Mobile devices, such as mobile phones, personal organisers and portable music players are usually powered by an internal rechargeable battery. These devices may be recharged using a mains or in-car battery charger. During charging the device can be powered from the mains or car battery instead of the internal battery. Often these mobile devices are also equipped with a secondary battery such that if the main battery fails or is removed from the mobile device, the secondary battery provides power to maintain internal settings and information (such as maintaining power to clock functions). This avoids the user the inconvenience of having to re-enter important information each time the main battery is removed or discharged.
Recently, some mobile devices have also incorporated USB connections or the like, which can also be used to supply power to the device or recharge the battery.
It is desirable that the mobile device optimises use of the internal rechargeable battery to increase the life of the battery and provide the user with an operational device for as much of the time as possible.
The USB connection, mains charger, and in-car charger can be considered as external power sources. Prior art devices treat the external power sources separately which leads to the device having several system supply circuits with duplicate functions. For example, the input node for receiving each supply will have voltage and current references, comparators, and regulators, which may all be arranged to handle high currents.
In one prior art implementation, shown in FIG. 1, a power management circuit 10 can be connected to a USB charger 20, wall charger 22, main battery 24 and secondary battery 26 (also referred to as button cell or backup battery). Various components 38 such as memory units and the real time clock system are connected to the main battery 24 or alternatively to the secondary battery 26 through a selector 40. The selector is arranged to connect the memory units and real time clock system to whichever of the main battery or secondary battery provides the higher voltage. However, this arrangement shortens the life of the secondary battery 26 as it can be forced to supply power to the various components 38 even if other power sources, e.g. USB or wall charger, can supply power to these components. Core circuits 36 (i.e. circuits other than the charging circuits 30 and those connected to selector 40) are connected directly to the power management circuit 10.
In WO 2006/102928 an improved prior art power management scheme is proposed. The power management scheme, as shown in FIG. 2, is able to receive power from four power sources: USB charger 20, wall charger 22, main battery 24, and secondary battery 26. The output of the power management circuit provides power through a main output voltage, Vcore, to the other components of the mobile device, such as the display, LED matrix, digital and analog circuits etc. The power management circuit includes a power source capabilities detection unit, a power source selection unit, and a regulator circuit. The regulator circuit has multiple inputs and a single output. The power source detection unit monitors the available power sources to determine their power supply capability. If a power source can supply voltage above a threshold level, then it will send an indication to the power source selection unit. The power source selection unit applies selection rules to determine which power source supplies power, as indicated in Tables 1 and 2 below.
TABLE 1TRUTH TABLE IN ON MODECHARGERVBUSBATTERYBack-up cellVcore powered byXX1XBATTERY1X0XCHARGER010XVBUS
TABLE 2TRUTH TABLE IN OFF MODECHARGERVBUSBATTERYBack-up cellVcore powered by1XXXCHARGER01XXVBUS00HighestBATTERY00HighestBack-up Cell
The selection rules are used to determine which power source supplies power based on the amount of current or voltage available from a source and the status of the mobile device. In the tables above, “0” indicates that the power source is not present, “1” indicates that the power source is selected to provide power, and “X” indicates that the value or status of a power source is not relevant. Therefore, the tables indicate that in the “On” mode with the main battery able to supply power, power will always be drawn from the main battery. In the “Off” mode with the wall charger 22 present, power is always drawn from the charger. In the “Off” mode with only the main battery 24, and secondary battery 26 present, power will be drawn from whichever of these two batteries has the highest voltage.
This prior art arrangement has a number of disadvantages as the input nodes for the USB charger and wall charger each have voltage references for detection of the charger. This arrangement of reference voltages etc for each voltage input takes up significant area on the semiconductor device used to implement the power management scheme. In addition, Vcore also requires a reference voltage, this is provided by a trimmed bandgap reference to get good supply accuracy. This takes up further room on the chip and is an additional power consuming component.
Furthermore, the various components mentioned above (e.g. display, LED matrix, digital and analog circuits etc) are all powered from a single voltage rail Vcore which supplies all of the analog and digital circuits. The magnitude of Vcore is required to be relatively large to power the analog circuits.
There is therefore a need to provide an improved power management arrangement that meets the requirements of providing core voltage but also minimises power consumption and optimises die area.