The Synchronous Buck Regulator
The synchronous buck DC/DC converter is illustrated in Figure 3. In this configuration, the diode is replaced with a switch. The switch is a field effect transistor (FET), which is designed to have very low on-resistance (RDSon) that allows the FET switch to exhibit lower voltage drop when current flows through it. This results in the circuit having much higher efficiency in comparison to when a diode is used. For example, if the average current in the system is 5A, the power loss in the diode would be 0.5 volts x 5A = 2.5 watts (this assumes a Schottky diode with a forward voltage of 0.5 volts at 5A), versus 5A x 5A x 0.011 ohms = 0.275 watts with a transistor having 11 mohm of on-resistance. The transistor achieves better than a 9x reduction in power dissipation.
However, with switch-2 integrated onto the die, the S2 losses will be on the die. This will require better thermal design of the die, but the overall improvement in efficiency will result in less total heat generated. The die will require more silicon area when switch S2 and its drive circuitry are included, but this will reduce the board area and component count since the external diode is no longer required.
There is another benefit to the synchronous circuit over the asynchronous one that is not obvious to many engineers. When the output load is very low, the inductor current may become discontinuous, meaning the current falls to zero. In the asynchronous configuration, the discontinuous current can result in electromagnetic interference (EMI) emissions. A minimum load may be required for the asynchronous