TI’s New Buck Converter and the Issue of Switching Regulator Conduction Modes
The buck converter operates in "forced continuous conduction mode (FCCM)." Let's break that term down.
The basic principle of switch-mode-regulator operation is fairly straightforward: we maintain a specified load voltage (despite varying load conditions) by periodically opening and closing a switch inserted between input and output.
The details, however, are more complex, in part because we can make a variety of adjustments to how exactly the switch is opened and closed.
The newly-announced TPS562207 from Texas Instruments always operates in something called forced continuous conduction mode (FCCM). So, in this article, I would like to briefly discuss the issue of continuous vs. discontinuous conduction in switch-mode regulation.
Buck Regulator Recap
We’re accustomed to the idea of capacitors charging and discharging: under certain conditions an external voltage causes charge to accumulate in the capacitor, and later the capacitor can act as a voltage source and supply charge to other components.
Inductors can function in essentially the same way, though the underlying mechanism is based on magnetic fields rather than electric charge. Thus, a switching regulator can supply consistent current to a load because it “charges” an inductor when the switch is closed and then allows the inductor to “discharge” current into the load when the switch is open.
To avoid the somewhat inaccurate charge/discharge terminology, we can say that the input voltage drives current through the inductor (and into the load) when the switch is closed and that the inductor, by means of its magnetic field, maintains current flow to the load when the switch is open.
This procedure leads to inductor current (IL) that looks something like this:
Diagram of an inductor current when a switch is closed and open.
When the switch is closed (labeled “ON” in the diagram), the inductor current is ramping up. When the switch is open (labeled “OFF”), the inductor current is ramping down.
Discontinuous Conduction Mode (DCM)
If a switching regulator allows the current through the inductor to ramp down all the way to zero, it is operating in discontinuous conduction mode. This doesn’t result in zero output current, though, because during the OFF portion of the cycle, both the inductor and an output capacitor are supplying current to the load.
The current delivered to the load circuitry is the average inductor current, not the instantaneous inductor current.
Maintaining nonzero inductor current during the OFF portion of the cycle requires a larger inductor with more storage capacity. Thus, discontinuous operation allows us to use a smaller inductor and thereby reduce overall solution size.
Lower inductance also helps to improve efficiency and transient response.
Continuous Conduction Mode (CCM)
If a switching regulator is operating in CCM, the inductor current does not ramp down to zero. Most buck regulators are designed such that they will be in CCM when delivering full-load current, and this mode generally provides superior overall performance.
A buck regulator doesn’t need to operate always in CCM or always in DCM. In fact, it is normal for a CCM converter to enter DCM in response to low load-current requirements (perhaps because a microcontroller has transitioned into a low-power standby mode).
Forced CCM is a technique that causes the converter to avoid discontinuous operation even during light-load conditions. This is accomplished by allowing the inductor current to be negative, such that current continues to flow and the average inductor current is reduced to a level that is consistent with the current required by the load:
The TPS562207 datasheet indicates that forced CCM is beneficial in two ways: it allows this converter to maintain a fixed switching frequency, and it reduces output ripple.
Compact, Low-Complexity DC/DC Conversion
The TPS562207 is a buck converter with integrated switching elements and a maximum output current of 2 A. It comes in a six-pin, 1.6 mm × 1.6 mm package and requires few external components.
Simplified TPS562207 schematic. Image used courtesy of Texas Instruments
Efficiency is quite good when the load current is not lower than the maximum value by more than a factor of ten, especially with higher output voltages.
Chart of the TPS562207's efficiency. Image used courtesy of Texas Instruments
TI says the part is optimized for low standby current, so this converter might be a good choice for battery-powered applications that spend a lot of time in an inactive state.
I don’t like output ripple, and consequently, I often prefer linear regulators in low-voltage analog and mixed-signal designs. However, the TPS562207 is a contender even in these types of applications: as shown in the following plot, the ripple at moderate output current is less than 10 mV peak-to-peak.
Plot depicting the ripple at moderate output current. Image used courtesy of Texas Instruments
Feature image (modified) used courtesy of Texas Instruments
Do you have any switching-regulator ICs that you like to use in space-constrained or noise-sensitive applications? Feel free to leave a comment and let us know.