Design Ideas: November 9, 1995

The circuit in Figure 1 uses a step-up (boost) dc/dc controller in a negative buck-regulator application. The circuit’s function is to power the laser diode in an optical amplifier/booster unit, a job for which no commercial IC is available. The anode of the laser diode connects to ground, so the supply voltage must be negative and must deliver 160 to 750 mA. Although the boost-regulator IC operates in a buck-regulator circuit, its standard connections permit proper control of Q₁. However, you need an op amp to invert the output voltage to obtain proper voltage feedback.

The load is referred to the most positive supply rail instead of to IC₁’s ground terminal, so the controller must increase its duty cycle as V_OUT increases. The op amp, therefore, inverts the feedback signal and shifts it to match the 1.5V threshold internal to IC₁. IC₁ is configured in its nonbootstrap mode, which provides an adequate gate-drive signal (ground to –5.2V) for the external MOSFET Q₁. With V_OUT set at –3V and the output current ranging from 160 to more than 700 mA, the circuit’s conversion efficiency ranges from 84 to 87% (Figure 2). (DI #1789)