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I'm designing a PCB and I need a voltage rail, roughly 3 volts above the main input (of 3 - 30V). This rail is to power some CMOS devices, so it won't need to supply much current.

Currently I plan on using 2 AA batteries in series with the input, but those batteries are huge compared to the PCB so I want to get rid of them. I have searched for multiple solutions, but none match my requirements:

  • Charge pumps probably won't work, because of the big voltage change in the input.
  • Boost converters deliver a steady voltage, but I need an addition to the input.
  • I have looked at isolated DC/DC converters, but these usually run on 5V or higher, or they are too expensive.

Do you have any ideas?

EDIT: I don't necessarily need a higher voltage. I need a stable 5v from the input. I'm not willing to use a buck boost converter, since that will take up too much space.

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    \$\begingroup\$ CMOS typically won't survive > 15 to 18 V. You are going to fry them with 30 + 3 V. You need to supply more details of what you are doing. Schematic? \$\endgroup\$ – Transistor Sep 18 '18 at 7:30
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    \$\begingroup\$ This sounds like an XY problem. Can you confirm that you don't actually need to "add" 3V to a 3-30V input, rather you need to create a +5V supply from a 3-30V input? Also, can you use a 3V3 CMOS NOR gate? That would remove your requirement for +5V... Even 74HC works on 3V (a bit slower though) \$\endgroup\$ – peufeu Sep 18 '18 at 11:50
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    \$\begingroup\$ @Transistor My input is rated for 3 to 30 volts and I want to add 3 volts to that, so 6 to 33 volts. \$\endgroup\$ – Bert van Rossem Sep 18 '18 at 12:10
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    \$\begingroup\$ You're not answering the question. If all you need is a 5V supply, you can use a buck boost or a charge pump, much simpler. And if you can run your 74 logic gate from 3V you don't need any special circuit. \$\endgroup\$ – peufeu Sep 18 '18 at 12:12
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    \$\begingroup\$ @peufeu Yes, I only need a 5 volt supply. But I can't use a buck boost converter because of space restrictions and I haven't had any luck in finding a charge pump solution to work at this wide range of the input voltage. \$\endgroup\$ – Bert van Rossem Sep 18 '18 at 12:18
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This sounds like a good application for a charge pump. All you need is a roughly 4 V square wave somewhere. It can be between ground and +4 V, for example.

The square wave goes directly into a capacitor, so there can be a arbitrary DC offset across it. As long as this offset changes little during individual pump cycles, it can be considered as "constant" for this purpose.

Here is the basic circuit:

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Vout will be the amplitude of the square wave, minus the two Schottky diode drops. For example, it would be about 4.3 V open circuit with a 5 V square wave as input.

C1 allows any arbitrary offset voltage between the square wave and Vout. Of course C1 needs to be rated for the voltage. C2 only needs to handle Vout.

Either side of Vout can be used as the reference. If Vout- is tied to your 30 V rail, than Vout+ will be a little higher. If Vout+ is tied to your 30 V rail, then Vout- will be a little lower.

The current capability of this charge pump is proportional to the pump frequency. A convenient place to get a square wave from can be a clock output of a microcontroller that is there already anyway. You can also make a square wave from a Schmitt trigger inverter with a R-C low pass filter feeding its output back to its input.

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  • \$\begingroup\$ How could I go about this clock. The popular 555 timer is specified from 4.5v, the icm7555 won't go over 18v. A discrete monostable multivibrator could be used, but its output should be limited to roughly 5 volts. And I can't use a regulator for this, because that circuit can not have a high voltage drop, since it must be able to function of off 3v. Do you have any ideas? \$\endgroup\$ – Bert van Rossem Sep 18 '18 at 12:13
  • \$\begingroup\$ @Bert: It's not clear what you are asking. As I said, you only need about a 4 V square wave. \$\endgroup\$ – Olin Lathrop Sep 18 '18 at 12:24
  • \$\begingroup\$ And my question is, how do I do that? I did some research but I can't find any chips that can do that (provide a square wave at a constant voltage with varying input voltage) \$\endgroup\$ – Bert van Rossem Sep 18 '18 at 12:30
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    \$\begingroup\$ @Bert: After noticing some of your other responses, it seems this is a X-Y question. You asked about a 3 V supply riding on the existing 3-30 V supply. However, you apparently only want a 5 V ground-referenced supply. Pop up a few levels and ask what you really want in a separate question. Leave out that you think you need a 3 V supply on top of the 3-30 V supply, because that's not likely a good solution. \$\endgroup\$ – Olin Lathrop Sep 18 '18 at 13:06
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3-30V is a big input range.

Assuming you're not up for custom magnetics, you could do it with two or three devices- a buck-boost regulator (eg. LM5118) to 5V then an isolated 5V:5V DC-DC, or an LDO + boost regulator + DC-DC. (Or use a charge pump in place of the DC-DC once you have 5V, but the 5V:5V isolated DC-DC converters are cheap).

The first would be much more efficient, but substantially more costly.

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  • \$\begingroup\$ Please have a look at my edit. I don't necessarily need an isolated voltage rail, simply a 5v rail from the 3-30v input. However, because of space restrictions I can't use a buck boost regulator. \$\endgroup\$ – Bert van Rossem Sep 18 '18 at 12:24
  • \$\begingroup\$ The isolated DC-DC is an easy way to stack it on top of the rail. I am not sure how space restrictions constrain converter topology, but anyway throw away 90%+ of the power at 30V in and use an LDO+boost converter if you prefer. \$\endgroup\$ – Spehro Pefhany Sep 18 '18 at 12:32
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A 74HC gate will work from 3V (or even lower) to 5V so you could simply use a 5V LDO to limit the input 3-30V to 3-5V and supply your 74HC gate with this. Since static supply current is very low, you should be able to find a LDO with tiny dropout voltage. Unless the gate supplies something that draws current.

If all you need is a NOR gate though, here is one that will work with your 3-30V input voltage.

enter image description here

(source)

This will not be as fast as a 74HC gate but it could be what you need. It is also small and cheap.

Logic can also be implemented with open-collector output comparators like LM339. The outputs can be combined.

Note that the output voltage is the same as the supply voltage, so if you want to limit it, simply add a zener.

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