xbee and 3.3v regulators

Anyone know why this regulator MC33269DTRK-3.3G does NOT work with an xbee?

As soon as I replaced my circuit with a standard linear 3.3V regulator (TO-92) it worked fine.

Seems strange to me....

I use the MC33269 on my boards (which have an XBee) without any problems. Is the pinout the same between the two regulators?

One thing you need to be careful of with the MC33269 is the decoupling capacitors. For stability On-Semi recommends capacitors with an ESR in the range of 0.2-10Ohms. Checkout the schematic and bill of materials in my ZB1 datasheet at http://tinyurl.com/5rfmrz

(* jcl *)


www: http://www.wiblocks.com twitter: http://twitter.com/wiblocks blog: http://luciani.org

After someone else pointed the filter peculiarities I did some experimenting:

Added 10uF to the output but it still didn’t solve the problem until I also removed the 100n caps on the output.

It is odd to me that it is ok to have the decoupling 100n at the power rails of the ICs in the circuit but they can’t be near the regulator output.

The critical specification for the decoupling capacitor is effective series resistance (ESR). What is the ESR of the capacitors that you had at the regulator?

Was the 100n capacitor a ceramic capacitor? If so then the ESR would almost certainly be well below the minimum ESR requirement. Moving the capacitor from the regulator to the IC increases the ESR.

What is the ESR of the 10uF capacitor? A generic capacitor most likely will not meet the ESR requirements. For my board I use a low ESR electrolytic designed for power supplies. The specific part number is in the BOM of my ZB1 datasheet (see http://www.wiblocks.com)

The capacitor dielectrics for power supplies can be critical. This regulator is particularly finicky.

(* jcl *)


www: http://www.wiblocks.com twitter: http://twitter.com/wiblocks blog: http://luciani.org

Yeah I had regular ceramic layered capacitor and they were very close to regulator.

Looking at the datasheet it says 10uF or greater on the output ( 0.2ohm < esr < 10ohm) but for the input it suggests a 0.33uf (or larger) as an example. I guess you saw no issue with using 10uf there too?

In my circuit I have two regulators very close to each other and both are MC33269T type.

So would it go:
DC in —> 10uF—>5V regulator ----> 10uF---->3.3V Regulator—>10uF

Only one 10uF between them or should I put two there and use the point in between as the place I take the 5V output from?

Larger (within reason) is usually not a problem. More important is the ESR and dielectric. I went with a larger cap since the voltage source (USB or wall adapter) is at the end of a long cable. I chose the same capacitor rather than having to purchase two different part numbers.

You do not need two capacitors since the parts are close.

Make sure to check the ESR of the capacitor you are using. As you change load and line conditions you could end up with instabilities.

(* jcl *)


www: http://www.wiblocks.com twitter: http://twitter.com/wiblocks blog: http://luciani.org

What about this cap? (mouser) 647-UHC1E100MDD

It's max ESR is 0.52 (so greater than 0.2)

The ones on your BOM are not in stock.

That one looks good to me.

(* jcl *)