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LM358B: Use it as voltage adder

Esteban Gimbernat
Prodigy 60 points
Part Number: LM358B
Other Parts Discussed in Thread: TPS61094

Hi, this is the first time I have to solve this type of issue os summing 3 independent voltages with the common ground before an LDO. I'm trying to add 3 voltages, each one with a different buck-down regulator with Iout of <500mA.

That is, I have 1.2V, 1.8V, and 2.7V and at the final circuit I need 3.3V, normally I will use a Step Up Regulator on the 2.7V rail, but since the 3.3V are needed with at least 500mA using one voltage is not an option. Since the design is for an external board to these voltages, moving anything on the supply board is not an option.

That lead me to add the voltages, on an ideal op-amp works as expected, but when I'm trying to simulate it with the LM358B crazy values are obtained. So I have two questions.

a) Is the correct approach to use an op-amp to add the voltages?

b) How can I get correct results on the simmulation?

  • +1
    TI__Guru*** 140136 points

    Op amps can add voltages together but they do not convert power or generate voltages greater than their supply voltages.

    You need 3.3V at least 500mA? That is 1.65 watts.  You have 1.2V , 1.8V and 2.7V at 0.5A max which is 2.85W total; so there is a theoretical solution.

    You could boost the (2 or 3) separate supplies with separate boost circuits to 3.3V ; I do not know how to make them share the load and still be efficient (I'm op amp support). 

    Therefore find a boost device, for example TPS61094, and start a new e2e post and ask how to boost 2 or more voltage to 3.3V with that IC. Power support will answer.

  • 0 in reply to Ron Michallick
    Prodigy 60 points

    Hi Ron, thank you for the response. I think I will opt to use normal power paths as regulators for this issue. From the theory seems that op-amp works, but in the practice seems difficult to accomplish if I have not greater voltage to supply.