Why Shrinking Sizes of RTCs Is Good News for Your Portable Designs

August 27, 2019

Tawni Lisseth Henderson  By: Tawni Lisseth Henderson
 Blogger, Maxim Integrated 

A common thought comes to mind every time a new version of a phone is released: it is even SMALLER? The same goes for new medical devices, smart watches, wearables. They’re really all getting smaller? How can technology that must perform so many functions, with new capabilities being added in almost every new update or version, keep getting SMALLER? It seems like a contradiction: shouldn’t a device get bigger with every new function that is added to it?

When a product is released and has a positive reception from consumers, companies often decide to release a variant of versions. The reasons can range from better efficiency, more functions, increased simplicity, lower cost, different application use, or SIZE. The catch is that, despite the smaller size, the new product must deliver the same (or more) capabilities as previous, larger counterparts. There are various ways to achieve a smaller device:

  • Integrate functions into the chip or parts being used to avoid external auxiliary components
  • Determine what is not necessary to the device and discard it in order to make room for other functions or to reduce the size in general
  • Use a different manufacturing process to decrease the feature size or develop a new innovative design methodology to create the same functional device in a different way

All these methods are valid, and it could be beneficial to apply more than just one approach to reduce the size.

Smartwatch clockFigure 1. Smartwatches are just one example of many portable applications that benefit from small, low-power RTCs.

For example, take a real-time clock (RTC). These chips are small to begin with. Not only do they take up little board space, they’re also easy to solder and can run off a separate power source for a long time. The MAX31341B and the MAX31342 are examples of RTCS that exemplify how some technologies are getting smaller. An RTC typically functions as a clock/calendar that provides time in seconds, minutes, and hours, or by day, date, month, and year (operating in a 24-hour mode). These RTCs have two alarms and are accessed on an I2C serial interface. They both operate with an external 32.768kHz crystal, with a load capacitance of 6pF and ESR up to 100Kohms (required for minimal current draw). The main differences between the two, however, are in power and size. The size difference is .5 mm in width. MAX31341B is 1.5x2.0mm2, while MAX31342 is 1x2mm2.

Sure, the difference is not that much, but how small is small? For comparison, a grain of rice is approximately 1mm. Reducing a size that is already small by 1/3 is incredible. Smaller size is beneficial for designers, since using this device enables designers to build the smaller, sleeker, low-power applications that are so in demand now. Smaller RTCs are desired in specific applications: wearable devices, portable devices, portable audio, and medical devices, for example. These applications require parts that take up the least amount of space possible on the board.

In each of the two parts, all important functions are integrated inside the chip; the only external part is the crystal (which keeps track of the time through oscillation). Having an external crystal might seem like it would result in lost board space, but it is designed this way to provide flexibility in crystal selection. There is no need to develop an external circuit for the crystal, since the MAX31342 has two integrated capacitors to help with the overall load capacitance of the oscillator. This improves accuracy while avoiding a mismatched crystal capacitive load. A typical layout of the crystal with the integrated capacitors is shown in the lower left of Figure 1, and the two integrated capacitors with a crystal equivalent circuit is displayed on the right.

Layout of crystalFigure 2. Layout of crystal with integrated capacitors (left) and two integrated capacitors with crystal equivalent circuit (right).

To reduce the size of the MAX31341B to create the MAX31342, the engineers discarded what was not necessary to the device in order to make room for other functions. They eliminated the battery backup, trickle charger, and the RAM, as not all applications need these functions or they could be acquired separately.

Focusing the MAX31342 on purely essential functions also resulted in lower power. The RTC can run with less current (150nA), minimizing power loss. The less power the device takes from the battery, the greater the extension of battery life, which results in lower battery replacement frequency. The MAX31341B, on the other hand, runs off 180nA current. The higher current draw is used to power the additional features that it provides.

SMALL is what everyone is trying to achieve these days. Typically, designers desire their devices to be the best, the longest lasting, and most easily used devices on the market. The MAX31342, with its small size, low current, and low power, helps achieve that. For applications requiring a backup battery, trickle charger, RAM, and/or power management, the MAX31341B provides those features along with a small package.

Tawni Henderson is a senior at Santa Clara University, where she is majoring in electrical engineering. She spent the summer interning at Maxim.