Design Regulated Dual Supply w/uA78L05 for Research Project

[gaganaut]

I am trying to build an opamp array to amplify some voltages simultaneously. The amplification part has been designed. But now I am trying to design a Regulated Dual Supply for all the opamps.

I found this circuit diagram in the Texas Instruments datasheet for uA78L05. This diagram shows an elaborate circuit, but this is for +/- 15V supply. I need a +/- 5V supply.

So can we just substitute the uA78L15 and uA79L15 with their 5V counterparts uA78L05 and uA79L05? Also will the capacitor in this circuit be retained or they need to be changed too?

I need to this for a research project and I have no Elec. background :(
I hope you peeps will be helpful as always.

Thanks

 

[berkeman]

Yes, that would be the correct substitution. You can also reduce the input voltages down to +/-8V or so if you want. Be sure to calculate the power dissipation in the regulators, to see if you will need heat sinks or larger physical packages.

 

[misgfool]

Try to keep the input voltage in the range defined in the specs. For example for x05 it is 7...20 V and with x15 17,5...30 V.

 

[Proton Soup]

the capacitor values are probably not that critical. i don't ever remember seeing a justification for the values chosen. i would definitely bring that input voltage down as suggested above, because otherwise, those regulators will be big heat radiators. i wouldn't get too close to the spec, though. berkeman's suggestion of 8 is probably good, but no more than 10.

and don't forget to check the current requirements of all your op amps to be sure your regulators and power source can handle it.

 

[gaganaut]

Yes, that would be the correct substitution. You can also reduce the input voltages down to +/-8V or so if you want. Be sure to calculate the power dissipation in the regulators, to see if you will need heat sinks or larger physical packages.

Thanks Berkeman. That was helpful. I am planning to use a 7.4V LiPo for powering up. So that fits perfectly to your suggestion.

Try to keep the input voltage in the range defined in the specs. For example for x05 it is 7...20 V and with x15 17,5...30 V.

Thanks Misgfool. The input range for the regulators, as you pointed out, is 7-20V for uA78L05. I am going to use a 7.4V LiPo, so it will be well within limits.

the capacitor values are probably not that critical. i don't ever remember seeing a justification for the values chosen. i would definitely bring that input voltage down as suggested above, because otherwise, those regulators will be big heat radiators. i wouldn't get too close to the spec, though. berkeman's suggestion of 8 is probably good, but no more than 10.

and don't forget to check the current requirements of all your op amps to be sure your regulators and power source can handle it.

Thanks Proton Soup. I could not find the explanation for the capacitors as well. Practically speaking, as you say, they should not make any difference as they will do their charging-discharging business in any case. But a quick corroboration from you was helpful.

With regards to the current rating of the op amps, each takes in 1.4mA typ. But I am using 128 of them, 64 for amplification and 64 for filtering. So that way current rating should not be a problem. I just need to change the fan-out now to be sure of being able to do this.

Thanks all for your help.

 

[berkeman]

Thanks Proton Soup. I could not find the explanation for the capacitors as well. Practically speaking, as you say, they should not make any difference as they will do their charging-discharging business in any case.

The caps will be mentioned in the voltage regulator datasheets. Some regulators require some minimum amount of output capacitance in order to guarantee stability (Low-Dropout LDO regulators typically have a minimum output C requirement). The jellybean regulators that you are talking about using don't have this requirement, but I believe the datasheet says something about the 0.1uF caps (or larger) helping transient response or something similar.

You should be sure to decouple each opamp's power pins to ground at each IC, and add a few bulk electrolytic caps sprinkled around the assembly.

BTW, when you mention a 7.4V power supply, you mean +/-7.4V, right? Why such an unusual number?

 

[gaganaut]

The caps will be mentioned in the voltage regulator datasheets. Some regulators require some minimum amount of output capacitance in order to guarantee stability (Low-Dropout LDO regulators typically have a minimum output C requirement). The jellybean regulators that you are talking about using don't have this requirement, but I believe the datasheet says something about the 0.1uF caps (or larger) helping transient response or something similar.

You should be sure to decouple each opamp's power pins to ground at each IC, and add a few bulk electrolytic caps sprinkled around the assembly.

BTW, when you mention a 7.4V power supply, you mean +/-7.4V, right? Why such an unusual number?

Actually, I did find some information about decoupling caps in the datasheet. They need to be 0.1uF.

Regarding the battery, I have a =/1 7.4V battery and it was picked up off the shelf. Actually, one of my labmates ordered 4 of these and I an planning to just use these to save some moolah.

Thanks

 

[Proton Soup]

batteries should be great in the sense that power supply noise is low and you can keep things isolated. but keep an eye on that battery voltage as they discharge. the datasheet is recommending 7V and at 6.7V it may fail to operate correctly.