Extend low-output-voltage switching regulator's input range

Hua (Walker) Bai, Linear Technology

EDN

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.

Figure 1.	For efficient operation at output voltages of 3.3 V or higher, a charge pump comprising D2and C5 provides a voltage boost that provides sufficient drive for IC1’s internal switching transistor.

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 3 V. A charge pump comprising diode D₂ and capacitor C₅ maintains the voltage at the Boost pin 3 V 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 3 V 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 + 3 V, which provides sufficient head room to drive the power transistor into saturation for greatest efficiency.

Figure 2.	At outputs as low as 1.8 V, efficient operation at low input voltages benefits from an added low-dropout regulator, IC2, for the Boost pin, which extends the circuit’s input-voltage range.

However, output voltages below 2.8 V 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 20 V to avoid exceeding the Boost pin's allowable maximum voltage. For outputs lower than 2.8 V, the circuit in Figure 2 extends V_IN's maximum voltage to 36 V. When the input exceeds 5.3 V, a Linear Technology LT3010-5 low-dropout voltage regulator maintains the voltage across C₉ at 5 V. As a result, for input voltages at V_IN of 5.3 to 36 V, the voltage at the Boost pin always remains at 5 V above V_IN.

Figure 3.	At a maximum input voltage of 36 V (VIN), waveforms at the SW and Boost pins show a 5 V boost-voltage margin for the circuit of Figure 2.

Figure 3 shows a 36 V input applied to V_IN and the resultant voltages at the SW and Boost pins. In Figure 3, the maximum Boost-pin voltage reaches 41 V, safely below the pin's 43 V maximum rating. For values of V_IN of 3.6 to 5.3 V, 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 3.6 V input, the Boost pin remains 3.3 V above V_IN, and IC₁'s internal NPN transistor receives sufficient drive voltage for saturated operation.

Figure 4.	At 3.6 V input (VIN) and 1.8 V output, a voltage of 3.3 V at IC1’s boost pin ensures that IC1’s internal switch still operates in saturated mode.

Materials on the topic