1.8 Power management  (Page 2/7)

For example, let's assume that the input power available to a system comes from an AC/DC wall adapter, with an output rating of 12 V at 1-A maximum load. (The design of high-voltage and AC/DC isolated power supply circuitry will not be discussed in this chapter.) The system in this example comprises the following major subsystems:

• A microprocessor circuit that requires 1.8 V at 1 A and 3.3 V at 500 mA.
• A display that requires a +5-V bias rail at 50 mA, and a backlight consisting of six series LEDs driven at up to 20 mA (white LEDs are in the range of 3.5 V/LED, so you can assume about 21 V for the series string to operate at full brightness).
• An audio amplifier that requires 5 V at 200 mA.

A quick estimation of the maximum system power gives you the following (Equation 1):

P _total = (1.8 x 1) + (3.3 x 0.5) + (5 x 0.05) + (21 x 0.02) + (5 x 0.2) W = 5.12 W (eqn 1)

Therefore, if all of the power-conversion circuitry is 100 percent efficient, it would take a little more than 5 W to operate the system at maximum load. In reality, of course, a typical power-conversion circuit will operate at less than 100 percent efficiency, but given that you have a total of 12 W available from the input power source, you should have more than enough to operate the system.

Important terminology/definitions

Linear regulator. The most basic form of voltage regulator. In this type of device (see Figure 3), a pass element (transistor) is connected between the input (power source) and output (load). The control circuit operates the pass device in its linear range to maintain a regulated voltage level at the output, even if the input voltage is variable.

The linear regulator is simple and relatively inexpensive; however, it has some limitations. Most notably, the regulated output must always be lower than the input voltage. Furthermore, if there is significant differential between the input and output (a high input voltage such as 12 V and a regulated output such as 3.3 V, for example), there will be significant loss in this type of circuit. Any excess voltage is simply lost (dissipated) across the pass element, so this type of regulator can be very inefficient and generate significant heat at high load currents or large input/output differentials.

Low-dropout (LDO) regulator. This is a special type of linear regulator that is designed to allow operation down to a relatively low differential between the input and output voltages. Traditional linear regulators (like the LM7805) require the input to be approximately 3 V higher than the output voltage to maintain regulation. LDOs can operate down to input voltages that are only 100-200 mV above the desired output setpoint. This type of circuit has become so popular that most integrated linear regulators are now generically referred to as LDOs.

Switch-mode power supply. A switch-mode power supply is a power-conversion circuit that operates in a completely different mode from a linear regulator. The pass element(s) are not operated in the linear (dissipative) range, but driven by a clocking (gate drive) signal to switch between fully on and off states. Instead of dissipating power across the pass elements, they operate at a controlled ratio of on to off time to transfer just the right amount of energy from the input source to generate a regulated output voltage as required by the system. The ratio of pass element on time to the total switching period is defined as the duty cycle . As the load conditions (and also possibly the input voltage) vary, the converter’s duty cycle is adjusted by its control circuit to maintain the desired output voltage. At lower duty cycles, less energy is transferred to the output; at higher duty cycles, more energy is transferred.