Internal operating voltages in electronic devices continue to decrease, but input-source voltages don't change. As the difference between input and output voltages increases, so does the improvement in efficiency that a switching regulator offers. Unfortunately, as a switched-mode step-down converter's output voltage decreases, the decrease imposes limitations on the circuit's input-voltage range. This Design Idea shows how to extend a low-output-voltage step-down converter's input-voltage range.

A switching-mode step-down regulator, such as Linear Technology's LT1936 (IC₁), includes an internal high-side NPN power transistor between its input, V_IN, and switched-output (SW) pin. For highest efficiency, the high-side NPN transistor requires a base voltage that's higher than the input voltage. The circuit of Figure 1 works best for output voltages greater than 3V. A charge pump comprising diode D₂ and capacitor C₅ maintains the voltage at the Boost pin 3V above V_IN. When IC₁'s internal power transistor switches off, the voltage at SW goes to ground through D₁. Boost capacitor C₅ charges to 3V supplied from V_OUT through D₂. When the power transistor turns on, the voltage at SW jumps to V_IN, and the voltage at the Boost pin jumps to V_IN + 3V, which provides sufficient head room to drive the power transistor into saturation for greatest efficiency.

However, output voltages below 2.8V no longer provide sufficient drive voltage to fully saturate IC₁'s switching transistor, and the circuit's efficiency suffers due to increased voltage drop across the transistor. In this situation, connecting D₂'s anode to V_IN instead of V_OUT doubles the Boost pin's voltage to twice the value of V_IN but limits V_IN to 20V to avoid exceeding the Boost pin's allowable maximum voltage. For outputs lower than 2.8V, the circuit in Figure 2 extends V_IN's maximum voltage to 36V. When the input exceeds 5.3V, a Linear Technology LT3010-5 low-dropout voltage regulator maintains the voltage across C₉ at 5V. As a result, for input voltages at V_IN of 5.3 to 36V, the voltage at the Boost pin always remains at 5V above V_IN. Figure 3 shows a 36V input applied to V_IN and the resultant voltages at the SW and Boost pins. In Figure 3, the maximum Boost-pin voltage reaches 41V, safely below the pin's 43V maximum rating. For values of V_IN of 3.6 to 5.3V, IC₂ operates in dropout mode and introduces only a 300-mV drop from its input to its output. Figure 4 shows that, even at the circuit's minimum 3.6V input, the Boost pin remains 3.3V above V_IN, and IC₁'s internal NPN transistor receives sufficient drive voltage for saturated operation.