Tiny capacitors for tiny IoT devices – a look at the latest

Some of the highest volume products for the IoT will be very small devices – and the smaller and lighter the better. Wearable computing will account for a huge chunk of this market and smartphones will form an important part of the infrastructure. For example, the Jawbone U3 wearable fitness tracker includes several sensors in one package. The sensors connect wirelessly to your phone and from there to the Internet. All of these IoT nodes need capacitors for coupling, decoupling and filtering. As device complexity grows, so does the need for capacitors, of which there are now between 400 and 500 in some of the latest smartphones. At the same time, designers demand ever-smaller components so that device size and weight can be kept within acceptable limits.

Power supplies derived from DC-DC switching regulators used in IoT products typically require several capacitors on inputs and outputs to provide stable operation and filter electrical noise. Capacitance values up to 100µF may be needed and it’s common for a supply rail to need several capacitors of different values connected in parallel to ensure effective decoupling across a range of frequencies. However, the most commonly used parts are small multilayer ceramic capacitors (MLCCs) used for signal coupling and decoupling. Typical values are in the range of 1000pF to 0.1µF. MLCCs of a few pF or 10s of pF are used for high frequency filters, temperature compensation and as loading capacitors for piezoelectric crystals.

 

Figure 1: Multilayer ceramic capacitors (MLCCs) are the most common types of capacitor in mobile and wearable battery-powered devices. There are up to 500 of them in some smartphones.

 

So who makes the smallest capacitors? Well, it depends on the technology in question. As MLCCs are the most common components, we’ll focus our attention on these. The greatest asset of MLCCs is their high capacitance per unit volume. One of their biggest drawbacks is relatively poor temperature stability in some types – the capacitance can vary widely with temperature, depending on the dielectric formulation used.

The other key factor that affects how much capacitance can be achieved within a given package size is the working voltage – a thicker dielectric layer is needed for higher voltage components, making the components larger. Battery operated IoT devices usually operate at low voltages, so this is not a major issue. Most MLCCs are rated at 10V or higher, whereas most semiconductors operate at 5V or below, although boost converters may sometimes be used to create higher voltages where piezoelectric components are present.

All MLCC manufacturers produce components using a variety of dielectrics. The more stable the dielectric material, the lower the capacitance that can be achieved within a given volume. General purpose decoupling is not particularly critical with respect to capacitance values, providing a minimum value is maintained across the operating temperature range, so less stable dielectrics are fine. Radio frequency filters and temperature compensating components need to be highly stable. There are a lot of dielectric designations but amongst the most common are COG (or NPO) and X7R. COG parts are very stable, X7R, X7S and X5R are ‘stable’ and Y5V or Z5U components are ‘general purpose’ but with higher capacitance per unit volume.

Surface mount capacitor sizes are expressed in industry standard terminology. 01005 components (0402 metric size – 0.4mm x 0.2mm) are the smallest in common use in mobile devices. You need to be careful with package sizes because the 0402 designation is used for both an imperial and metric size, the imperial version being the larger at 1.0mm x 0.5mm.

RS Components has an online dynamic product selection tool that allows you to find the right MLCC for your application by filtering on up to 19 parameters. The selection tool displays imperial package sizes in the ‘Package/Case’ section.

 

In order of the number of individual parts stocked, we carry MLCCs from:

Murata
AVX

Kemet

Samsung Electromechanics

TDK

Vishay

Syfer Technology

Yageo
Phycomp

Taiyo Yuden
Cornell-Dubilier

From the Murata range, the maximum capacitance in stock is 47µF in a 1210 package with X7R dielectric. Values up to 100nF are available in COG dielectric and a 1206 package. Also in COG, capacitors with values up to 1000pF can be supplied in 0402 packages, and a 100pF COG component is offered in a very tiny 0201 package (0603 metric) that measures just 0.6mm x 0.3mm.