Mobile Phone Power Management System Project
Cell phone power management requires small regulators with fast response
The notion of power management means different things to different people. It can describe the software controlled turn-on and turn-off of different blocks of a complex system, like a cellular phone or notebook PC, to optimize the power consumption and battery life of the whole box. It can mean the collection of different voltage regulators that supply many different voltages these systems require, including their sequencing and supervisory functions. The term can also include battery charging, monitoring and fuel gauging. This paper will concentrate on the voltage regulators and their role within the cellular phone.
Voltage regulators are used in cellular phones for different reasons. A couple of phone generations ago, when the different sub-circuits were built on 0.5u-0.6u CMOS processes, everything could theoretically operate from the battery directly. However, it is easy to show that adding a regulator between a CMOS digital circuit, or a switched capacitor based mixed signal circuit, and the battery will significantly increase battery life. This is true even using regulators with relatively low efficiency, like linear regulators. The reason for this phenomenon is that the power consumption of a mostly digital CMOS circuit is proportional to the square of the supply voltage (i.e. the battery voltage if the circuit is operated from the battery directly), while if one adds a linear regulator in front of this circuit the power consumption will change only linearly with the battery voltage. The paradigm has shifted in the past 2 years. Most phones today operate on a single cell Li-Ion battery, which has a 4.2V maximum fully charged voltage. If the cellular phone manufacturer requires the phone to operate with removed battery and plugged in charger the maximum input voltage of the system can be higher, depending on how the battery charger is implemented. As the process evolution path moves down the main circuit components to 0.25u, 0.18u and below, it is clear that there has to be a voltage regulator between most of the phone circuits and the battery.
Initially the voltage regulator function was implemented using discrete low dropout linear regulators, LDOs. Today most phones are built using more integrated power management solutions, that include a large number of regulators, LDOs and switching regulators, battery chargers, sequencing circuitry, supervisory and house keeping circuitry.
The role of voltage regulators
The voltage regulators in cellular phones are used to accomplish different goals, aside form their basic task, which is to increase battery life:
1. To step down the voltage between the battery and the different sub-circuits that require lower supply voltage, or to step up the voltage for sub-circuits that need higher voltage than the battery (like SIM card, backlight LED, etc.). Occasionally a buck-boost type regulator is required to generate a voltage that is between the maximum and minimum value of the battery voltage.
2. To isolate the different subsystems from each other. This is important in the RF section, and also between digital and analog/mixed signal circuits. Using LDOs can be cheaper and more area efficient than adding the traditional LC isolation filters that are used in the supply lines of RF circuits.
3. To isolate sensitive circuitry from the transient voltage changes of the battery. This is especially relevant in GSM phones where the PA operates in a 217 Hz pulsed mode with 12.5% duty cycle. The high current of the PA, typically up to 1.6A, can cause a voltage transient of up to 0.5V due to the combined effect of the battery's ESR and protection circuitry. The PSSR of the voltage regulator significantly reduces the supply transient seen by the phone circuits.