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Substitute for LM2577

Mosi
Intellectual 735 points
Other Parts Discussed in Thread: TPS61040, LM2577, LM27313, TPS61046

I was using LM2577 to boost a 3.6V battery voltage to 8.0V to power a sensor board. It was working just fine but has it's own issues since LM2577 is pretty old and has a big footprint and external components and is expensive and I don't need that much output current so I decided to use TPS61040 instead. First It looked like what I wanted with 400mA swtich current and everything cause I just needed 50mA output current but turns out it is 400mA SWITCH current NOT the actual OUTPUT current and apparently they are different!

It's a shame I didn't know that and never noticed that. By the way if anybody can clear this for me I would really appreciate that. How there two currents are different from each other and how was I supposed to know the output current? There should be some sort of an equation I think.

Now the output voltage decreases from 8.0V after connecting the sensor board to the output and I blame output current cause once I increase the input voltage from 3.6V to 6.0V it works fine.

So after wasting my time and money I'm looking for a better substitute for LM2577 and my requirements are:

Vin(max) = 3.6V

Vout(max) = 7.5V ~ 8.0V

Iout(min) = 50mA

Iout(max) = 80mA

I want it to have a small footprint and few external component requirements. (oh and no shocking price!)

Does anybody have an idea?

Edit: Considering the fact that this is a battery powered device it is better to draw less power from the input terminal while the converter is ON. (It's working only for a minute or two in entire day)

  • 0
    TI__Guru**** 191445 points
    The current difference between switch and load is inherent for boost converters. During the switch on time, the output is supplied only by the output capacitance whale energy is stored in the inductor. During the off time, energy from the inductor is used to both supply the load and re-charge the output capacitance. Switch current = load current / (1-D).

    I ran your requirements in webench:

    www.ti.com/.../power_portal.page

    It returned 37 possible solutions. I would look at LM27313 or TPS61046.
  • 0 in reply to JohnTucker
    Intellectual 735 points
    Thank you.
    When you said Switch current = load current / (1-D) . What did you mean by D ?

    Also I was reading TPS61040 datasheet and realized with its 400mA switch current I should be able to get what I want (60mA). Wouldn't you say so?
    It seems like I just have to change some component values cause I just entered my requirements into Webench Designer and it gave me a schematic and new values and simulations were just fine.
    So I think I'm just gonna have to give it one more chance.
    Although something is weird. For instance I used 10uA inductor and didn't get the right answer and now the Webench tells me to use 15uA inductor.
    I seriously doubt that 5uH more inductance could make that much difference, Right? (although some capacitor values have been changed too but I still have my doubts!)
  • 0 in reply to Mosi
    TI__Guru**** 191445 points
    D = duty cycle. Not sure why, but TPS61040 was not on the list of webench selected parts. I am not real familiar with that family as I do not directly support them. I often answer posts like this that have general questions though. And yes inductor value makes a big difference with regard to peak switch current.