6.5v supply for IC

[level6]

I need to supply an IC with -6.5V on a supply pin, max -21mA. Would a -8V regulator with a voltage divider after it work okay?

[JustMeHere]

Have you looked for an adjustable regulator?

[Zero999]

I need to supply an IC with -6.5V on a supply pin, max -21mA. Would a -8V regulator with a voltage divider after it work okay?

That sounds odd. What IC is that? Please post a schematic.

[magic]

LM337L appears to exist

[greenpossum]

He meant the IC that needs -6.5V.

[Gyro]

At that sort of current, a TL431 shunt regulator might be better (more accurate) than a zener - it depends how close a tolerance is needed.

I'm curious what the IC is too, something PMOS maybe?

[GerryR]

How about 4 AA or AAA batteries in series??

[Fraser]

How about a LM79L06 -6V linear regulator with a silicon diode in series with the 0V pin to increase the output to -6.6V ?

https://www.alldatasheet.net/datasheet-pdf/pdf/446014/HTSEMI/LM79L06.html

Much depends upon the required accuracy of the -6V rail.

This thread details the issues with the approach I have suggested.....

https://www.eevblog.com/forum/beginners/diode-in-gnd-pin-of-voltage-regulator/

If it is a case of using the right part for the job to ensure accuracy.... go for a low current adjustable regulator that can be set to exactly the voltage you require.

Fraser

[level6]

I need to supply an IC with -6.5V on a supply pin, max -21mA. Would a -8V regulator with a voltage divider after it work okay?

That sounds odd. What IC is that? Please post a schematic.

It's a specialty IC for analog music synthesis.

http://www.alfarzpp.lv/eng/sc/AS3394.pdf

I'm thinking a variable voltage regulator is probably the best way to go. Would I use a trim pot with the regulator or are the specs for variable regulators dependable enought to just use a static resistor value?

[Siwastaja]

Any adjustable negative regulator. Do not use trimpots, just calculate the resistor values to get the voltage you need. Most cheap regulator ICs have approx. 2-3% accuracy which is very likely more than good enough for such IC.

In any case, whatever regulator you end up using, read the datasheet carefully. They may have specific requirements for the output capacitor type (amount of ESR) and value.

[wraper]

I need to supply an IC with -6.5V on a supply pin, max -21mA. Would a -8V regulator with a voltage divider after it work okay?

That's a wrong way to approach the problem. Normally you should use adjustable regulator with resistive divider on it's adjust pin. Once you attach divider on the output, voltage will be no longer stable and will change under load, not to say it's inefficient in regards of wasted energy.

[Gyro]

Maybe not as exotic as we might have thought. Could be just a dual rail opamp perhaps?

Well a little bit exotic then 

The Application Hints on page 5 of the datasheet seem pretty comprehensive. The -6.5V supply is pretty non critical and is used to feed the internal current mirrors.... "Any one of the readily available 3-terminal regulators may be used to supply the -6.5V negative supply". Fraser's 79L06 or even a 79L05 with the required number of diode drops in the ground lead or an LM337 will be fine (check the datasheets for capacitor value and type requirements).

Note that the positive supply is critical "Since the stability and jitter of the VCO are directly affected by noise on the positive supply, a supply as stable and clean as possible should be use (not the +5Vdigital supply)". So you should dedicate a 7805 or well filtered LM317 regulator just for this one chip.

[level6]

Thanks, everyone, for all your help! I think I'll go with the LM337 and a 10uf electrolytic stability capacitor.

[magic]

Consider LM337L unless power dissipation capability is insufficient.

[mariush]

I just checked the datasheet.

It looks to me like you don't HAVE TO power it with -6.5v

Positive power supply range : min 4.75v  max 8v  ( typ. 16mA , max 21mA )
Negative power supply range :  min -4.75v, max 16v (typ. 16mA , max 21 mA)

Page 5 says: The AS3394E was designed to operate from +5V and -6.5V supplies. The non-standard negative supply was necessary not to compromise the VCO frequency resolution, which ranges from -4 to +4 volts, in favor of a -5V supply (this  is  because  there  needs  to  be  4  diode  drops  for  the  current  mirrors).Any  one  of  the  readily available 3-terminal regulators may be used to supply the -6.5V negative supply. Since the stability and jitter of the VCO are directly affected by noise on the positive supply, a supply as stable and clean as possible should be use (not the +5Vdigital supply) Maximum supply allowable across the device is 25 volts

So you could power it with 5v (a 7805) for the positive side, and you could power it with -9v from a 7809 (or low current variants) and it should work fine.
Another option would be to use an adjustable regulator to output 7.5v for the positive side, and then use an inverter IC to produce -7.5v for the negative side .. see for example TC7660 from Microchip (and maybe others) which can take in 1.5v..10v and do -Vin / 2 Vout at up to 20mA : https://www.digikey.com/product-detail/en/microchip-technology/TC7660CPA/TC7660CPA-ND/115285

The max 10v input would make it possible to also power your device with  a 7.5v or 9v wallwart adapter : shove 9v into the inverter chip and you get -9v, shove the 9v into a 7805 and you get +5v ....

