Hello all. My Arduino project requires to be battery powered and have +/-30V and +/-18V rails available for op amps etc.

I was thinking of using a 2 cell LiPo battery that can be recharged over USB/5V wall wart by using this board:
https://wiki.dfrobot.com/USB_Charger_for_7.4V_LiPo_Battery_SKU__DFR0564

This would give me a 7.4V rail which I would feed through a 7805 regulator to get 5V (and bypass any issues with the battery voltage depleting as it is used). For the 2 pairs of bipolar supplies, I was thinking of using 2 LT1072 switching regulators: one as a boost converter to boost 5V to 30V, the other as a buck-boost converter to convert 5V to -30V. This +/-30V supply would then go through a 7818/7918 regulator pair to give me the +/-18V supply.

The problem I have is in the LT1072 data sheet I can't find any formula to figure out how to get the boosted voltage from 5V to 30V and 5V to -30V. I have attached my schematic. I assume it will have something to do with the ratio of the resistors connected to the FB pin - R1:R4 for the boost converter, R8:R5 for the buck-boost converter...?)

This is my first SMPS design, so if anyone can help me I'd really appreciate it

 

Which OP Amps are you using that require +/- 30 or +/-18V rails?

 

How much current on the ±18V and ±30V supplies?
How accurate do the voltages have to be?
1) Don't produce your ±30V from after the 7805. The efficiency of the 7805 on a 7.4V supply is only 67.5% - you have just lost 32.5% of your battery energy as heat.
2) A 7805 won't work below 7V, so your 5V supply will reduce as your battery goes flat.

 

Thank you for the quick replies!

Which OP Amps are you using that require +/- 30 or +/-18V rails?

AD633 multipliers and a LM358 are used which work on +/-18V, and 10x yet to be determined op amps to deliver constant current @30V (similar to a TENS machine but with lower uA current requirements)

How much current on the ±18V and ±30V supplies?
How accurate do the voltages have to be?
1) Don't produce your ±30V from after the 7805. The efficiency of the 7805 on a 7.4V supply is only 67.5% - you have just lost 32.5% of your battery energy as heat.
2) A 7805 won't work below 7V, so your 5V supply will reduce as your battery goes flat.

I can't see the current on the +/-18V going higher than 400mA. The +/-30V will need 5mA + whatever the 10x op amps on the 30V circuit use (ideally not more than 100mA, but this is a total guess as I'm not sure what op amps usually consume - LM358 that I use on the 18V circuit seems to be 340mA max)

The Arduino Mega also has a st7920 LCD screen and I can't find the current consumption specs on that.

The +/-30V needs to be pretty accurate (say +/- 1%...?)

Good point about the 7805 - I was only thinking of keeping that 5V line stable for the +/-18V and +/-30V rails as the battery drains - the Arduino can run straight off the 7.4V from the battery and use its own regulator and power the LCD from there.... but since the 7805 doesn't work under 7V, its a moot point anyway!

I'm open to suggestions

 

1% is accurate for a power supply! The good old 7805 only manages ±2%.
400mA from the ±18V rails will require 2A from the battery.
Where are you getting an op-amp which will stand a ±30V supply? I can't think of many which will take more than ±22V. LM358 will only manage ±16V

I would suggest that you make a flyback converter with four outputs running directly off the battery. Aim to make ±18V and ±36V. That would mean all the secondary windings could have the same number of turns which will fit the bobbin neatly. Feed back from the +18V supply, then put linear regulators (LM317/337) on the ±36V outputs to give ±30V ±1.5%.
TI's webbench power supply designer will probably design it for you.
https://webench.ti.com/power-designer/switching-regulator

 

AD633 multipliers and a LM358 are used which work on +/-18V

No. Look at the data sheet.
The AD633 is specified at ±15V with ±18V being the Absolute Maximum.
You never want to operate at the maximum rating of a device.
It should be run at ±15V not ±18V.

 

The problem I have is in the LT1072 data sheet I can't find any formula to figure out how to get the boosted voltage from 5V to 30V and 5V to -30V.

I didn't see a formula either.

You can calculate using this block diagram:


\(V_{out}=1.24V*(1+\frac{98k}{1.24k})=99.24V\)
I guess that's close enough to 100V for LT...

 

The charger circuit says in the translation that it has a cutoff voltage of 8.4V when a 2-cells Lithium battery is far from being fully charged. 8.4V is when charging voltage should stay at 8.4V until an hour or two later when the charging current drops to a low amount and it is fully charged. Maybe the translation is wrong.

I have used thousands of opamps and have never powered them from +/-15V. Many of them used a little 9V battery without a voltage regulator.