A buck regulator is a step-down DC-to-DC converter and is useful in many circuit applications, particularly high-power circuits. A power field effect transistor (“FET” or “MOSFET”) is designed to handle higher power levels than other semiconductor devices, and is therefore often used as a switch in a buck regulator.
For example, a synchronous buck regulator has a high-side power FET for charging an inductor and a low-side power FET for re-circulating the stored inductor current. Typically, the high-side power FET is sensed for control of the buck regulator. To avoid shoot-through, the low-side FET cannot turn on until high-side FET turns off. Thus, a circuit is required to sense when the high-side FET is off. Accurate and fast sensing is critical to achieve the best circuit performance. The sense circuit needs to be fast to minimize dead time when both FETs are off, and it needs to be accurate to prevent the low-side FET from turning on too early and causing shoot-through.
Prior solutions usually sense the gate-source voltage VGS of the high-side FET. However, this generates a sense signal in the high-voltage domain, and therefore requires level-shifting circuitry to bring the signal down to the low-voltage domain in order to be processed by other circuitry. This approach is accurate but slow due to the delay introduced by the level shift circuit. To compensate for this delay, the low-side FET can be made to turn on a little bit earlier. However, accuracy will be compromised because delay from the level shift varies with different operating conditions and process variations. Thus, under certain conditions, the low-side FET could turn on too early and cause shoot-through.
Therefore, it remains desirable to find an improved solution for generating a sense signal that is fast and accurate for controlling buck regulators and other useful power circuits.