Positive-stepdown switching regulators abound in the electronics industry, but the complementary negative-stepdown function (converting a negative voltage to a smaller negative voltage) is rare by comparison.
An IC controller can build a negative-stepdown switching regulator that delivers l A at -7.5 V from -15 V, with efficiency better than 90 percent. The IC is a stepup switching controller referenced to the negative rail.
When the power switch (Q3) turns on, the current in the inductor (L1) increases as long as VOUT is less negative than VIN. When the inductor current reaches the limit set by R6, Q3 turns off. Q3's drain flies high and is clamped to the positive supply rail (ground) by the catch diode (D2). Inductor current then decreases until Q3 turns on again. The output capacitor (C3) integrates inductor current to produce the output voltage.
Output voltage is referenced to the positive supply rail, and Q4 reflects the error signal from the positive rail to the negative rail. When the error signal falls below the feedback threshold, the power switch turns on and initiates another switching cycle.
Dl, Q1, Q2 and R1-R4 form a startup circuit that holds IC1 in shutdown until VIN reaches approximately -11 V. That action ensures that Q4 has sufficient voltage compliance to regulate the feedback loop. The minimum input voltage necessary to maintain a regulated output voltage IS VOUT +(-VFB), where VFB is the feedback threshold voltage (l.5 V in this case). Efficiency varies with load current.
Additional technical information on power conversion and other topics by Maxim Integrated Products' engineers can be found on the company's Web site at www.maxim-ic.com/technical.htm.