When the 3.3V rail is too low to properly operate IC1, the switching power MOSFET (Q1) is forced off by Q2, D2, and a SOT23 reset (IC2). If the 3.3V rail dips below the allowed minimum, IC2 and Q2 shut down the circuit by turning off Q1. Powered by the 3.3V boost controller IC3, this step-down controller (IC1) generates 1.5V from inputs as low as 1.8V. Note that IC1 is biased by 3.3V, but power for the 1.5V output comes directly from the battery.įigure 1. The 3.3V rail that powers this step-down controller is taken from a high-current, synchronous-rectified boost controller (IC3), which is otherwise included to provide power for external logic and the CPU's I/O blocks. The upper switch-mode DC-DC converter in Figure 1 (IC1) generates over 600mA at 1.5V, from a 2-AA-cell input that varies from +3.4V to +1.8V. This battery output can drop to 1.8V as the battery discharges. Thus, providing efficient power for the low-voltage CPU core in a hand-held product can be a problem if the power source is a 2-cell AA battery. Step-up ICs that operate from less than +1V are available, but step-down ICs that accept input voltages near +2V are not. This circuit provides one solution to generate 1.5V at 600mA and also supplies 3.3V at 200mA.ĭC-DC conversion is particularly challenging when both the input and output voltages are low. Stepping down from low voltage inputs like 2 AA cells is challenging.
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