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LM2577: Can LM2577 be used to create 24V output from 6-30V input

Carsten Thuesen
Prodigy 20 points
Part Number: LM2577
Other Parts Discussed in Thread: LM5118, LM5156, LM5155

Hi, we have a varying voltage source (from a generator) that can have a wide voltage range from 6-30V. This voltage has to be regulated to a stable 24V output for an external controlboard that use 10-12W of power.

Can we use the LM2577 in a forward converter mode to achieve this? I only see that the LM2577 is a boost converter, so if the voltage input is > 24V then it needs to operate as a buck converter... will it automatically handle this?

If this is not possible can it then be achieved with a flyback converter? If yes which transformer can I use? In the datasheet the proposed transformers are in the range 4-6V and 18-36V, so not really feasable for my application - but maybe the 18-36V can also go lower?

Lastly is it true that the built-in current limiter ofLM2577 is not working in forward converter mode (based on note 3 on page 4 of the datasheet).

FYI: I have been looking at LM5118, but it is much more complicated than the LM2577. 


BR
Carsten

  • 0
    TI__Guru* 76745 points

    Hi Carsten,

    Thank you for reaching out.  The LM2577 is not suitable for a forward converter because its max duty cycle is 80%.  You may consider the LM5156 in a sepic or non-isolated flyback design, better than a forward topology.

    Thanks,

    Youhao Xi, Applications Engineering

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Hi Youhao

    Thank you for the quick response. I qill look at the LM5156.

    BR,

    Carsten

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    One more thing, I have looked briefly in the LM5156 datasheet and can see that there is a link to a very detailed design guide  for making a boost converter with the LM5156 (https://www.ti.com/lit/an/snva941/snva941.pdf?ts=1619434675145). Since I have an input voltage that can either be below, near equal or above the output voltage I need to use a SEPIC or flyback converter - right? Do you happen to have a similar design guide for using the LM5156 in SEPIC mode... and ideally for 24V output applications?! This would really help a lot.

    I know of the WEBENCH power designer tool... but I'm a little novice when it comes to power regulators.

    BR,

    Carsten

  • 0 in reply to Carsten Thuesen
    TI__Guru* 76745 points

    HI Carsten,

    Yes you should try a SEPIC.  Please refer to https://www.ti.com/lit/zip/snvr481 for SEPIC design.   You may also use the LM5155 SEPIC Webench.  It is not a typo here.  The LM5155 Webench can also be used for LM5156 design but you need to note that LM5155's AGND pin becomes LM5156's DITHOFF pin, and grounding it will enable the spread spectrum, and pull it high would disable the spread spectrum. 

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 20 points

    Hi Youhao

    Thank you very much... this is exactly what I need :-)

    Have a nice day!

    BR/ Carten