Comparing Voltages in (Very) Tight Spaces: A New Comparator from Maxim

Components that use wafer-level packaging (WLP), such as the MAX4000x series, are valuable options when size reduction is a critical design goal.

I’m going to say it right now: This article is not intended for those who wish that they could assemble everything with a soldering iron (I include myself here). It is not intended for those who have a sentimental attachment to DIP components (that would be me again). And it is not intended for those who think that it should be possible to evaluate a device without the aid of an electron microscope (I suppose I wouldn’t mind using a TEM if I had one...).

Whether we like it or not, the worldwide trend is miniaturization, at least in the realm of electronics. If you want small devices, you need small components. And if you want small devices with lots of functionality, you need extremely small components.

It turns out that the incomprehensibly sophisticated technology that goes into “old-fashioned” integrated circuits just isn’t good enough these days. The passé approach to IC fabrication involves a wafer that is “diced” into individual units, and these units are then encapsulated into the IC packages that we all know so well.

Wafer-level packaging (WLP) is a space-saving alternative that essentially does away with the IC package. Solder balls or “bumps” are attached directly to the semiconductor die, and consequently the finished device is the same size as the die—all that extra bulk associated with the package is eliminated.

Image courtesy of Mouser.

It turns out, though, that reduced component area is not the only benefit. With WLP, the signals on the die are connected directly to the balls/bumps, which are in turn soldered to the PCB. This arrangement avoids the use of bond wires, which in “traditional” IC fabrication are needed as interconnections between the die and the IC package. The elimination of the bond wires reduces inductance and therefore improves high-frequency performance.

Image courtesy of Volkan Yuksel [CC BY-SA 4.0]

Component Consolidation

When designing small-form-factor electronic devices, we usually attempt to use as few ICs as possible. For example, instead of an ADC, a DAC, a processor, and some discrete logic, we’d much rather have a single microcontroller that incorporates modules with equivalent functionality.
Nevertheless, there are times when it is very difficult to avoid the use of a single-function device. Maxim obviously believes this to be the case, because they recently released the MAX4000x series of ultra-tiny analog comparators. (Yes, “ultra-tiny” is the official terminology used in Maxim’s part description.) These devices are available as WLP and in an SOT23 package, but in the context of this article, we’re more interested in the WLP option.

Innovative Inputs

An interesting feature of the MAX4000x devices is that they have only four connections: input, output, V_CC/reference, and ground.

Diagram taken from the MAX4000x datasheet.

This is not so surprising if the device has a fixed internal reference voltage, and the MAX4000x parts do indeed offer this feature (the options are 0.2 V, 0.5 V, and 1.2 V). This is rather restrictive, though—what if you need a different threshold voltage?

It turns out that the MAX4000x series allows you to use the V_CC pin as the reference voltage (i.e., the voltage to which the input is compared). The part can operate with a supply voltage anywhere from 1.7 V to 5.5 V, and thus your threshold voltage can also be any value from 1.7 V to 5.5 V.

I know what you’re thinking: Doesn’t this place serious restrictions on the acceptable voltage range for the input signals? For example, what if I need a 1.7 V threshold, but my analog signal can go as high as 4 V? Maxim’s clever solution is to incorporate a special input stage that allows the input voltage to exceed the supply voltage by as much as 4.3 V (because the minimum V_CC is 1.7 V, and the input voltage is limited only by the absolute maximum rating, which is 6 V).

Versatile Outputs

There’s another problem, though. If you lower your V_CC to 2 V because that’s the threshold you need, your output voltage is now limited to 2 V—the chip can’t drive 5 V from a 2 V supply.
Not surprisingly, Maxim has an answer for this one as well: Two of the devices in the MAX4000x series (the MAX40002 and MAX40003) come with open-drain outputs. Thus, with the help of a pull-up resistor, you can work with output voltages as high as 5.5 V, even if your V_CC is only 1.7 V. And you can even dispense with the resistor if the output is going to a microcontroller that has an internal pull-up:

Diagram taken from the MAX4000x datasheet.

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Do you have any experience with WLP devices? Do you think there are significant advantages or disadvantages in terms of performance? Feel free to share your thoughts in the comments.