mA. But the sigma will be smaller than 0.235mA/3.

Regards

Lei

Great information, this is really helpful!

Is it possible that the current limit values are dependent on the exact implementation?  Input/output capacitance, inductor specs, etc?  Is it known exactly what passive components were used when measuring to obtain the 3 sigma values?

The reason I ask is because we are getting about 2.8mA on average with these.  It may be that our measurement setup is not very accurate.

Also, we are using this to charge a supercapacitor (several 10s of Farads), so the output capacitance of the TPS63900 is effectively very high.  Could that also be causing this deviation we see?

Hi Karl,

As the information I can find, the major factor that impacts the average input current limit is the Vin and Vout. The effective output capacitance won't cause the deviation.

A higher average input current limit, such as 2.8mA is possible. 

Regards

Lei

Very interesting.  Do you know what Vin/Vout voltages were used when the 2.5+/-0.235 data was measured?

Hi Karl,

The sigma data I provided was tested under Vin = 3V, Vout = 2.5V. And no data from other Vin Vout conditions.

Regards

Lei