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LM2585-12 Voltage vs Current

Other Parts Discussed in Thread: LM2733, LM2735, LM2731, LM2588, LM2585, LM2587

Hello,

I am using a LM2585-12 to convert a 5V DC to 12V DC. I need about 1A at the load. When I use a 12-Ohm resistor as a load the voltage goes down to 8.5V. Since load current is 1/3 of the switch current, I believe it should have worked. Changing Inductor to various values has no effect. Do I need to use a regulator with higher switch current? If so which one would be a better choice for me? Also can you please suggest a regulator which would convert a 5V DC to 30V DC at 150ma?

Best Regards

Ilyas Mirza

  • Hello Ilyas,

    For your second question. Look at LM2735; a SOT23-5 device with a 2A / ~20V  switch. (There are also other package choices.) I think if you put a capacitor voltage doubler on the switch node, you can get there. Most of our offerings will have inadequate max duty cycle limit to be able to reach 30V directly from 5Vin. The control section of this product is designed for 5V supply input. The other tiny boosts in that family (LM2731 and LM2733) have higher Vin ratings on the controller section but lower switch current limits. If you need to see a schematic on this approach, let me know. There is a small added resistor that won't be obvious.

    Your LM2585 question: LM2585 does not have adequate peak current limit for this application. It is true that the average current makes it look like it will work, but you'll need the next larger device LM2587 or LM2588 (Enable input). BTW: Webench is a good tool for vetting these applications.

    Your bench experiment that shows Vout sagging down to 8.5V, This is predictable behavior on boosts that hit peak current limit in operation. They are still attempting to boost but the peak current limit intervenes and terminates the switch ON-time early and it drops out of regulation.

    Alan Martin

  • Hello Alan,
    Thank you for your response. For 5V to 12V @ 1A I will try LM2588 and get back to you. In anticipation I had ordered two LM2588s to try. For 30V it would be nice if you could share schematic with me.
    Best Regards
    Ilyas Mirza
  • 5to30Boost.pdfHere is a sketch of the approach.

    Alan Martin

  • Also, you may want to look at the attached app note for tips regarding boost regulators.

    6740.snva731.pdf