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In any system incorporating batteries one of the major design goals is to maximise the time between charges. Battery design engineers try to achieve this goal by providing cells with the maximum possible capacity. The product design engineer however can play a major role in helping the battery charge to last longer by using power management schemes in their designs which reduce the current drain on the battery. Since a given charge can be made to last longer or to do more useful work, a consequential benefit is that for the same cycle life, the actual battery lifetime will also be extended.
In general power management schemes reduce the overall power consumption by supplying power only to parts of the circuit which have an immediate demand for it and slowing down, or switching off the power to, circuits which may be temporarily idle. This may be done progressively as in laptop computers or according to a preprogrammed sampling scheme which allows power on demand as in mobile phones.
Progressive Circuit Shut Down
This technique is used extensively in laptop computer designs. By sensing the keyboard activity the computer knows when the machine is idle. After set intervals of inactivity certain functions are switched off. A measure of user control over the length of the intervals is usually provided to enable the user to match the shut down scheme to his work pattern. The shorter the intervals the more power will be saved. The functions are usually restored immediately when the computer senses that the mouse or keyboard have been used again.
Shut Down Sequence
- First the monitor or screen is switched off.
- If there is still no activity the hard disks may be switched off.
- Standby Mode is the next stage in the shut down. - This places the entire system in a low-power state by slowing down the clock speed and hence all the circuit functions. All the open applications remain in place and ready to restart on demand. This gives the benefit of getting right back to work when required without waiting for the computer to restart.
- Hibernation is the last shut down stage which occurs when there has been no activity for a prolonged period. In this state all the contents of memory are first stored in a special file on the hard disk and then the central processing unit or the entire computer is switched off. When the computer is powered up again it must first reboot but everything will be restored exactly as it was left including programs and documents which may not have saved or closed.
There are other things the computer user can do to preserve battery life. The most obvious one is to disconnect the battery when mains power is available. It is a pity laptop manufacturers don't provide a switch for this purpose. It usually requires removing the battery.
LCD screens are notoriously power hungry devices. Their appetite can however be reduced by turning down the brightness control.
PDAs, unlike laptop computers, have no hard drives.They can only store data in volatile memory and so can never be completely switched off. The power management circuitry must still supply keep-alive voltage to the sleeping processor and memory when the PDA is not in use, and it must do it at extremely low quiescent current.
Power On Demand
Mobile phones need to be switched on at all times to be able to receive calls, but they don't need to be transmitting unless there is some information to transmit. The receiver is a low power circuit processing low received signal levels, but the transmitter has to send out high power signals strong enough to reach the base station. The audio amplifier and display circuits also consume high power but are only needed during actual call. The following techniques are used to conserve power:-
- The transmitter has to keep the base station informed about the location of the handset and to check if there are any incoming calls so it must send out regular signals to the base station to keep it updated .The transmitter is therefore only switched on for about half a millisecond every second (the exact period depends on the system design) to tell the base station where it is.
- The audio, display and other feature circuits are switched to a low power sleep mode between calls. In response to the transmitter paging the base station, if there is an incoming call, a wake up circuit will turn on all of these functions again.
- During a call, when it needs to transmit speech, it only uses power if there is information to be transmitted.
- Automatic gain control ensures that, when it is transmitting, the communications link is maintained with just sufficient transmitter power consistent with acceptable signal quality.
Intelligent Energy Manager (IEM)
New developments in microprocessor and software technology recently announced by ARM enable dynamic tuning of the processor's operating voltage and frequency to match the processing workload. Reductions in the processor's energy consumption by up to 25% are claimed for applications using this technique, extending the battery life for portable systems.