Synchronous Buck Converter Overview

The figure shown is an idealized version of a buck converter topology and two basic modes of operation: continuous and discontinuous modes.

The basic operation of the buck converter can be illustrated by looking at the two current paths represented by the state of the two switches:

  • S1: the high-side switch
  • S2: the low-side switch

When the high-side switch is turned on, a DC voltage is applied to the inductor equal to VIN - VOUT, resulting in a positive linear ramp of inductor current. Conversely, when the high-side switch turns off and the low-side switch turns on, the applied inductor voltage is equal to -VOUT, which results in a negative linear ramp of inductor current. In order to make sure there is no shoot-through current, a dead time where both switches are off is implemented between the high-side switch turning off and the low-side switch turning on and vice-versa.

A buck converter operates in Continuous Inductor Current mode if the current through the inductor never falls to zero during the commutation cycle. Provided that the inductor current reaches zero, the buck converter operates in Discontinuous Inductor Current mode.

continuous-inductor-current-mode.PNG
discontinuous-inductor-current-mode.PNG

Zero Current Comparator

Typically, by using a synchronous solution, the converter is forced to run in Continuous Inductor Current mode no matter the load at the output. This, in turn, causes losses at low loads as the output is being discharged. One solution to this problem, which is also applied in the design of the MCP16311/2, is to use a zero-current comparator. This comparator monitors the current through the low-side switch, and when it reaches zero, the switch is turned off. This feature is called diode emulation and, by implementing it, the converter will have the advantages of both Synchronous and Asynchronous modes of operation.

In a traditional converter, the S2 switch would have been a catch diode (Schottky diode). This is still practiced in many of today’s buck converters, as it offers increased simplicity in terms of control while being cost effective at the same time. Although such an asynchronous solution may seem simpler and cheaper, it can also prove ineffective, especially when targeting low output voltages. For this reason, a synchronous solution was developed which involves replacing the S2 switch with a MOSFET, thus increasing efficiency and output current capabilities.

PFM/PWM Operation

In order to further increase the efficiency at light loads, in addition to diode emulation, the MCP16311 features a Pulse-Frequency Modulation (PFM) mode of operation. When in this mode, compared to the traditional Pulse-Width Modulation (PWM), the MCP16311 increases the output voltage just up to the point after which it enters a sleep mode. During this dormant state, the device stops switching and consumes only 44 μA of the input. When the output voltage drops below its nominal value, the device restarts switching and brings the output back into regulation. An instance of PFM operation is represented in the figure shown. It can be easily identified by the triangular waveform at the output of the converter.

PFM-operations.PNG

The PFM mode of operation considerably increases the efficiency of the converter at light loads, while also adding a lower-frequency component at the output which varies with the input voltage, output voltage and output current.

Once the output load increases, the converter transitions to normal PWM operation. The threshold point is determined by the input-to-output voltage ratio and by the output current.

Because of the triangular waveform at the output, the use of the MCP16312 is recommended because it runs in PWM mode.

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