Switch-mode buck-regulator ICs can deliver high power with high efficiency, but they seldom operate with input voltages below 3 V. That restriction excludes the use of single-lithium-ion-cell power supplies, for example, because their output ranges from 4.2 V down to 2.6 V. The input can be boosted to 5 V and then regulated back down to the desired level, but this approach yields a larger, more expensive, and less efficient circuit. Low-voltage applications also are complicated by the need for low-threshold MOSFETs.
In Figure 1, a step-down switchmode controller (IC1) generates the regulated low-voltage output (1.7 V in this case). To maintain proper operation for low VIN, a regulated charge pump (IC2) boosts the input to 5 V so that it can power the logic, analog circuitry, and MOSFET gate drivers internal to IC1. These loads represent only a few milliamps. The main output current (up to 3 A) flows from VIN via Q1, L1, and R3.
The circuit’s minimum-guaranteed supply voltage is determined by the input-undervoltage lockout of IC2, which has a typical value of 2.35 V. IC2 also determines the maximum operating voltage. Pulling its SK —IP —pin high sets the input range at 2.6 V to 5.2V, while pulling it low sets the range at 2.6 V to 3.6 V with a lower quiescent current. The circuit typically starts at 2.4 V with a 3-A load, and draws about 2.5 mA with no load. For a 2.6-V input, the efficiency versus load current peaks at 88% (Fig. 2).
The capacitors connected to IC2 are sized to deliver only the current needed by IC1. By scaling up these capacitors (per instructions in the data sheet), you can draw as much as 50 mA from the charge pump for use in an auxiliary 5-V supply. R4 sets the internal switching frequency for IC2. For higher charge-pump current, substitute the MAX683 (100 mA) or the MAX682 (250 mA).
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