Some parts are available as fixed output voltage options. Can I adjust the output voltage anyway?
This is a question that comes up often with parts like the ADP2108, ADP2109 or ADP2138, ADP2139 that do not have an adjustable version available.
As a general rule if the the parts have a separate Vout sensing pin it is possible to change the fixed output voltage to have the part generate a higher output voltage.
For example the ADP2108 has a 1.8V version and a 2.3V version, but there is no 2.0V version.
In this case it is possible to use the 1.8V version with an additional 2 resistors to set its output voltage between 1.8V and 2.3V.
The internal node impedance is around 400K (+/-30%) for a 2.5V output, so a Rb value around 4K is advisable to limit the error due to the variation of the internal divider. Calculate the value of Rt to achieve the desired output voltage (1.8V = Rb/(Rt+Rb) * Vout)
This is the nominal value of the FB impedance for the available preset voltages of the ADP2108/9:
I am adding here a little excel-sheet with math for optimizing the Rb Value and to see the tolerances of the desired output voltage.
that is a good start.
A few sources of error to add to this spreadsheet would be:
- initial and over temperature accuracy of the ADP21xx (about 2%)
- accuracy of the external resistors (1% or 0.1% each)
- available value of external resistors from the standard set (E24 or E96 etc)
How to modify an ADP2138CB fixed voltage eval board to adjust the output voltage.
The board we are modifying is the ADP2138CB. In this case we want to set the output to 2.3V. Remember you can only set the voltage higher. We are using the fixed 1.8V ADP2138 board.
On this IC the lower right hand bump is the feedback node. The board is laid out is such a way that you can cut the copper plane and isolate the feedback pin from the main Vout plane.
Once the plane is cut, verify with an ohm meter that it is no longer connected. Remove some of the solder mask on either side of the cut. This board has 4 vias and a small plane located to the left of the cut. That plane happens to be GND. Remove solder mask there as well. Rb (4K) will be placed across that small Gnd plane and the feedback node. Rt(1.1K) will sit across the cut in the plane.
Here is a calculator that can used to calculate the variation in the output voltage when the output voltage of a fixed output voltage part is modified by using an external resisitive divider