Have you come across a need for a split-rail, low-power supply from a single input voltage? For example, this could be +12/-12 volts from 5 volts or +/-5 volts from 12 volts. This Power Tip presents three power-supply topology options to meet these types of requirements with varying degrees of performance and costs.
Figure 1 presents a simple method, which involves a charge-pump integrated with a boost converter. The boost output is a regulated positive voltage while the charge pump supplies the negative voltage. Referring to Figure 1, when MOSFET Q18 turns off, capacitor C86 charges through D18 to a voltage equal to the positive output voltage plus a diode drop. When Q18 is turned on, C86 discharges through D17 into output capacitor C87. D15 and D16 add a diode drop to the charge on C86 to offset the voltage drops of D17 and D18 in the charge pump. Removing D15 from the circuit results in the -12 volts, being a diode drop less than the +12 volts. This circuit needs to have equal or greater loading on the positive output, or the ripple voltage becomes excessive on the negative output. For instance, if the positive output is unloaded, the power supply switching stops and the negative output capacitor voltage decays until the next switching cycle.
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Figure 1: A charge-pump added to a boost creates a negative output when VO>VIN.
presents an alternative approach using a buck-boost converter with a coupled inductor. This circuit is useful for a wide range of input and output voltages and is not constrained to just a boost relationship. This circuit uses an integrated-FET buck controller in a buck-boost power stage. The controller is referenced to the negative output voltage, but starts through the output diode D2. As current in the power inductor's primary winding builds, the negative voltage is driven lower. In this circuit, the sum of both the positive and negative output voltages is regulated. This improves regulation of the outputs, compared to only regulating one. If you only regulate one output it will be tightly controlled, and you may have a +/-10% variation on the other. In this case, regulating the sum improves the regulation of each to +/-5%. The controller's return is connected to the negative output, which has advantages and disadvantages. It eliminates a differential amplifier that would be needed, if the return were connected to ground. However, it requires level shifting signals like power good, enable and clocks. The other choice that you have to make with this circuit is whether to design it so that it always operates with continuous inductor current. For continuous operation, D2 and perhaps D1 are often replaced with MOSFETs, which allows the current to reverse in them during the 1 - D off time. If D1 is not replaced with a FET and the turns-ratio of the inductor is 1:1, the positive output will be about a diode drop less than the negative. Continuous conduction provides better efficiency and cross regulation, but at the expense of complexity and cost.
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Figure 2: Buck-boost produces two outputs with no restriction on VIN to VO with a coupled inductor.