# Best method for generating low current (5 - 10 mA) -2.5 V supply

I am designing a digital system with various power supplies. I need to generate positive and negative supplies for a component. The voltages are +6.5 V and -2.5 V. Both supplies will pull in the ~5 mA to 10 mA load currents. I also will have a +2.5 voltage rail in the system that will have higher current in the couple of amps range.

I have an idea on how to do the +6.5 V with a buck converter, but the -2 V supply is giving me some trouble. Most solutions I have been finding have a maximum negative voltage ≥ 3 V.

Some have suggested split supplies, but I don't see many in the ±2 V range. regulating to a "higher" negative supply like -5 V and then regulating to -2.5 V would be an option, but seems like it would add additional space and power to my design, which I would like to avoid.

I would gladly take any recommendations.

Cheers!

• Best method, using which limitations? Which space and power limitations you then have? Any budget how much it can cost? Also, asking for suggestions what to buy is off-topic. Oct 21, 2021 at 21:53
• When I do a parametric search at digikey, negative supply regulator "min output voltage" and "max output voltage" refer to the minimum and maximum MAGNITUDE respectively. "maximum magnitude of output" being greater than 3V isn't a problem at all, as long as the "minimum magnitude of output" is less than 2.5V Oct 21, 2021 at 21:53
• So do your search again, with your definition of "minimum output voltage" and "maximum output voltage" flipped from what simple arithmetic says they should be. Oct 21, 2021 at 21:56
• Simple charge pump with one 555 can be a solution. Or look for buck-boost converter. Or isolated converter that can be connected in series with your 6.5v source.
– user208862
Oct 21, 2021 at 22:11
• Charge pump inverters cost about 2 dollars in small quantities. Add a linear regulator if needed. Oct 21, 2021 at 22:52

Your application is a good case for the inverting mode of the classic MC34063 switching regulator.

Only 200mA saturation current is necessary, so a good, inexpensive inductor would be the \$0.50 USD (at 1 qty) Bourns RLB0914-331KL 1.25A Sat, 330 µH Unshielded Wirewound Inductor 740 mA, 760mOhm Max.

Here are the design parameters: 1.4 Vsat output sw, 0.5 Vf diode, 5 Vin, -2.5 Vout, 4.2 Vin(min), 0.050 Iout, 24KHz Fmin, 0.01 Vripple(pp)

And here's the worked-out design: MC34063A design, Rsc=1.6 Ohm, L=330uH 200mA Sat. min., Ct=750pF, Cin=Cout=1000uF Low ESR, Vin=4.2v-6.0v, Vout=-2.5v, suggest the optional filter of 1uH + 100uF, R1=R2=4.7K Ohm

I did not verify the design (but I'm actually working on it -- could not get my LTSpice files for the MC34063 working, and I misplaced a chip that I had). For the beginners happening on this information, a good Youtube video to watch is made by Dave of EEVblog: "EEVblog #110 - Let's Design a DC to DC Switchmode Converter" which I enjoy.

If you derive this -2.5 V supply from the 6.5 V supply which you mentioned that you will create, then the components need to change to the following:

• Ct = 620 pF
• Rsc = 1,870 mOhm
• L1 = 430 uH (still >= 200mA Sat)
• Cout >= 680 uF
• MC34063 is an oldie but a goodie. There are more modern devices that work well too.
– Frog
Oct 22, 2021 at 19:29
• Since I didn't specify the input voltage this answer is technically correct. Thank you for taking the time to answer my question. Oct 22, 2021 at 21:32
• @MiamiBatman - You're welcome. Did you see at the bottom that I also included values for your 6.5 volt supply? If you have another voltage, I will recalculate the values for you and give you updated information. Just ask. Oct 25, 2021 at 2:42