Boost regulator from 3V to 14V; then two LDO to make a +5/-5 power supply?

Question

I am designing a mixed signal circuit and I need +5/-5 dual supply for my opamps. Could the following configuration work, else what are my alternatives?

 Li battery 3V--> Boost regulator--> 14V-->14 to 7---> Linear reg(e.g.,7805)--->+5V    
 (coin or LiPo)                             0 to 7---->Linear reg(e.g.,7905)--->-5V
 The 7V would be the actual ground throughout the circuit for both analog and digital

Given that, I have a microcontroller and other digital components in the circuit, I will be drawing more current from the positive supply than I would be sinking from the negative supply, so a net positive current is drawn from the boost regulator (OR is there something wrong with this logic?)

Would this scheme work? Any other suggestions for achieving the same results? Any specific battery specs or otherwise I need to worry about?

Answer 1

I would use a boost regulator form 3v to 5v, and then a voltage inverter like the Analog Devices ADM8829 or the Maxim MAX1697, about $2.30 in single quantities, to get the -5v.

Both take an input up to 5.5v, and produce a negative voltage of the same magnitude, with an output of either 25 mA or 60 mA which should be enough for op-amps. They only need a minimum of external components (two caps). Quiescent current is 600 uA for the AD part, and a few hundred µA for the Maxim part. Very simple, you don't have to deal with virtual grounds or the like.

Answer 2

1. The 78XX and 79XX series are (to my knowledge and a quick google check) NOT Low Drop Out (LDO) regulators. They require (as you note) some 2 Volts above the regulated voltage on the input.
2. You would do better to use a switching power supply to generate your negative voltage - this way, you can have one ground level for all of your circuit rather than having a virtual audio ground 7 Volts above your digital and other ground planes.

Answer 3

Yes, it will work. Or at least it can be made to work with some extra effort. But it's probably not the best way to do it; the best solution would probably be an inverting switching regulator for the negative rail. But I will discuss your idea here.

As @JRE said, the 7805 and 7905 aren't LDO regulators; they're linear regulators (and LDO is another type of linear regulator, "low drop-out").

But that's beside the point. The 78xx can only source current and the 79xx can only sink current. But to create the virtual ground at 7V, you'd need to do it with a regulator that can both sink and source current, because at different times either the high side or the low side might be using more current. An op amp wired as a voltage follower would be one example (following the 7V from a resistive voltage divider), but it needs to be beefy enough for the difference between the high side and low side currents.

At least that's the theory if you want a robust ±5V regulator for generic use. If your circuit always uses more current on the high side, the virtual-ground regulator could be a sink-only type.

Since your power source is a battery, the virtual ground is perfectly okay, but if you connect a grounded battery charger and at the same time your circuit's output is connected to some grounded equipment, suddenly your virtual ground could appear like +7V and blue smoke may be released from somewhere.