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LM3224 low efficiency

c_ 64
Intellectual 630 points

Hi,

I'm using the LM3224MM-ADJ to convert a 3.7 V input to a +/- 10V output (using the design example in the data sheet to get the negative output voltage). With output currents of 60-80 mA I get a terrible efficiency of only 30%, and I think the layout, the component choice, or a combination thereof might be a problem (I can't see any other reason). I'm using a 4.7 uH inductor with Farnell number 186-9753. I thought this was suitable, but could this choice be a problem?

Also, something that deviates from the suggested circuit are two 0 Ohm resistors, one between pin 7 and Vcc, and one between pin 7 and GND (in order to be able to choose the switching frequency by removing one of them). To check if maybe the 0 Ohm resistors were a problem, I removed both, and soldered on a piece of copper wire from pin 7 to Vcc, but didn't notice any difference in current draw.

 

Thanks!

  • 0
    TI__Mastermind 25500 points

    I would suspect the inductor is the culprit without any additional information. I see it states that it is rated for 1A, but that is misleading as that is only for a 40C temp rise. Looking at the datasheet more closely you will see that the more important spec, the saturation current rating, is only 140mA. So you are running this inductor in saturation at all times which will result in excessive switch current and very poor efficiency. Try using an inductor with a saturation current above the peak switch current you expect at min Vin and max Iout. If you are concerned about current limiting situations you will want to use an inductor rated for the device current limit.

  • 0 in reply to Clinton Jensen
    Intellectual 630 points

    Hi,

    Thanks for the response! It cleared things up. I guess I've been of the false understanding that the inductor's saturation current should exceed the expected output current. But a closer look at the data sheet, and what you are saying in your response, the inductor should be rated according to peak internal switch current. So if one were to be on the safest side, one could pick an inductor with saturation value above 2.8A, right? (according to the I_CL in the Electrical Characteristics, given min Vin of 2.7V). But then in what cases should I be worried about the I_DC of the inductor?

  • 0 in reply to c_ 64
    TI__Mastermind 25500 points

    Yes, the safest bet is to use an inductor with a saturation current rating of at least the current limit of the device. This will allow the device to hit current limit in the event of an output short or hard load transient without saturating the inductor. As for the DC current rating of the inductor, that is usually specified simply for a given temperature rise. In the case that the rating is higher than the saturation current rating it is usually a useless spec, but some inductors will have a 20C or 40C temp rise rating lower than the saturation current rating. In that case you need to be careful that you are not going to overheat the inductor in a particular application, especially in high ambient temperature situations.