Looking for some charge pump experience.

[RobA]

Since this is my first post, I've got to start off by saying thanks for making these PCB's available. I've built the Current Lover, Pork Barrel, and Slow Loris so far and they've all been great.

Now to the question(s). I've been playing around with a circuit that is essentially the negative voltage generation from the Road Rage. I've got a modified polarity protection scheme on the voltage input and I've changed the capacitors a bit. For the caps, I'm using low ESR electrolytics paralleled with smaller tantalums. Although, this doesn't really seem to have much to do with the issue since changing caps around doesn't fix the issue (other than changing the volume).  I'm testing with an op-amp based tone control circuit that can give about 20 dB of gain.

The issue is the oscillator frequency from the various converter chips. So far, I've tested the Microchip TC1044SCPA, Intersil ICL7660S, and the LT1054CP from TI. I've put a scope on the output and here is what I hear and see for the whine (frequency multipliers on) : TC1044SCPA 10800 Hz @ -68dB, ICL7660S fractionally below 20kHz @ -68dB, LT1054CP 38 kHz @ -64dB. I have either 2 or four samples of each of these and the numbers are pretty close.

So, the TC1044SCPA looks unusable on anything with much gain. The ICL7660S is usable but I'm a bit worried about marginal samples causing problems. The LT1054CP looks really good but is at the higher end price wise.

The TC1044SCPA's have a non-multiplied frequency of around 3kHz which is well below the spec'd typical value. So, I was wondering if anyone else had scoped any of these and had more data. 

Also, I see that Microchip has a device, the TC7660H that is spec'd at 150kHz at 20mA supply with a 10.5V limit. Has anyone tried one of these?

The last question I had was if anyone had tried using an external clock to drive the TC1044SCPA and if so what were your results?

Thanks, Rob

[nzCdog]

Hey Rob, Welcome to the forum and great first post!
Interesting research you're doing here, I don't have anything brilliant to add other than I'm jealous you have a scope at home!    I was thinking if you can't eliminate the noise of the TC1044 through filtering, maybe trying a different Opamp may help...  something with better CMRR?  I'm very interested to hear how your project pans out 

[RobA]

Hi Corey,

Thanks for the input. I love your country. I got to spend two weeks on the South Island about 35 years ago. Absolutely beautiful!

About the scope, it's not really a scope. I have a MOTU MicroBook II. The software that comes with it has several features that are passable for this purpose. Actually, the combination of the signal generator and the FFT view have been really useful for debugging designs and builds.

I haven't tried putting a choke(s) in the circuit yet. I suppose that might help.  The opamp is an OPA2134 with the power rails bypassed using 2.2µF tantalums and 0.1µF ceramics located at the pins on the breadboard. If I have to go much farther than that with the 1044, I'll give up and stick with the LT1054. The LT1054 does work perfectly (even without the bypass caps). Really, my curiosity was piqued by how low the base frequency was for the 1044 samples I have, so I was trying to investigate what was going on.

I'm going to try an external clock on the TC1044SCP and I'm going to order some samples of the TC7660H and give them a try. I'll update with those results when I get them.

Rob

[nzCdog]

Thanks Rob, I love NZ too the South Island is very beautiful
Those Motu's look nifty... I have been trauling for a half decent scope interface for my PC, keeps getting bumped down the priority list however... :S  I saw this thread on the subject a while ago, might be of interest.

Be great to hear your musings on the TC1044 and others when you get done.

[RobA]

For anyone who might find this useful, I ordered a few more parts to test with and they arrived today. So, here are the results:

1) I got a few of the TC7660H and tested them in a boost/overdrive circuit that was rigged to draw right at 20mA. The high frequency oscillator noise showed up at over 40kHz for each of them. So definitely useable for this situation (-9V output).

2) I got 10 more samples of the LT1054CP and put them in a circuit that was drawing about 40mA. Every one of them had the whine at over 35kHz. So, this look to be a good choice.

3) I tested driving the TC1044SCPA with a 555 timer. This works really well for setting the frequency to whatever you want it to be. I was able to drive it to 48kHz. The only problem with this is that the 555 timer does introduce noise. The CMOS version of the 555 is supposed to take care of this and I've now got a couple to test with but I haven't tried them out yet. This does look like a good path to look into if you really want to be sure the frequency is held above audible.

Once I get around to testing the CMOS 555 timer, I'll update with those results too.

Rob

[croquet hoop]

I know this thread is a bit old, but I found Rob's experiments interesting. I'm thinking of adding a charge pump to a high gain pedal (Riot clone, possibly an Upper Decker later), so should I infer from this thread that the 7660H is a good choice to feed these builds with 18v without adding noise?

[RobA]

I've done some further testing with the TC7660H and I think it works the best of the low current charge pumps I've tried. It seems to be good for currents that are less than 20mA. Overall though, I still prefer the LT1054. I've yet to come across one that has an audible oscillator and it measures out to be more efficient voltage wise too.

With that said though, it can depend on other factors like clocks in the power supply or other pedals in the chain. and the frequency that the other clocks are running at. It could be that one or the other of the TC7660H or the LT0154 could be better at not interacting with a given external clock frequency. They have very different nominal frequency specs, so it could make a big difference.

[croquet hoop]

Noted, thanks