Appropriate Voltage Regulator/How circuit works?

[Gwow]

I have a project in which i need to minimize weight (= powered from 2 AA batteries), but need voltages of +-30 V, 12V (for op amp circuitry), and 3.3 volts (for an atxmega microcontroller).

Looking i have considered the LT1944-1 in order to produce +-30 Volts and the 12 volt reference
https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1003,C1042,C1031,C1060,P1704,D1574

And heres the part I'm not sure about: can i use a voltage divider from the 12 volts to get 3.3 volts and then have an op amp buffer separating the output voltage from the Vcc pin of the atxmega microcontroller? Assuming the output can handle 15 ma? Or should i use a separate step up converter?

Also, for the last application note for the LT, im not sure how they achieve the -15 volts from the 15 volt reference. Can someone point me out to some material to explain this? Im not sure how two diodes can cause -15 from 15 volts, but i don't know how these boost up converters work anyways.

My requirements are:
3.3 V @ 15 ma. Microcontroller uses built in 12 bit adc and internal precision 1.0 voltage reference.
+-30 V @ 5000 fa
12 V @ whatever an op amp draws for supply.

 

[nschutten]

Hi Gwow,

I think you should be able to use an opamp with the resistive voltage divider you describe, as long as current ratings are all OK. Please note though, that the efficiency of this voltage regulation will be very low (you will always loose 12V-3.3V across the regulation; so at 15mA you would unnecessarily dissipate 130mW). If this is unacceptable to your application, then you should indeed consider a DCDC converter.

Regarding your other question about the diodes: please have a look at the following that explains it in detail: https://en.wikipedia.org/wiki/Voltage_multiplier

Good luck!

 

[Gwow]

Thanks for that link!
As for the efficiency: isn't that assuming that the 15 ma flows through the resistor divide network and drops from 12 to 3.5 volts?

I was thinking that the op amp's supply would be the 12 volt output and that the resistors would be chosen large (k-ohm m-ohm) so that current is very small. Then the unecessary power dissipated would involve the internals of the op amp (?).

Or, lets say instead of a voltage divider I simply used an op amp connected directly to 12 volts but which dropped the voltage by 1/2?

That power dissipation would indeed would kill my application. The best approach would probably be just to buy another dc-dc regulator. Just curioius - what would be the most efficient regulator to convert 2.5 volts to 3.3 volts?

Edit: Oh, i forgot, theres another thing about that diagram i do not understand. By each voltage output they have a certain current. But the sum of the currents does not even come close to the max current they say each side of the step up converter can handle (175 and 100 ma)! For example, on the very front page they have 40 ma @ 5 volts (for the 175 ma handling side) and 2.5 ma@15V and -10V @ 1 ma for the 100 ma handling side. I assume this means the max current that each output can handle??? I have no clue why though, and its important to know what the circuit can handle.

 

[nschutten]

About your opamp and resistor bias circuit: the resistors will indeed consume only little current; however, the opamp will need to regulate its output to continue to output 3.5V, regardless of its output loading. The only way it can do that, is draw from the 12V supply; so the opamp will behave effectively like a pass through transistor, dissipating the difference between 12V and 3.5V. If you want to be sure what current flows where exactly, you can take some opamp circuit spice model from a vendor's website and perform some spice simulations. This may be very insightful.

About your second question: I am not sure what you mean exactly, but at least the following should hold: the power is constant; so with higher voltage, less current will be available. Maybe this helps a little already.

 

[Gwow]

If you look at the typical application section, you can see stuff like '40 ma' next to the voltage outputs. I already tried the constant power thing, didn't work. I assumed the current by the output voltage was the max current allowable...