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Buck boost converter


Advanced Member level 3
Jun 24, 2012
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My input voltage is 4V to 36V DC
Output voltage required is 12V, 700 mA required for other circuits. Then 12V to 5V with LDO for some circuits.

I found the IC L7986TR but it required more external components. Do you suggest any IC with very small footprint with minimal external components.

Just a thought for buck boost.. Assume there is a voltage sensing circuitry. When the voltage is lesser than 12V, the boost circuit should operate and produce the constant voltage of 12V, then the LDO to 5V. When the voltage is greater than 12V, the boost circuit to disable and only the LDO to operate. I think, in this way, it is possible to find the small footprint IC for boost circuit with one small inductor.

Thanks in advance.

IC manufacturers have online selection tools. That´s how I search for devices. Also distributors.

I searched but couldn't get the satisfied results. Thought someone had similar experience and share some results.

your text is a bit confusing:
So basically ou need a
* non isolating power supply
* 4..36V DC input
* 12V / 700mA output.
That´s it. search finds 5 ICs buck-boost and 17 devices in section "switching regulators". And this is only one vendor.

* 5V LDO does not relate to part you are looking for. (Why LDO at all? 12V to 5V means 7V dropout. This is not considered as LOW)

Meaningless because you can´t do a search for your phrases: (Better: Specify numbers and everyone exactly knows what you mean)
* more external components
* small footprint
* minimal external components
* voltage sensing circuitry... (because already implemented in every buck boost)
When the voltage is greater than 12V, the boost circuit to disable and only the LDO to operate. I think, in this way, it is possible to find the small footprint
Makes no sense. Then if the input is 36V and your output is 5V you "dissipate" 31V x 0.7A = almost 22W as heat. This will be a rather big device with big heatsink, maybe even a fan.

To make there are many free design tools online. Otherwise buy just one for less.

I have not built anything with it and if it does not a layout package, that will rely on user experience. There are at least a dozen PSU design tools online. TI's had everything for some parts but not all now. TI also has a generic set of calculators tool.


it depends on all the needs and experience of a PS designer. PowerEsim has a lot of info not normally found in simulators with real hardware links, BOM's and DVT reports.

Free Microcap 12 has some fundamental designs by Christophe Basso, author and former On Semi guru. MicroCap has many tools including Smoke/Stress analysis, and a 3D analysis tool. Everything has a learning curve and time investment is an unknown factor.
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What is the maximum output current that you require from the 5V output?
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A flyback is an isolating buck-boost - that could be made to work, as is:

a sepic - non isolating converter that can buck and boost ( also single fet )

both need external power components to the controlling IC to work properly - especially at your 8.4 watts out

4VDC in is problematic - as there are few control IC that will work well at this low Vin - but you can make a boost from 4V to 36V say

that will give you a nearly fixed 36Vout regardless of Vin, and then buck down to your 12V at 700mA (?) 8.5 watts

note that 8.5 watts out is 2.2 amps in at 4V - can your source supply this ?

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