Design Ideas: August 15, 1996

Applications deriving power from solar cells often require a 5V power supply. However, the cells typically provide a terminal voltage of only 0.8 to 1.4V with current capacity of 3 to 4A. Most dc/dc converters can neither start with such low voltages, nor can they start under full-load conditions. The two-IC approach in Figure 1 allows the converter to start up with low input voltages and to produce a 5V supply voltage under full-load conditions.

IC₁ operates in bootstrapped mode, deriving power from its output. The device boosts the input voltage from 0.8V min to 5V. Deriving its power from the 5V output, the second converter, IC₂, delivers output currents as high as 0.5A. R₂ and R₃ program IC₂'s output voltage. IC₁, thus, enables IC₂ to start up, regardless of the load conditions. Providing IC₂ with a full 5V supply also minimizes r_DS(ON) in the external n-channel MOSFET, by providing a maximum gate-drive voltage swing of 0 to 5V.

To suppress input ripple arising from power-supply switching, select a low-ESR, 220-µF capacitor for C₁. This capacitor also minimizes supply-voltage fluctuations by lowering the solar cell's output impedance. The 330-µH inductor, L₁, allows a low start-up voltage for IC₁. The low-ESR, 15-µF capacitor (C₂) at the output of IC₁ minimizes ripple in the supply voltage for IC₂.

Be sure that the output-stage inductor, L₂, is rated properly for maximum peak inductor and saturation current. The current-sense resistor, R₃, limits the peak current in this inductor to 100 mV/R₃. IC₂'s low-ESR, 470-µF output capacitor, C₃ lowers the output ripple to less than 80 mV p-p for load currents as high as 600 mA. Smaller values of output-load current permit lower values for C₁ and C₂. Figure 2 shows the overall conversion efficiency vs load current for different input voltages. The circuit delivers 200 mA or higher for V_IN=0.8V and 450 mA or more for V_IN=1.5V. (DI 1901)