I built a battery powered device that requires the use of two different voltages. It uses 3.3V to power general electronics components, at very low power, no more than 100 µA or so. And16 volts to power a valve actuator solenoid.

For this purpose, I found an excellent 3.3V regulator that draws a ridiculously low amount of power when idle, about 0.8 µA. And I'm quite happy with it. The problem is that this regulator accepts a maximum input voltage of 7V. And so I decided to arrange the array of 10 batteries according to the following schematic:

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What makes me unhappy, though, is pretty obvious. The lower 4 batteries (which deliver 1.6V each when new) will deplete faster than the upper 6 batteries. This because the electronics draws a continuous amount of current, while the solenoid is only powered sporadically.

I would very much like to find a way to regulate the entire 16V array down to 3.3V, while also maintaining the minimum possible power draw form the regulator itself. I've already browsed through dozens of component candidates and so far I haven't found a single one that meets my requirements. Any ideas?

 

A Google search turned up this one, which has low ground current.

EDIT: What about putting a 12V Zener in series with the 7V max input regulator?

 

See my edit above.

 

A Google search turned up this one, which has low ground current.

EDIT: What about putting a 12V Zener in series with the 7V max input regulator?

Many thanks for your help Crutschow. If I understand the datasheet correctly, the "Ground Current" is the device's basic power draw, is that right?:



If that's the case, then it's a little too high for my taste.

 

What s the current and duty cycle of the solenoids? Unless the average current is same magnitude as the 100uA drawn by the electronics, it makes no sense to to optimize the power to the electronics.

Say, for example the solenoids are powered for 1 minute per day at 1A. They would have an average draw of 690uA and reducing the power to the electronics to zero would only save you 13% of the operating power.

 

I'm going to ask a possibly stupid question. But I'd rather look stupid than stay stupid.

What if I were to use a diode with a large forward voltage drop connected between the 16V source and the regulator's input? For example (see attached sheet) this diode says it has 10V forward voltage. Wouldn't the regulator "see" only 6V at its input when connected this way? Or would the diode's internal resistance ruin the low power draw requirement?

 

Use a depletion MOSFET, with its gate to the 3.3V output and its source to the regulator input, and drain to 16V.

 

What s the current and duty cycle of the solenoids? Unless the average current is same magnitude as the 100uA drawn by the electronics, it makes no sense to to optimize the power to the electronics.

Say, for example the solenoids are powered for 1 minute per day at 1A. They would have an average draw of 690uA and reducing the power to the electronics to zero would only save you 13% of the operating power.

The solenoid itself draws about 4A through a 50ms pulse each time it's activated. And the solenoid will be activated about 300 times a day, at the very least.

 

Use a depletion MOSFET, with its gate to the 3.3V output and its source to the regulator input, and drain to 16V.

That's interesting ... a depletion mosfet is one that is turned off when a voltage is applied to its gate, right?

 

That's interesting ... a depletion mosfet is one that is turned off when a voltage is applied to its gate, right?

Correct. You'll see the technique a lot in the startup circuit for flyback converters. When the Aux winding gets going it pulls source higher than gate and turns off the startup circuit, saving power.
If you use a MOSFET with Vgs(th) of -1.5V, it will settle with the source about 1.5V above the gate, which will give your regulator adequate headroom.

 

Here's the application note:
http://notes-application.abcelectronique.com/137/137-47514.pdf

 

Correct. You'll see the technique a lot in the startup circuit for flyback converters. When the Aux winding gets going it pulls source higher than gate and turns off the startup circuit, saving power.
If you use a MOSFET with Vgs(th) of -1.5V, it will settle with the source about 1.5V above the gate, which will give your regulator adequate headroom.

I found this one .. but it has too large of an internal resistance for my taste...

 

Another application note:
https://ww1.microchip.com/downloads/en/Appnotes/AN-D66.pdf
This is the one I use:
https://www.mouser.co.uk/datasheet/2/196/Infineon_BSP129_DS_v01_42_en-3160739.pdf

 

I found this one .. but it has too large of an internal resistance for my taste...

How low does it need to be for a 100uA supply? It will drop 800uV at 100uA and you need it to drop about 11V

 

I built a battery powered device that requires the use of two different voltages. It uses 3.3V to power general electronics components, at very low power, no more than 100 µA or so. And16 volts to power a valve actuator solenoid.

For this purpose, I found an excellent 3.3V regulator that draws a ridiculously low amount of power when idle, about 0.8 µA. And I'm quite happy with it. The problem is that this regulator accepts a maximum input voltage of 7V. And so I decided to arrange the array of 10 batteries according to the following schematic:

View attachment 295976
​

What makes me unhappy, though, is pretty obvious. The lower 4 batteries (which deliver 1.6V each when new) will deplete faster than the upper 6 batteries. This because the electronics draws a continuous amount of current, while the solenoid is only powered sporadically.

I would very much like to find a way to regulate the entire 16V array down to 3.3V, while also maintaining the minimum possible power draw form the regulator itself. I've already browsed through dozens of component candidates and so far I haven't found a single one that meets my requirements. Any ideas?

You've said that you have already found an excellent regulator in the one quoted above, so just go with it! Why waste more power by dropping any more and wasting it in zeners or by any other means? You may even be able to run it from 4.5V if the electronics it's powering is frugal enough!

 

This is the one I use:
https://www.mouser.co.uk/datasheet/2/196/Infineon_BSP129_DS_v01_42_en-3160739.pdf

Wow ... this one's a real gold mine ... many thanks!

 

You've said that you have already found an excellent regulator in the one quoted above, so just go with it! Why waste more power by dropping any more and wasting it in zeners or by any other means? You may even be able to run it from 4.5V if the electronics it's powering is frugal enough!

Thanks for chiming in, Sarah. Perhaps I wasn't clear in my requirements. I want to use the entire array of batteries to feed both the regulator and the solenoid at the same time. That is, I'd like to have a single 16V output from the array, instead of the two that I've had to use at this point. This because I'd like for all the batteries to discharge evenly at the same time.

 

Use a depletion MOSFET, with its gate to the 3.3V output and its source to the regulator input, and drain to 16V.

Is this the arrangement that you were referring to? ... it looks very promising.

​

 

Is this the arrangement that you were referring to? ... it looks very promising.

View attachment 295983​

That's the one.
It will probably work better than the single regulator as it puts another RC filter between the noise on the battery and your 3.3V supply.
Also if you also need a not-too-clever 5V supply, you can use the voltage on the source.

 

That's the one.
It will probably work better than the single regulator as it puts another RC filter between the noise on the battery and your 3.3V supply.
Also if you also need a not-too-clever 5V supply, you can use the voltage on the source.

Many thanks ... I can't find the DN2540N8 available anywhere, but I'm going to try and see if I can find an equivalent.