Adjustable vs. fixed voltage option

Discussion created by LucaV Employee on Aug 16, 2012
Latest reply on Jan 10, 2014 by kevintompsett

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.


ADP2138 adj.png

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)


Additional information:

This is the nominal value of the FB impedance for the available preset voltages of the ADP2108/9:

1.0    160K

1.1    176k

1.2    192k

1.3    208k

1.5    240k

1.8    288k

1.82  291k

2.3    368k

2.5    400k

3.0    480k

3.3    528k