I need a wide-ranging input switching regulator

[Speakerguy]

Hey guys,

I need a part, or a circuit idea, to use for a 5V switching supply. The Vin range will be 8 volts max down to perhaps as low as 1 or 2 volts. Output must be 5V tol 5% for a powered USB system. I've found several small switcher chips but none that fit my needs Thanks for any help.

PS this one is for work, so don't go out of your way to help me on this, since I doubt my boss will write you a check too. I just feel I should note in my posts when my questions relate to work, so that folks know what exactly they're helping out with.

 

[Blatman Bond]

As low as 1 Volt? That's really hard to built. Might be there a solution here. Small boost converter (active when input too low) for driver and another buck-boost converter for 5V load. But no warranty.

 

[ericgibbs]

hi,
You dont say what sort of current output you are looking for.

Have a look at the Linear Technology range LT3489, LT3477 HF Boost Regs.

 

[Speakerguy]

Sorry, 600ma typ 1amp max

 

[Hero999]

I wonder if it's possible to bootstrap a SEPIC converter IC?

I wonder if you could increas R1 to something like 100hm: then stick a diode from the output to the power supply of the IC?

This will work if the IC will start working at 1V and step up the voltage sligtly above 1.6V which will cause a run away effect until the IC is being powered from 5V.

Note that I haven't looked at the datasheet, this is just a random idea.

Also, thanks for having the decency for saying it's work related.

 

[Hero999]

Bump.

Well did you do it?

Or did it get put to the back burner?

I'm interested to see what you did.

 

[Speakerguy]

Ended up going with the Linear LTC3780 until they can get me samples of LTC3785. That'll get me down to 2.5V in IIRC, which after talking with the project coordinator another round or two should be OK.

 

[kjennejohn]

Takes less then a Volt in (needs a startup V of 1.5V,sigh), 500mA inrush current control, 95% efficient, true shutdown, puts 1Amp out, adjustable for 3V to 5V.
Looks OK? Data sheet: https://www.electro-tech-online.com/custompdfs/2007/07/MAX8627.pdf

Hope that helped.
kenjj

 

[kjennejohn]

Oops. Max input is 5.5V. Poo!
kenjj

 

[kjennejohn]

This looks closer. 1.5V to kick over, 2V after that. Will take an input up to 16.5V! Needs minimal support parts, a power pass FET to get the 1Amp out:
https://www.electro-tech-online.com/custompdfs/2007/07/MAX641-MAX643.pdf

'Luck on your project!
kenjj

 

[Hero999]

That's fine for stepping 1.5V up to 5V but it won't step 8V down to 5V.

The probelm with the SEPIC converter is the IC won't work when the input voltage is too low but it's possible to power it from a blocking oscillator type boost converter that will work down to under 1V and give 3V on the output. I'll post a schematic if you're interested.

 

[Speakerguy]

That's ok, thanks though. The bill of materials was already drawn up, approved and submitted with the proposal this week. So long as their power device works and they don't push it too hard we should be fine with the LTC chips.

Thanks everyone! I will keep all these other options in mind, in all probability we will need a more capable solution at some point.