LMR36520: What is the real current limit?

Hello

The meaning of the min, typ, and max are the same as for any specification in the data sheet.

This device has two current limits.  When the peak inductor current reaches Isc, the output voltage will fall, and this represents 

the largest output current capability.  If the valley of the current limit does not reach ILS-LIMIT before the next clock cycle, then

the frequency will drop until it does.  This mode is designed to prevent inductor current run-away during a short circuit.

The inductor should be rated for at least the typical value of Isc.

The attached document will help to clarify some of these concepts.

Also, the attached spread sheet will allow you to enter in your values, along with the current limits, and it will calculate

the maximum output current before the output falls out of regulation, etc.

In the spread sheet: "peak current limit" = Isc and "valley current limit" = ILS-LIMIT

Thanks

Ilimit_blog.docx

Buck_peak_valley_ilim_calc_revA0.xlsx

Thank you so much for your professional reply.

I have understood the relationship between ISC, ILS-LIMIT and output current. While I don't know which value should I choose for a new design, in other words, which value should I bring into the formula shown in the word document. For example, if the expected load current is 3A, if I design with the maximum value of ISC and ILS-LIMIT, this device may satisfy the requirement, while if I design with the minimum or typical value of ISC and ILS-LIMIT, this device can not satisfy the load requirement any more, which really confused me. Another question I want to know: Are the ISC and ILS-LIMIT one-to-one correspondence? For example, the minimum ISC match minimum ILS-LIMIT.  Is there such a situation that the minimum ISC match maximum ILS-LIMIT for one single device. Thanks.

Hello

For determining if the regulator can provide your load current, you would use the minimum values of current limit.

The spreadsheet we provided will calculate the maximum load current for you, when you put the current limit values in 

the spreadsheet.

There may be some correlation between the high-side and low-side current limit; that would depend on the particular device.

However, the spreadsheet can be used to experiment with different values to ensure that the device you choose can provide

your load current.

Thanks

Thank you so much for your reply.

Since we use the minimum values of current limit to determine if the regulator can provide your load current, what the meaning of typical value and maximum value. In other words, when would I consider these two values. Thanks.

Hi Frank,

Thank you for your reply.

I want to confirm if the conditions for Hiccup mode are: 1. FB voltage < 0.4V; 2.ILS-LIMIT is achieved.

If the ISC is achieved while ILS-LIMIT is not achieved due to large inductor current ripple, even FB voltage is less than 0.4V, it will not enter into Hiccup mode?

Hello,

If the inductor is small, and the ripple current would be large, the ISC would be tripped while the ILS-LIMIT would never be achieved, right?

Under the circumstances, when VFB<0.4V, the device would still enter into hiccup mode or not?