Regular goings onSwitching regulators are not always more efficient or more effective than low-dropout linear regulators explains Dipl.-Ing. Werner Obermaier of National Semiconductor Continuing developments in rechargeable battery technology is resulting in much longer talk and stand-by times for portable systems including laptops, mobile phones, camcorders, despite their ever increasing functionality.
For example, one of the biggest mobile phone manufacturers has recently announced its next generation product with talk times of up to seven hours and stand-by times of up to 200 hours.
Better battery technology is not the only reason for this and the use of power-saving silicon makes its contribution.
System voltage regulators are essential and switching types are presented as the efficient, modern, way to produce a stable voltage, but linear regulators should not be overlooked.
A wireless phone probably illustrates this best. All the semiconductors required by this application are available today in a 3V technology with specifications guaranteed for a supply voltage down to 2.7V.
A battery, built out of three nickel-cadmium or nickel-metal hydride cells in series, provides a supply voltage between 3.6V and 3.0V. Modern low-dropout-regulators have a typical dropout voltage of 120mV with an output current up to 50mA. A tolerance of 1 per cent across the whole temperature range prevents the minimum voltage from falling below 2.7V, with a fixed output voltage of 2.8V. At the same time, the energy supplied by the battery is being completely used.
The efficiency of the regulator can easily be calculated assuming that there is a linear voltage drop between the fully charged and discharged states. The quiescent current of the regulator reduces the supply available to the load resulting in additional current losses. However, this is low in micropower regulators.
The above example results in 84 per cent efficiency, demonstrating that linear regulators do not necessarily cause low efficiency. Careful selection of the battery, the output voltage and the linear regulator facilitate lengthy talk times without having to use more costly switching regulators. Furthermore, surface mount packages make these regulators very small, requiring only a fraction of the space of a comparable switching regulator.
In most applications, the regulator is not required for the stabilisation of the output voltage, but for the mutual decoupling of the different sections of the circuit. So long as they are not operated in drop-out mode, switching regulators can provide this isolation. But the electrical noise generated by the pulsating load on the battery, especially during high current demand, can mean that shielding is required on sensitive circuit parts.
Despite continuous development, linear regulators come up against limiting factors under certain circumstances. For example, they cannot create output voltages that are higher than the input voltage.
Another reason for choosing a switching regulator can be a source with a wide supply voltage range, for example re-chargeable lithium-ion-batteries. A typical application case for a step-down switching regulator is one which creates 5.0V or 3.3V out of 2-cell lithium-ion batteries.
Both linear and switching regulators have their advantages and disadvantages. Only when considering the power supply at an early stage of development, is it possible to design a cost effective product with the least design effort. Clearly, the feeling that linear regulators are inefficient has been countered
Switching regulators often seem to be the easiest solution but may not always be the most cost-effective. However, in most cases it is possible to avoid higher costs if an initial overall power supply concept including source, conditioning and load, is taken into account.
National Semiconductor makes both switching and linear regulator chips.