Hello,
in the past, I used quite a lot of the LM2776 charge pumps. In a recent project, I used the LM2776 to convert +5V to -5V and applied both voltages to three LT1994CMS8 OPVs.
The output of the LM2776 dropped to +720mV. This behavior I had in rare cases also in the past.
When I disable two of the three OPVs via the enable pin of the OPV, the -5V appears and then, I can enable the other two OPVs as well. For other projects, I increased the output capacitor and was able to get rid of this effect. So far I hat no luck with this improvement.
From various post, I assume that the LM2776 has problems when some voltage is present at the output before the LM2776 has reached its nominal voltage. In case of direct connection to the power rails of an OPV such a situation is not uncommon. If I connect 30R resistor in series, the LM2776 output voltage starts even when all OPVs are enabled by default. But this is just a hack as there is a voltage drop of about 1,5V then.
When connecting a low voltage dropout regulator with e.g. -4V which is then used to drive the OPV, the LM2776 works well at all time ...
My question is now: Is this a common behavior of the LM2776 and is there any possibility to get rid of it?
Best regards
Steffen
