We're using the ADP3605 on one of our evaluations boards and we need to get a voltage of -5V.

Seeing as our input voltage can go below 5V we're using the circuit shown in Figure 17 of the datasheet to get a scaling factor bigger than -1

However the Vout = (1.5/9.5KOhn)*R1 seems to apply only when the normal circuit is used without the charge pumps.

When using the circuit below with R = 31.6k we get close to -5V, but when we use the circuit above we get -2.6V.

I'll use a potentiometer to see what the resistor needs to be in the meantime, but I'd prefer if I could use a formula to determine an acurrate value.

Is there such a formula or how would I go about deriving one?

Thank you