Add constant DC voltage to a variable DC input

Question

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.

Answer 1

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:

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.

Answer 2

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.

Answer 3

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.

(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.