In this article Robert Keim from “All About Circuits” website presents and discusses a schematic design for a ±5 V inductorless power supply.
I recently wrote an article on charge-pump DC/DC converters, i.e., DC/DC converters that create output voltages by periodically pumping charge onto a capacitor instead of switching current through an inductor. Charge-pump-based voltage regulation is an important alternative to the more common inductor-based approach; charge-pump circuits
The primary limitation with charge-pump regulators is output current; inductor-based switchers are a better choice when you need more than about 50–100 mA. However, 50 mA is plenty of current for many low-power electronic devices or subcircuits, and it seems to me that the focus on inductor-based DC/DC conversion has caused many designers to ignore a potentially superior alternative.
I created a reference design for a power supply block that takes a 5 V input and generates +5 V and –5 V output rails. It would not be difficult to modify this circuit for different voltages, but I think that the 5 V to ±5 V configuration could be useful in many applications, because 5 V is what you get from USB power (which is conveniently available almost everywhere) and because ±5 V is suitable for a wide range of analog circuits. Also, 5 V is a good place to start if you want to generate 3.3 V using an LDO, so maybe you could use the positive 5 V rail for analog circuitry and also regulate it down to 3.3 V for digital circuitry.
A note regarding the dual supplies: There is no doubt that many analog circuits can be implemented in a single-supply environment, and this approach can be advantageous. However, my personal opinion is that analog circuits are more straightforward and more intuitive when bipolar supplies are used. I am the last person who would want to complicate a design with unnecessary power-supply circuitry, but the charge-pump circuit presented in this article is so simple and compact that it makes bipolar supplies a feasible option for many analog and mixed-signal devices.
The central component in this circuit is the LTC3265 from Linear Tech/Analog Devices.
It’s a highly integrated part that incorporates a voltage-doubling charge pump, a voltage-inverting charge pump, and two linear regulators. Here’s how I go about generating symmetric, low-noise rails:
There are other ways to implement the LTC3265. You could invert the input voltage and then use the input voltage and the inverted voltage as your bipolar rails, or invert and double the input voltage and then use an LDO to regulate only the doubled voltage, or use the doubled voltage to feed the inverter and connect the doubled and inverted outputs directly to the load (i.e., without using the LDOs).
However, the configuration that I use in the reference design is preferable in most situations:
I should mention one detail before we discuss other aspects of the schematic: I’ve referred to the charge pumps as “doubling” and “inverting,” but the full story is a bit more complicated. The LTC3265 can operate either in burst mode or in open-loop mode. In open-loop mode, the boost charge pump increases its input voltage by a factor of two and the inverting charge pump multiplies its input voltage by negative one. In burst mode, however, the factors are slightly smaller: VBOOST = 0.94 × 2 × VIN_BOOST, and VINV = –0.94 × VIN_INV. This doesn’t really affect my circuit, though, because the small difference won’t change the voltage generated by the LDO.
Here is the entire schematic for my inductorless bipolar power supply:
As you can see from the schematic, a part like the LTC3265 allows you to generate low-noise bipolar power supplies without extensive design effort and without a long list of components. (I’m assuming that the LDOs will remove most of the switching noise; I’ll know for sure after I have a chance to test the board.) Though certainly not a high-current power supply, the circuit can provide up to 100 mA (50 mA from each LDO), which is more than enough for many applications.