I´m trying to calculate my LD1086PUR to output 5v and 3.3V

MOssw.1 · ‎2021-12-21

I´m trying to calculate my LD1086PUR to output 5v and 3.3V for an ESP32 and somehow can´t get it right. Does someone have an idea which resistor I should use or how I can calculate the resistor? I tried the Vo=Vref * (1+R2/R1) but it doesn't seem to add up.

Peter BENSCH · ‎2021-12-21

Welcome, @MOssw.1, to the community!

The LD1086 is a quasi LDO that can derive exactly an output voltage from an input voltage. If you want to generate two voltages with a single LD1086, i.e. 5V and 3.3V at the same time, this will therefore not work.

Otherwise the formula Vo=Vref*(1+R2/R1) is correct and fits perfectly - if you use the reference voltage (mentioned in the data sheet) and the given resistance. Right next to the formula, the resistor R1 is already specified as 120 ohms, so that its current is large enough not to disturb the Adjust Pin Current IADJ (see datasheet, table 8). Of course you also have to change the formula to R2, but that shouldn't be a problem, should it?

Good luck!

If the problem is resolved, please mark this topic as answered by selecting Select as best. This will help other users find that answer faster.

/Peter

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View solution in original post

Peter BENSCH · ‎2021-12-21

Welcome, @MOssw.1, to the community!

The LD1086 is a quasi LDO that can derive exactly an output voltage from an input voltage. If you want to generate two voltages with a single LD1086, i.e. 5V and 3.3V at the same time, this will therefore not work.

Otherwise the formula Vo=Vref*(1+R2/R1) is correct and fits perfectly - if you use the reference voltage (mentioned in the data sheet) and the given resistance. Right next to the formula, the resistor R1 is already specified as 120 ohms, so that its current is large enough not to disturb the Adjust Pin Current IADJ (see datasheet, table 8). Of course you also have to change the formula to R2, but that shouldn't be a problem, should it?

Good luck!

If the problem is resolved, please mark this topic as answered by selecting Select as best. This will help other users find that answer faster.

/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

MOssw.1 · ‎2021-12-22

Ah, Thank you so much. I was looking for the reference voltage but didn't find it. Now it makes sense and everything works fine. First time for me to design a PCB and sort through a datasheet and catching every important information is something I have to and want to learn!

Peter BENSCH · ‎2021-12-22

Fine!

Good luck!

If the problem is resolved, please mark this topic as answered by selecting Select as best. This will help other users find that answer faster.

/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.