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

• Thanks for the quick reply.

Just to be clear are you saying I should power my external components as shown in the drawing below?

I need -5V and around 50mA.

I'll resolder my circuit and try it out.

Regards,

Alex

• That works. I've tested it under a load and the voltage holds pretty well.

Thanks for your time.

Alex

• The equation in the datasheet is a little confusing, because for the -2x doubler, the Vout is not the "VOUT" pin....  Its actually the node connecting R1 to the 10uF cap.  So that node will be about 2x the voltage on the VOUT pin. So let me know if that works.  Otherwise, can you give me some more info:  what is the voltage that you want to achieve?  And what is your output current?

Thanks.

- Jon

• Yes, what you've drawn is correct.  Let me know if you have any trouble with it.