L6902 component choice

[Tfive]

Hi, I'm designing a DIY switching power supply around the L6902. Intended output voltage is <22V, input voltage 24V, output current in constant current mode between 0.6 and 0.8A.

My questions:

• the datasheet shows an inductor value of 22uH, is this value independent of output voltage and current? Currently I chose the part SRU1048-220Y, datasheet: https://www.mouser.de/datasheet/2/54/SRU1048-1391431.pdf
• Is the compensation network also independent of output voltage and current?

For neither of the parts I found any further hints in the datasheet, so I thought I'd better ask...

Thanks very much in advance for your help!

[Peter BENSCH]

Welcome @Tfive, to the community!

You should calculate and simulate the design with the eDesignSuite. However, you must apply at least 3V more to the input than you need at the output, i.e. at least 25V at the input with 22V at the output.

• For 25V input voltage and 0.8A, the eDesignSuite calculates a coil of 47µH, for which you should use a saturation current (not working current!) of at least 1.5A, 2A to be on the safe side, so that its magnetic field does not collapse at the peak currents.
• The compensation network depends on the operating point of the switching regulator, but the eDesignSuite will also take care of this for you.

Hope that helps?

Good luck!
/Peter

 

[Tfive]

Thanks for your recommendations, very much appreciated. Actually I can turn up the voltage on the input supply up to 28V if needed. So that should not be a problem. I will have a look at eDesignSuite tomorrow.

[Tfive]

Kudos to the engineers who designed this tool! First of all I love that it's web based, no need to install anything. Secondly it seems to just work. I work as a web dev myself, so I pretty much know what I'm talking about ;)

I was able fine tune the circuit as now the current and voltage demand are known more precisely, i.e. 21.5V @ 600mA.

I have one last question:
What voltage rating do I need for the compensation network? The design tool calculated some not-so-common values for the capacitors and with 50V rating these get rather expensive. Will I get away with 10V caps? Normally I do want to overrate voltage rating with at least 50%. as these are quite likely to come out as X7R or X8R types their capacity will vary with voltage, so how tolerant is the network to changes in capacitance? 

From my gut feeling I would suspect this is a rather low voltage node here, so probably no concern?!