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: