Edit - Corrected the gainHi,
I have not built this circuit, but I have run the LTspice simulation, which is normally very accurate.
I am going to explain some of the subtle features of the circuit that I presented.
2.2 M Ohm resistorIf you look at the partial block diagram:
You can see that there is a 1.9uA current source on the TRK/SS pin. This is required in normal operation to charge the SS capacitor. However, in the circuit that I presented this current is undesirable. It creates an offset on the TRK/SS PIN.
Consider these models:
And the modelling result:
You can see that the voltage is higher on the circuit with the 1.9uA current source.
The offset can be calculated as the Thevenin equivalent resistance of the divider multiplied by the SS current. With the values chosen the offset is 15mV.
The whole circuit has a gain of
5 25 from the TRK/SS pin to the output, so the 15mV offset becomes
75 375mV on the output.
(The gain is from the TRK/SS PIN to the output. The gain is 5 from the from V_CTRL to the output)
CompensationIdeally you would want to create a current source of the appropriate value and inject this current into the output voltage divider to create a matching offset.
The Thevenin resistance of the output divider is 10K//249K = 9.6 k Ohm
So to get 15mV of offset we need a current of 15mV/9.6k = 1.56 uA
You could go to a lot of trouble and design a 1.56uA current source, or you can make a first order approximation using a voltage and a resistor.
The INTVcc, at a nominal 3.1V is a suitable voltage.
ModelModelling ResultsSo in the first order the offset caused by the SS current source has been cancelled. There is little point in having a better solution because the SS current source is specified as 1.2 to 2.6 uA over the full temperature range of the part.
Regards,
Jay_Diddy_B