Design Ideas February 3, 1997

Boost controller drives buck converter

Michael Keagy, Maxim Integrated Products, Sunnyvale, CA

  The usual way to step down a low voltage to a still lower one is to use a low-dropout (LDO) linear regulator. However, in battery-powered systems, the LDO usually doesn't deliver the maximum energy available. A cell count chosen for near-dropout operation when the battery is empty delivers too much voltage over most of the battery's discharge cycle, and a cell count chosen for maximum efficiency allows dropout well before the battery is empty. The highly efficient buck dc/dc converter in Figure 1 can solve the cell-count problem.

  The circuit can step down inputs as low as 2V to outputs as low as 1.25V, with efficiency as high as 80%. Figure 2 shows the efficiency characteristics. Like an LDO regulator, the circuit works well with low input voltages. Unlike an LDO regulator, its efficiency remains fairly high, with inputs as high as the allowable maximum (6.5V). You make the step-up switching regulator, IC₁, step down by adding the external switching transistor, Q₁. Through LX (pin 8), Q₁ receives drive from the IC's internal switching transistor—an open-drain, n-channel power MOSFET connected to ground.

  R₂ limits the base current to Q₁, and R₁ turns off Q₁ when LX floats. The R₁ and R₂ values are chosen for maximum efficiency at light loads (1 to 10 mA), thus limiting the maximum available output current. Lower values for R₁ and R₂ allow higher output current but cause the circuit to draw higher levels of quiescent current. R₃ and R₄ determine V_OUT, which equals V_REF(R₃+R₄)/R₄, where V_REF is 1.25V.

  The minimum V_OUT is 1.25V (with R₃=0 and R₄ absent). R₅ and R₆ similarly determine the threshold for low battery voltage. The input and output capacitors can be inexpensive aluminum electrolytic or tantalum types. For best efficiency, the inductor should have a rating greater than the desired output current and should have low series resistance. D₁ should be a Schottky type, because losses are proportional to the diode's forward voltage (this voltage is a substantial fraction of V_OUT). (DI#1984)

FIGURE 1

By adding an external switching transistor, you can use a step-up dc/dc converter to step down voltages to produce an efficient battery-powered power supply.

FIGURE 2

The circuit in Figure 1 can yield an efficiency greater than 80% for output currents from 4 to 10 mA with low battery voltage.

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