Design Ideas September 28, 1995
A discrete-component external charge pump enables the PFM-controlled dc/dc converter of Fig 1a to generate dual outputs with moderate regulation and high efficiency. The circuit accepts input voltages between 2 and 12V (typically 5V). The device delivers simultaneous 0- to 100-mA outputs of ±12V. Efficiencies range from 80 and 90%.
IC1 regulates the +12V output through its V+ terminal, but the 12V output has no direct feedback connection. Nevertheless, flying capacitor C1 couples changes in 12V load current to the IC. These load-current changes affect the switching frequency just as +12V load changes do, that is, by current-limited, minimum-off-time, pulse-frequency modulation of the IC's internal switching MOSFET. The resulting pseudoregulation produces a load change of 10 to 100 mA at either output. This output causes only a 4% change in the negative output, from 11.36 to 10.96V (Fig 1b).