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LM27762 is a charge pump combined with an LDO. It outputs a +2.5 V and a -2.5 V. The charge pump portion switches at 2 MHz.

I am not worried about the ripple noise at 2 MHz + harmonics I will take care of that with pi filters and LC filters.

What I'm primarily concerned about is the noise created from the rise time. I can't figure out if the switching rise time noises path is my outputs?

This is to power a very sensitive analog EEG. ADS1299. Can anyone figure out if the rise time affected loop is constrained in the LM27762 or is its path the out+? The out- is technically pulling into the LM27762 so it's not going to the ADS1299 anyway.

I simply don't want the rise time of the charge pump 2 MHz switching to affect the power that hits the ADS1299.

Datasheet

pcb

Schematic lm27662

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  • \$\begingroup\$ In charge pump system, the 2 MHz ripple is caused by the rise/fall time of the pump switching the capacitor polarity around, the ripple essentially is many rise/fall events repeated one after the other. Posting this as a comment because I'm not sure if it addresses your question. \$\endgroup\$ Commented Nov 26 at 18:02
  • \$\begingroup\$ Rise time of what exactly? Ideally, charge pumps work by shorting a capacitor to input which charges it and then shorting it to the output capacitor to charge it, repeating at 2 MHz. \$\endgroup\$ Commented Nov 26 at 18:02
  • \$\begingroup\$ @Justme. Well you know how there is a rise time associated with switching? Thats what im worried about \$\endgroup\$ Commented Nov 26 at 19:29

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What I'm primarily concerned about is the noise created from the rise time.

The noise created from the "rise time" of the switching in the charge pump is the cause of the 2 MHz ripple + harmonics. You will take care of this by filtering the ripple.

I can't figure out if the switching rise time noises path is my outputs?

I drew a very crude/oversimplified drawing over your layout of where I would generally expect the switching noise current to flow (yellow) and the positive LDO output current (pink). For the switching noise, the current is mostly supplied by the input capacitor. Because you're sharing this capacitor for the LDO, I would expect variation at the input pin voltage to affect the LDO output, especially at such high frequencies where the LDO PSRR is not very effective.

enter image description here

It looks like the rest of the circuit does not share much of a current path. Note the return current for the charge pump is flowing on the ground plane mostly directly above the forward current path. I did not draw the return current for the LDO because it's not clear from your picture where the load is, but as long as you control the return current path away from the switching current path, you don't have much to worry about.

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  • \$\begingroup\$ ive concluded that the only way im gonna get the noise supppresion i need is by having a seperate charge pump and separate ldo and putting a pi filter between them and a bunch of other lc filters and stuff on the inputs and outputs and im gonna have to adjust these filters by hand after the pcb is manufactured \$\endgroup\$ Commented 2 days ago

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