1.8 Power management  (Page 3/7)

The switch-mode concept has the significant advantage of being able to efficiently convert power from any input voltage to any desired output voltage. A buck or step-down converter (Figure 4) can convert a given input voltage to a lower output voltage. A boost or step-up converter can convert an input voltage into a higher output voltage. A buck-boost converter can generate a regulated output voltage from an input that may vary both below and above the desired output level. (For example, if using a variable input source such as a battery, the input voltage may range from 2.5 V up to 6 V depending on the battery. If a steady 5-V output is required across the full battery range, a buck-boost design can provide the needed output.)

The trade-off for the performance advantages of the switch-mode converter is that you must follow a more complex design process; cost and complexity increase in exchange for performance and flexibility. However, many integrated catalog solutions now exist to simplify this task. A more complete explanation of the basic operation of switching converters is provided in the references.

Efficiency . Efficiency ( Η ) (Equation 2) is the ratio of power delivered to the load versus power drawn from the input. For an ideal (perfect) power supply, 100 percent of the power taken from an input source is delivered to the load to perform useful work. In reality, today’s high-efficiency switch-mode converters may have efficiency ratings in excess of 90 percent; however, the values may vary widely depending on operating conditions.

Η = P _output / P _input (eqn 2)

Ripple . In switch-mode power supplies (which operate by turning pass devices on and off at a high frequency), the output voltage will have a small AC component (typically mV) superimposed on the DC. This AC component, measured as a peak-to-peak voltage, is the ripple voltage .

Regulation accuracy . An ideal voltage regulator will produce a fixed-output DC value that stays at the exact setpoint (such as 3.3000 VDC) under all operating conditions. In practice, the output voltage may vary slightly above and below the setpoint by some finite percentage as a result of input voltage changes, output current (load) changes, reference signal accuracy variation and temperature change. Most voltage regulator data sheets have electrical specification tables that will express output voltage accuracy either as absolute values (+/- mV) or percentage change from the nominal output setpoint value.

Transient response . Any voltage regulator circuit is, at its core, a control system. Its purpose is to maintain a stable output voltage regardless of variations in input voltage, load current or temperature. As with any control system design, there must be a balance between speed (the ability to respond quickly to any perturbation) and stability. In most systems, when some perturbation occurs at the input, there will be a brief change in the output until the control loop has time to respond. The load transient response of a power supply refers to the change in output voltage as a result of a step change in the load current. The line transient response refers to the change in output voltage as a result of a step change in input voltage. A well-designed voltage regulator will have minimal deviation from the steady state (regulated) value of its output and quickly settle back to its desired regulation point when any type of line or load transient occurs. A badly designed power supply, on the other hand, can have significant oscillation or instability on the output due to certain types of input or load transients. The following section will show examples of these responses.