Wednesday, September 19, 2012

What means "guaranteed" in a datasheet?

This last summer I spent some time rebuilding my workbench power supply. I chose the LM350K regulator as the heart of the power supply because its 3A guaranteed output capabilities and short circuit protections. I bought two units, just to have one lying around in case something went wrong.

This is a photo of the two units. For clarity:

SH99AF               SH57AG
LM350K               LM350K
 STEEL P+             STEEL P+

When I tested the new power supply with the LM350K at the left, I quickly noticed something: Maximum output current was just under 2 amps. Of course I checked all parts, and of course the heat sink, but I found nothing: It was the regulator itself who was limiting the maximum output current just under two amps. Why?

Once of the features announced in every LM350 datasheet is:

Guaranteed 3A output current

The same datasheet explained how the maximum output current is available only with a limited range of input/output voltages:

All conditions were meet, but output current never went over 1.9 amps, so I decided to test the second unit, the one at the right of the above photo. The second unit worked just as expected: Maximum output current is 3.6 amps, more than the guaranteed value (3 amps) but somehow under the graph values (4-5 amps). Because 3.6 amps is above the guaranteed value, I declared finished the power supply rebuilding.

Why the first LM350 did not reached the "guaranteed" output current? I don't know. Maybe is a defective part, or maybe is a counterfeit. With a limit current of 1.9 amps it resembles to me more like a LM317 than a LM350.

Finally, some advice: LM317, LM350, LM338 and similar adjustable 3 terminal voltage regulators are prone to self-oscillations when bypassed with ceramic capacitors. Use always electrolytic capacitors with these regulators.

1 comment:

  1. Most of the linear regulators prefer a little bit of ESR (equivalent series resistance) at the output bypass capacitor, so as to avoid inestability issues. Almost any low ESR electrolytic or tantalum capacitor will do.

    However, you can also use a ceramic capacitor provided you add a very small resistance in series with it. A 0.1 ohm resistor is usually a good compromise.

    Do not forget to check the recommendations for the output capacitor at the datasheet of the particular linear regulator you want to use.

    Nice blog and nice article!
    Jose Angel Jimenez