4 Power supply

Let’s start with the power supply requirements of an MCU. An MCU requires very little “juice” to run. This part is often known as the “Electrical Characteristic” of an MCU. Most MCUs accept a somewhat wide range of power supply voltages, but almost none can accept a voltage more than 5V. The current consumption of an MCU is typically less than 20mA.

Although an MCU can accept a range of input voltages, it is best to regulate the power supply of an MCU. This makes sure the actual voltage of the power supply to an MCU remains constant regardless of other factors such as battery conditions, minor noises of transformers and etc.

The simplest type of voltage regulators is a linear type. A common regulator is called the “7805”. It is a three-pin device with the pin outs as follows:

• Pin 1: input voltage. This pin needs to have at least 7.5V if the output is 5V.
• Pin 2: ground. This pin connects to the 0V node of the entire system.
• Pin 3: output voltage. This is the regulated output pin. Minor low frequency variations of the input voltage does not affect this output voltage.

A linear regulator does not work by itself. It requires an input capacitor and an output capacitor. These two capacitors connect from pin 1 to pin 2, and from pin 3 to pin 2 of the regulator. These are typically called “by-pass” capacitors, they act as “shock absorbers” of voltage changes due to change of load or supply variations.

The input by-pass capacitor typically has a value of 10uF, while the output by-pass capacitor can range from 10uF to 22uF. When a capacitor is specified, there are several major factors to consider:

• capacitance: this is measured in Farahs, (F). The most common unit, however, is a micro farah (μF, also represented by uF). If a capacitor is a balloon (to contain electrons, not air!), the capacitance is “how many electrons can it contain at a particular electron pressure (voltage)”. The higher the capacitance, the more electrons (charge) the capacitor can store at a fixed voltage (electrical potential).
• rated voltage: this is measured in volts, (V). This parameter specifies the maximum voltage that a capacitor can handle. In a balloon analogy, this specifies the maximum air pressure a balloon can handle before it bursts. In practice, if a capacitor needs to handle 12V, it is best to specify one that can handle 15V or more.
• equivalent series resistance (ESR), measured in ohms, (Ω or spelled out “Ohm”). This specifies the relative difficulty to charge and discharge a capacitor. A high ESR means a capacitor is more restrictive, kind of like a balloon that has a small opening. In electronics, using a capacitor of the wrong ESR specification can lead to “flaming caps”.
• ripple current, measured in amperes (A). This specifies the average sustained charge and discharge current that a capacitor can handle. This characteristic is related to the ESR. Some capacitors are only specified by their ESR, while some are only specified by their ripple current.

When a by-pass capacitor is specified, it is best to choose one that has a low ESR or high ripple current. Note that all capacitors of this range of value are electrolyte type, which means they are usually polarized. It is important to make sure the poles of an electrolyte capacitor are connected correctly.

In terms of physical placement, a by-pass capacitor should be placed as close as possible to the pins that they are connected. This minimizes any effect of a long trace (resistance and inductance).