If you’ve read this article, you know more or less how I feel about switching regulators. The bottom line: I don’t like them. I don’t care how efficient they are. The circuits are complicated, the layout is sensitive to imperfections, the output voltage is noisy.
Nevertheless, I realize that this opinion belongs in a museum. The world of modern electronics could not exist without the efficiency of switching regulators—the linear approach to regulating voltage wastes too much power for small, portable devices. And even if size and battery life are not major priorities, there’s another thing to consider: output current.
The output noise of the T/90 devices is actually quite low. All images from Murata (PDF).
Let’s say you have virtually unlimited power available from a wall outlet. In this case perhaps you can tolerate the inefficiency of your linear regulator. But high output currents are a problem whether or not you are trying to conserve power. Recall that the approximate power dissipation of a linear regulator is the output current multiplied by the difference between the input voltage and the output voltage. So if VIN = 9 V, VOUT = 5 V, and your load draws 50 A, the regulator is dissipating 200 W. You might need a supply of liquid nitrogen to keep your regulator functional under these conditions.
The 50 A example might seem like overkill, but low-voltage digital ICs can draw some serious current these days. This is evident in the specs for Murata’s OKDx-T/90 series of DC/DC converter modules. The output voltage range is 0.6–1.8 V. And the max output current? Oh, just 90 A.
What sort of a device operates on 1.8 V and needs 90 A? Well, the general category would be high-performance (and presumably rather large) processors and programmable logic devices. For example, Intel processors used in PCs generate around 65 W. If we simplify by assuming a single supply voltage of 1.4 V, we’re looking at 46 A. Forty-six amps.
So far we’ve established two things: 1) I don’t like switching regulators; 2) Sometimes you need a lot of current, even with low supply voltages. The first issue can be resolved by using a DC/DC module that eliminates the majority of the difficult design work. And we can address the second issue by choosing a module that can handle high currents. I assume there are various options; I personally like the µModule devices from Linear Tech. In this article, though, we’re focusing on the Murata parts mentioned above.
The OKDx-T/90-W12-xxx-C Series (. . . We’ll Call It the T/90 Series)
First, these are not ICs; they’re circuit boards:
Just a few basic components are needed in addition to the module; the typical application circuit (which I don’t want to reproduce here because its visual quality is rather abysmal) shows just two capacitors and some resistors.
If a regulator is going to supply 90 A without overheating, it has to be efficient. Efficiency tells us how much power the regulator itself will dissipate relative to the power delivered to the load:
In the case of the T/90 series, the efficiency maxes out at around 90%.
So if your output voltage is 1.2 V and you’re supplying 40 A, your output power is 48 W. With an efficiency of 90%, only 5.3 W will be wasted as heat dissipated by the switcher. That’s pretty good, but as you can see in the plot, you have to be careful with switching-regulator efficiency specs.
With a linear regulator, it’s straightforward: the efficiency is simply the ratio of output voltage to input voltage. With a switcher, efficiency varies greatly depending on the load current, and it also is affected by the difference between input voltage and output voltage. You have to be especially careful with low load currents; in the case of the T/90 series, efficiency plummets for currents below 8 A.
If you don’t consider 8 A to be a “low” current, then you need a different module. Or you could just tolerate the reduced efficiency—there’s a natural balancing effect here because lower load current means less output power and therefore less power dissipated by the regulator.
Another Communication Protocol . . .
The T/90 series does more than just regulate. One interesting feature is the Power Management Bus (PMBus) interface. PMBus is a standardized protocol for communication among components in a power system; it’s based on SMBus, which in turn is an extension of I2C. The PMBus interface allows you to monitor and configure the T/90 module; the list of commands begins on page 36 of the datasheet.
Do you have a preferred line of DC/DC converter modules? Let us know in the comments.