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I'm currently trying to create a converter 20V-26V to 5V with an 2.1A output. So I use the LM2489 chip (datasheet: here).

My circuit is the following:

LM3489 converter

I make this circuit but with I power up everything and I don't get 5V output, I only have 1.8V. I verified all my circuit but nothing is wrong. I made research on the internet but I don't find solutions. My questions are the following:

  • Is the LM3489 a good solution ?
  • Is my circuit right? If not, what I must change to get 5V output?
  • What can be the reason of the 1.85 output?

I want to add that when my input vary from 20V to 5V the output increase from 1.8V to 4.5V

Thanks in advance!

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  • \$\begingroup\$ How are you measuring the output? Hook a scope there. I suspect your output cap is too low, since there is none. \$\endgroup\$
    – Vladimir Cravero
    Commented Nov 2, 2016 at 11:18

1 Answer 1

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I verified all my circuit but nothing is wrong

I beg to differ.....

Using a 0.1 uF capacitor across the feedback resistor is asking for trouble. The data sheet says that it should be 100 pF i.e. 1000 times smaller than what you have. The output is probably going unstable and oscillating and your meter is averaging the output and giving you a number that you think means it has 1.8 volts DC. Your output capacitor is also missing: -

enter image description here

Is the LM3489 a good solution ?

I think the FDC5614P is a little too lightweight for a 2.1 A output - how are you heatsinking this tiny little device? Note that the application diagram in my answer uses the same FET but the output is only rated for 0.5 amps.

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  • \$\begingroup\$ Thank for your answer! I changed Cff and add Cout and I get the wanted value Which component can I use to replace the FDC5614P and support 2.1A ? \$\endgroup\$
    – Majonsi
    Commented Nov 2, 2016 at 13:57
  • \$\begingroup\$ Your best bet is to use supplier search engines and home in on a device with significantly lower on resistance but in the same package (if that is still important). I have no recommendation off the top of my head but if you think you have chosen one, add a comment with a link to the data sheet and I'll take a look. \$\endgroup\$
    – Andy aka
    Commented Nov 2, 2016 at 14:00

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