The schematic in Figure 1 shows a way to increase the power available from a current-limited 5V supply by adding power from a –5V supply. The dc/dc converter generates a single 12V, 150-mA (1.8W) output from two regulated and current-limited input sources at 5V, 300 mA (1.5W) and –5V, 300 mA (1.5W). Because the input uses different-polarity voltage sources, the design uses a flyback dc/dc converter to avoid a system-grounding problem. Level-shifted feedback sensing using a pnp transistor, Q1, references the feedback signal to the negative input voltage. You calculate the feedback-resistor divider by using the formula R1=R4(VOUT–VBE)/VREF, where R1connects to the emitter of Q1, R4 connects to the collector of Q1, VBE is the base-emitter voltage of Q1, and VREF is the feedback reference voltage of the switching regulator.
To simplify the circuit, the flyback converter in Figure 1 uses an LT1946 monolithic switching regulator. The voltage rating of the monolithic regulator has to be greater than the maximum switching voltage of the flyback converter, calculated by [(VIN1+|–VIN2|)MAX+VOUT(MAX)/(T1 turns ratio)]+VSPIKE. The maximum switching voltage is approximately 25V for the circuit in Figure 1. Note also that the input capacitor and dc/dc regulator input must be able to handle a maximum input voltage of 10V, resulting from the calculation +VIN1(MAX)+|–VIN2(MAX)|. In an event of fault-current conditions, such as shorted input or output, a zener diode, D2, creates the undervoltage-lockout threshold to turn off the LT1946 whenever either input source is in current limit or the input voltage (+VIN1+|–VIN2|) drops below 6V to help the input supply recover when the fault condition is removed. In a system with two available current-limited power supplies, you can convert the two supplies into a single supply that has more power-handling capability than either of the two inputs. A flyback topology based on an LT1946 monolithic converter offers a simple approach to the grounding problem and the feedback-sensing problem inherent in a dual-input power supply. Sharing the power between two input sources not only adds output-power capability, but also increases the overall flexibility of the system.
