Switching regulators, as the name implies, accomplish DC/DC conversion by switching something on and off. In a typical inductor-based circuit, switches control the current flowing through an inductor; in charge-pump-based regulators, charge from the input supply is “pumped,” via switches, onto a capacitor. Switching is a periodic thing, and consequently switch-mode regulation is never far from the question of frequency.
I’m sure you’ve noticed that switching-regulator ICs come in a wide variety of frequencies. I would say that the typical range is 100 kHz to 2 MHz, though you can find frequencies below 100 kHz and there are parts that go significantly higher (up to 3 or 4 MHz). You may also have noticed that many switch-mode ICs allow you to choose the operational frequency (within a specified range) via an external resistor. Why do parts offer such a wide range of frequencies? And what’s the correct frequency for a given application? Let’s take a look.
The Effects of Switching Frequency
When you start to get into the details, switch-mode DC/DC converters are not exactly simple. In fact, they’re not even close to simple, and switching frequency is a fundamental parameter that in one way or another affects just about every aspect of the circuit’s functionality and performance. Thus, I’m going to focus on the most important and practical considerations, and I’ll try to present the information accurately but without burying myself in complicated details.
The following subsections are written from the perspective of inductor-based switching, but this doesn’t mean that none of the information is applicable to charge-pump regulators.
When I think about switching frequency, the first thing that comes to mind is noise, both conducted and radiated. You can’t make switching noise disappear by moving your frequency up or down, but you can make the noise less problematic.
The basic idea here is that your switcher is going to generate noise at the switching frequency and at harmonics of the switching frequency.
This plot is used courtesy of Analog Devices, taken from an article on switching-regulator output artifacts. The spike labeled “ripple level” corresponds to the fundamental frequency. We'll discuss output ripple toward the end of this article.
By adjusting the fundamental, you can “steer” the noise so that it doesn’t collide with sensitive analog circuitry or FCC emission limits.
For example: Let’s say that your converter is in close proximity to an ADC that is sampling a 50 kHz baseband waveform. If your switcher is operating at 1 MHz, you’ll be able to suppress the noise associated with the fundamental and all the harmonics using a single-pole or (better) double-pole low-pass filter.
Using a higher switching frequency is sometimes an easy way to deal with noise, because you can more effectively address problems by adding a low-pass filter. However, you need to make sure that you’re not pushing the noise up into frequency bands that have lower emission limits or that are being used by nearby RF circuitry.
We don’t want to reduce the efficiency of our highly efficient switching regulators simply by choosing an inappropriate switching frequency. The basic idea here is that higher frequency means lower efficiency.
This makes sense if you think about it: Switch-mode regulation is efficient because it takes advantage of the low power dissipation associated with a transistor’s “fully on” and “fully off” states. Significant power dissipation occurs only in the intermediate region between on and off, and if a transistor switches between on and off more frequently, more power is wasted and efficiency drops.
The following plot provides an example of the relationship between switching frequency and efficiency.
Plot taken from the datasheet for the LT8610 from Linear Tech/Analog Devices.
The versatility and high efficiency of switching regulators make them attractive options for small, battery-powered devices. This means that PCB real estate is sometimes a critical factor in the design process. An important vote in favor of higher switching frequency is reduced board space: Generally speaking, a higher switching frequency allows the converter’s output filter to achieve comparable performance with lower capacitance and inductance values, and lower capacitance and inductance values correspond to smaller capacitors and inductors.
The first subsection (entitled “Noise”) deals with interference that is created by the converter’s switching action and then coupled or radiated into other portions of the system or into nearby electronic devices. This is distinct from “ripple,” which refers to the periodic variation that is directly present in the converter’s output voltage.
The importance of ripple varies from application to application. Digital circuitry is highly resistant to supply ripple, but some analog components are also highly resistant—namely, those with good power-supply rejection ratio (PSRR) at the relevant frequency.
The basic relationship here is that higher switching frequency means lower ripple. The following plot gives you an example of this effect:
The plot is from this app note (page 10) published by Texas Instruments. You can see that both theoretical calculations and SPICE simulations indicate that higher switching frequency leads to lower ripple.
You need to be careful, though, because if you increase the switching frequency, you might move the ripple frequency up into a band in which your analog components have lower PSRR. Actually, this is quite likely, since PSRR tends to decrease as frequency increases. However, you might get lucky and end up with a component like the MAX40018 from Maxim:
Plot taken from this datasheet.
The PSRR initially decreases with frequency, as expected, but then it increases after 10 kHz. (Though we don’t know what happens after 100 kHz.)
The switching frequency of a DC/DC converter affects numerous aspects of the circuit’s functionality. It seems to me that the most important relationships are as follows:
- With higher switching frequency, noise is easier to control (via filtering), but in general the frequency should be adjusted according to the noise characteristics and requirements of each system.
- Higher frequency leads to lower efficiency.
- Less board space is required when higher frequencies are used (because passive components can be smaller).
- Increased switching frequency leads to decreased ripple amplitude.
If you have any thoughts or practical tips on choosing the switching frequency for a DC/DC converter circuit, feel free to let us know in the comments.