LT1054/7660s whistling - joining pins 1 and 8 works on the LT1054?

[BluffChill]

I've been building a distortion pedal which uses a charge pump to boost the voltage to 18V. I have a load of 7660s chips lying around but they all give an unbearable whine, even with the HF boost (bridging pins 1 and . I gave up and bought three LT1054 chips to experiment with.

I replaced the 7660s with the LT1054 and cut the jumper between pins 1 and 8. It's way, way better but there's a very very high pitched frequency whine, very quiet and barely on the threshold of hearing, coming through. If you turn your guitar volume down you can hear it and it's quite distracting. So I read several threads here and other places on the web and found two suggestions:

- ground pin 7
- 5-25pF cap between pins 2 and 7.

I tried both. Neither worked. Sending pin 7 to ground did nothing, and the cap between 2 and 7 made the frequency lower, more audible (and possibly louder), even worse than with the 7660s.

I was about to quit when I accidentally rejoined the pins 1-8 jumper, and the squealing stopped and it seems to work perfectly, and after five minutes or so of play, didn't catch fire.

Is this a known trick with the LT1054? I can't find any reference to anyone using this trick with this specific chip, and really just want to know if it's safe as this is a custom job for someone and I'd hate to give them something that would break or catch fire.

[Kevin Mitchell]

While I'm unsure of how it solved the issue - none of the application notes on the datasheet suggest tying the connections. I wouldn't recommend doing so for the longevity of the IC.

I've actually never had noise issues with the LT1054 and I've used them in a dozen designs. I wonder if something fishy is going on with yours. You've got a few so I assume you've swapped them around?

What effect is it running?

-KM

[BluffChill]

Kind of a Klon/DRV1981 hybrid. Pretty basic distortion sort of setup.

[Rob Strand]

Root cause is perhaps no cap across the input supply (ie. to pin letting noise get into the ckt.
Cap ESRs could come into play as well.

Assuming you have the cap on pin 8 the only way forward is a workaround. 

Noise is probably getting into the feedback opamp input (via pin 1).  Connecting pin1 to pin 8 forcefully disables the feedback opamp.  That might be OK but it's not possible to tell from the datasheet.   Ideally connecting the opamp to internal 2.5V (Vref) would have less risk of side effects but you can't do that.   Perhaps the safest workaround/experiment would be put a 100nF cap from pin 1 to ground, that ensure the Feedback opamp input is clean.

The problem with these types of "fixes" is you need to prove you don't have some other problem before you go and add a fix which could be just hiding the real problem.

[bluebunny]

Pin 1 isn't a frequency boost on the LT1054, unlike its lookalikes (it seems to get used when the chip is used for regulation: "FB" = feedback).  None of the voltage boost circuits I can see make any use of it.  Leave it unconnected.

[duck_arse]

I have a rude question - what is the output voltage of the LT1054 now?

I was about to quit when I accidentally rejoined the pins 1-8 jumper, and the squealing stopped and it seems to work perfectly, and after five minutes or so of play, didn't catch fire.

[Rob Strand]

FWIW,  the LT datasheet indicates pin 1 can be set to a voltage equal to the input supply rail.  It appears under *Absolute Maximum Ratings*.  Setting a pin to the absolute maximum limit is probably a bad idea.  Also, while the voltage can be set that high it doesn't say if it needs to go through a current limiting resistor  to avoid damage under all conditions.

I'm not agreeing with connecting pin 1 to pin 8. All I'm saying is the datasheet say *something* about the voltage on pin 1.
You need to find the root cause of the problem. 

[BluffChill]

OK well in the end I managed to get rid of almost all the whistling with a basic cap to ground on the output - I don't lose any tone, but the whistling has pretty much disappeared. I'm wondering if I got a bad batch of LT1054s (they were cheap) or even some re-labelled MAX1044s!

[Rob Strand]

I'm wondering if I got a bad batch of LT1054s (they were cheap) or even some re-labelled MAX1044s

Maybe they are all MAX1044's which have been relabeled (ie. fake LT1054's). 
Your problem seems to imply they could be.

If they are MAX1044's you should be able to see the switch frequency change when pin 1 is connected or not.
The LT1054 won't change and it's a different frequency.
The frequencies of the parts are all different so it shouldn't be too hard to confirm what part it is.

[BluffChill]

Hmmmm. If the frequency shouldn't change when I connect pins 1 and 8 of an LT1054 then these are almost definitely fake LT1054s. Thanks all for your help!