September 12, 1997

Wall-cube dc/dc converter is 85% efficient

Steve Pietkiewicz, Linear Technology Corp, Milpitas, CA

The ubiquitous 12V wall cube, power source of countless electronic products, generates an unregulated dc voltage of 8 to 18V, depending on line voltage and load. If you use a linear regulator to drop the voltage to 5V, a 400-mA load translates to 5W worst-case dissipation in the regulator. To deal with the heat, you must provide adequate heat sinking, thereby increasing your product's size and weight. Moreover, customers may object to the heat. These factors can negate the cost advantage of a linear regulator. The circuit in Figure 1, a negative buck converter, delivers 5V at loads as high as 400 mA from a 7 to 25V input with peak efficiency of 85%. The circuit requires no heat sinking.

Because the LT1307B (IC₁) is designed to operate with a low input voltage, you use Q₁ and Q₂ to form a simple preregulator, providing 1.9V to IC₁'s V_IN pin. The IC switches at 600 kHz; you can thus use a low-cost 22-µH inductor and a 10-µF ceramic output capacitor. You need Q₃ to level-shift the output voltage, because IC₁'s feedback pin uses the negative input as a reference. Output ripple measures 10 mV p-p with a 400-mA load. Figure 2 shows the efficiency of the circuit; Figure 3 gives the response of a load step from 150 to 300 mA.

Input bypass capacitor C₁ encounters worst-case rms ripple current equal to half the output current; its ESR should be lower than 0.5 ohms. During construction, keep R₁ through R₃ close to IC₁'s FB pin and away from the SW pin to prevent undesirable coupling. Use a ground plane and keep the traces for the power components short and direct. Although the fact that the negative side of the wall cube is not grounded might seem unsettling, remember that the 9V wall cube floats. The circuit merely regulates the negative side, rather than the more conventional positive side. (DI #2079)

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