Discrete Charge Pump Adds -12V Output to +12V Boost Converter
Abstract: A discrete-component external charge pump enables the PFM-controlled DC-DC converter of Figure 1 to generate dual outputs with moderate regulation and high efficiency. The circuit accepts input voltages between 2V and 12V (typically 5V) and delivers simultaneous 0mA to 100mA outputs at ±12V (Figure 2). Efficiencies range between 80% and 90%.
A discrete-component external charge pump enables the PFM-controlled dc-dc converter of Figure 1 to generate dual outputs with moderate regulation and high efficiency. The circuit accepts input voltages between 2V and 12V (typically 5V) and delivers simultaneous 0mA to 100mA outputs at ±12V (Figure 2). Efficiencies range between 80% and 90%.
Figure 1. An external charge pump (C3, C5, D1, and D3) enables this dc-dc step-up converter to generate ±12V dual outputs.
Figure 2. "Pseudoregulation" stabilizes the -12V output in Figure 1.
IC1 regulates 12V via its V+ terminal, but the -12V output has no direct feedback connection. Nevertheless, changes in -12V load current are coupled via "flying capacitor" C3, where they affect the switching frequency just as 12V load changes do-via current-limited, minimum-off-time, pulse-frequency modulation of the chip's internal switching MOSFET. The resulting pseudo-regulation is impressive: a load change of 10mA to 100mA at either output causes only a 4% change in the negative output (from -11.36V to -10.96V).
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