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Exploring a four-quadrant DC/DC switching regulator

Posted: 21 Apr 2014 ?? ?Print Version ?Bookmark and Share

Keywords:bipolar? voltage? current? FPGA? voltage?

The four-quadrant operating capability of the converter is shown in figure 3. Here, a sinusoidal control signal is used to generate a sinusoidal output voltage centred on 0V. The inductor currents can go positive or negative; whatever is necessary to have the output voltage go to the commanded level. The operating waveforms show clean and smooth operation through the ground potential. The choice of using a sine wave control signal is arbitrary; a DC signal, square wave signal or any other type of signal could also be used.

Applications
Many applications exist that can take advantage of this four-quadrant DC/DC converter. In high performance digital circuits, such as an FPGA, body back biasing can be used to significantly reduce static power dissipation while maintaining or improving dynamic performance. The voltage of the body of the PMOS and NMOS devices can be independently controlled in order to adjust the threshold (VT) of the devices.

Figure 3: Sine wave output voltage passes through 0V.

When the demands of the FPGA are low, the thresholds can be adjusted higher, significantly reducing leakage currents in the digital blocks. When demands are high, the thresholds can be decreased, increasing the speed and thus increasing the performance of the FPGA. Figure 4 shows a high level diagram of this application. Note that for the NMOS body bias, the voltages are typically 0V300mV and this is well suited to the four-quadrant topology.

Figure 4: FPGA body bias application.

Another application that can benefit from the four-quadrant topology is DC motor drives. In many cases, DC motors need speed adjustment as well as reverse capability. The LT8710 used in the four-quadrant converter can do both. Figure 5 shows such an application. Note that the negative terminal of the DC motor can be simply tied to ground while the positive terminal can be adjusted between positive and negative 10V. Similar to the DC motor drive application, the four-quadrant topology can also be used to drive thermoelectric coolers (TECs), audio speakers, and many other applications.

Figure 4: DC motor drive with reversible drive direction.

Conclusion
The LT8710 used in the four-quadrant DC/DC converter topology is a circuit capable of producing positive and negative output voltages as well as positive and negative output currents. The inductor (L2 in figure 2) in series with the output results in reduced output voltage ripple. Generating output voltages close to ground is also simplified since the duty cycle in this case is close to 50%. Many applications can benefit from this circuit, including but not limited to FPGA body biasing, DC motor drives, thermoelectric coolers, and audio drivers.

About the author
Albert Wu is design engineering manager for power products at Linear Technology Corp.

To download the PDF version of this article, click here.


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