How to get voltage follower?

Hi,
I cannot create or find a precise voltage follower. I want to make my programmer like USBtinyISP but with "proper" level shifting to the target. In the Adafruit's design the level shifter is powered from the target. Since the power source of the target may be weak (e.g. coin battery) I am afraid it may not be able to provide programming current and current for the voltage buffer and all the fancy circuitry I plan to add. So I wanted to power the circuitry interfacing with the target from USB 5V reduced to the target's voltage. Standard solutions are not handy:

Transistor voltage follower will provide V_BE drop less voltage than needed (may be problem when target is 1.8V).
Op amp will need to be rail to rail and may not be able supply enough current.

I expected there will be plenty simple circuits for such purpose (something like current mirrors) but I cannot find any. Is there a trick to get rid of the V_BE drop (but still be able to supply 5V when the target is at 5V)? Or do I need power it from transistor driven by an opamp somehow?

Thanks for tips.

Maybe you need a power op-amp. There are such things that can source more current than the usual op-amp.

http://playground.arduino.cc/Main/I2CBi-directionalLevelShifter

See the circuit about half way down the page.

Hi,

Since the power source of the target may be weak (e.g. coin battery)

But Adafruit says..

  • Powered off of 5V USB bus at less than 100mA to allow it to be used with unpowered USB hubs
  • Easily accessable jumper to power target project off of USB (target must be 5V tolerant, of course)
  • Remove the jumper and it will self-power but buffer the I/O to match the target device. (v2)

Where does it say the programmer is powered off the target?

Tom... :slight_smile:

I know how to do level shifting. I don't know how to power the low voltage part other way than from the target.

TomGeorge:
Where does it say the programmer is powered off the target?

Tom... :slight_smile:

If you look at the schematic you will see

  1. AHC125 does the level shifting down and is powered only from the target (or both are powered from USB)
  2. Nothing does level shifting up so it will probably fail for 1.8V target

Since I didn't find better way I am driving a transistor with opamp. Maybe overkill but it is working well.

Is this topic about voltage following .... or is it about level shifting? Or both?

Also...as mentioned.... there are such things as power opamps.... which can source relatively large current.

Smajdalf,

Are you trying to level shift a power line or logic signals? What are the ranges of the input voltage and the output voltage that you are trying to shift?