[level6]

Consider LM337L unless power dissipation capability is insufficient.

Yes, it's the LM337L I'm considering. I already have a few of these to work with.

[Zero999]

I need to supply an IC with -6.5V on a supply pin, max -21mA. Would a -8V regulator with a voltage divider after it work okay?

That sounds odd. What IC is that? Please post a schematic.

It's a specialty IC for analog music synthesis.

http://www.alfarzpp.lv/eng/sc/AS3394.pdf

I'm thinking a variable voltage regulator is probably the best way to go. Would I use a trim pot with the regulator or are the specs for variable regulators dependable enought to just use a static resistor value?

That looks like an unusual IC. An analogue synth, designed to be controlled by a digital circuit. Today, most synthesisers are digital.

Anyway, the LM337L is a good choice. The schematic is on the data sheet. For -6.5V out:
R1 = 240R
R2 = 1k
https://www.ti.com/lit/ds/symlink/lm337l.pdf

The LM337 will also work, but is overkill. The LM337L is smaller, cheaper and the lower current limit of <320mA, 220mA typical, provides more short circuit protection, than the LM337, which might pass over 3A, before shutting down.

[level6]

I just checked the datasheet.

It looks to me like you don't HAVE TO power it with -6.5v

Positive power supply range : min 4.75v  max 8v  ( typ. 16mA , max 21mA )
Negative power supply range :  min -4.75v, max 16v (typ. 16mA , max 21 mA)

Page 5 says: The AS3394E was designed to operate from +5V and -6.5V supplies. The non-standard negative supply was necessary not to compromise the VCO frequency resolution, which ranges from -4 to +4 volts, in favor of a -5V supply (this  is  because  there  needs  to  be  4  diode  drops  for  the  current  mirrors).Any  one  of  the  readily available 3-terminal regulators may be used to supply the -6.5V negative supply. Since the stability and jitter of the VCO are directly affected by noise on the positive supply, a supply as stable and clean as possible should be use (not the +5Vdigital supply) Maximum supply allowable across the device is 25 volts

So you could power it with 5v (a 7805) for the positive side, and you could power it with -9v from a 7809 (or low current variants) and it should work fine.
Another option would be to use an adjustable regulator to output 7.5v for the positive side, and then use an inverter IC to produce -7.5v for the negative side .. see for example TC7660 from Microchip (and maybe others) which can take in 1.5v..10v and do -Vin / 2 Vout at up to 20mA : https://www.digikey.com/product-detail/en/microchip-technology/TC7660CPA/TC7660CPA-ND/115285

The max 10v input would make it possible to also power your device with  a 7.5v or 9v wallwart adapter : shove 9v into the inverter chip and you get -9v, shove the 9v into a 7805 and you get +5v ....

I wondered about that statement in the datasheet. If it was designed for specific voltages wouldn't that mean it is best to use those voltages, even though the limits are higher?

[level6]

That looks like an unusual IC. An analogue synth, designed to be controlled by a digital circuit. Today, most synthesisers are digital.

Oh, there's a whole world of analog synthesizers out there!

[Siwastaja]

Note that the positive supply is critical "Since the stability and jitter of the VCO are directly affected by noise on the positive supply, a supply as stable and clean as possible should be use (not the +5Vdigital supply)". So you should dedicate a 7805 or well filtered LM317 regulator just for this one chip.

Note that most of the "digital noise" just goes through 7805 or LM317 with little attenuation, so the keyword here is "well filtered". Another option is to use a high-bandwidth regulator which shows good PSRR values even at high frequencies (to tens of MHz).

[exe]

I wondered about that statement in the datasheet. If it was designed for specific voltages wouldn't that mean it is best to use those voltages, even though the limits are higher?

The only way to know is to test.  My guts feeling tells me that you won't even hear the difference. Afaik the whole idea of current mirrors is that their performance doesn't vary much with the voltage (unless it's a very crappy design), as long as there is enough voltage headroom (that's why the datasheet specifies that it should be at least 4 diode drops). So, if they did it "right", then there is no problem. May be this will even improve performance, but I doubt you will hear it.

Anyway, I don't see a reason why not using an off-the-shelf regulator to produce -6.5 output. All you need is to calculate a voltage divider that will set the output voltage. Afaik, you can even use any three-terminal regulator (if it's voltage in [-6.5:0] range), and trim it's output just like you do with lm337 (unless it has significant ground current). I'd use a trim-pot to set the output.