Hi Alex,
This is a common question that I think we can help you with. Consider the two phases of the inverting charge pump operation:
Phase 1: C1 (the flying cap, which is C38 on your schematic) is applied in parallel to the input cap (C35 on your schematic). So C1 charges to Vin.
Phase 2: C1 polarity is reversed and C1 is applied in parallel to Cout (C31 on your schematic). So Cout charges to -Vin.
During Phase 1, C1 must be charged from the input caps (C38). And if those input caps don't have a good charge path from their source (LM317 Vout), then the load on them that C1 represents (do to the change in input capacitance-->C1||Cin) will cause a drop in the input voltage during phase 1. If Cin=1uF and C1=10uF, then the new input capacitance is basically increased by 10x during Phase 1--requiring a lot of current and time to recharge Cin during Phase 1. Does all that make sense?
There are a couple of easy fixes:
1. Change the switching frequency to 120kHz and decrease the value of C1 to 2.2uF. This will dramatically reduce the load that C1 places on Cin.
2. Add more input cap. I think the Cin is far too low (its only about 1.1uF now). Try 10uF or so. Then when C1 is placed in parallel with Cin, the change in input capacitance will be much smaller.
3. Change to ceramic capacitors. Unless your application requires the use of tantalums, I highly recommend switching to ceramics. The ceramic capacitors have a much lower ESR (then both tantalums and electrolytics) which will help tremendously in charge pump applications. I would recommend switching Cin, C1, and Cout to ceramics.
I hope that helps. Let us know if you are still having trouble or if you need anything else.
Best Regards,
- Jon