Most common dc-dc power application circuits are designed for a positive buck or a boost circuit. However, certain designs may require a negative buck or boost topology. The problem usually arises because most board designers aren't accustomed to thinking in terms of negative voltages.
The transfer function for a positive buck is VOUT/VIN = D, and interestingly, the transfer function for a negative buck is –VOUT/ –VIN = D. Similarly, the transfer function for a positive boost is VOUT/VIN = 1/(1 D), and for a negative boost it's –VOUT/ –VIN = 1/(1–D).
A negative buck regulator can easily be configured by starting with a standard positive buck-regulator circuit and flipping the polarity of all the voltage rails and power switches (Fig. 1). The circuit in Figure 2 is an actual negative buck regulator that converts a 7-V input to a –5.2-V output at 500 mA.
The purpose of the small-signal transistor (Q1) is to level-shift the feedback voltage so that the output is regulated at –5.2 V. The MIC2288 is a 1.2-MHz pulse-width-modulation (PWM) boost regulator in an SOT23-5 package with an internal 1.2-A peak current switch between the SW and the GND pins. So, there's no need to connect an external npn power transistor Q2 as shown in Figure 1b.
The chip GND is sitting on –7 V (VIN), and feedback on the MIC2288 will be 1.24-V higher than GND, so R3 will program 100 µA in the collector lead. This current will produce approximately –4.6 V on the emitter, and the base will go down to approximately –5.2 V (assuming Q1 s VBE = 0.6 V). R1 and C1 program an enable delay on the MIC2288 to make sure that input supply is fully stable before the negative buck converter starts switching.
The maximum load current is limited by the switch handling capability on the MIC2288. For higher load current applications, one can certainly employ a boost controller that uses an external MOSFET or a bipolar switch. This circuit's efficiency at 500 mA was just over 87%.
A negative boost converter can easily be configured by following the same steps as for a negative buck converter. Figures 3a, 3b, and 4 show the positive boost, negative boost after source and switch transformation, and an actual circuit for a –5.2-VIN to –7-VOUT, 500-mA application. The MIC4690 is a 500-kHz PWM non-synchronous buck regulator in an SO-8 package, with an internal 1.3-A peak current switch between the VIN and the SW pins on the regulator. Thus, there s no need to connect an external npn power transistor Q3 as shown in Figure 3b.
The output voltage is programmed when 124 µA (1.24 V/10 kΩ) flows through R1 and R2. Therefore, VOUT = –1.24 V + (46.4 kΩ X 124 µA) = –7 V. The MIC4690 is enabled by pulling the SHDN pin to the lowest potential voltage.
Again, the maximum load current is limited by the switch handling capability of the MIC4690. For higher load-current applications, one can use a buck controller that uses an external MOSFET or a bipolar switch. The efficiency of this circuit at 500 mA was about 75%.
Ed. note: Ajmal Godil has moved on from Micrel Corp. as of publication. All queries can be sent to Marty Galinski at [email protected].