1. Field of the Invention
This invention relates to an electronic device, such as a cellular phone, and an expansion unit, such as a card type of modem, that can be plugged into the cellular phone, and more specifically to an electronic device and an expansion unit which increase the efficiency in supply of electric power from the electronic device to the expansion unit and thus significantly reduce the power dissipation of the electronic device.
2. Description of the Related Art
In recent years, there have been significant advances in information processing and communications technologies. In addition, there have also been marked improvements in techniques to manufacture precision parts, typically semiconductor integrated circuits. Based on these technical advances and improvements, various battery-powered electronic devices, such as personal digital assistants (PDAs), that have radio communications functions built in have recently been developed.
Most of such electronic devices are equipped with an expansion slot in order to meet the needs of users. That is, by allowing each user to plug one of various expansion units into the slot according to his or her requirements, the devices are enabled to have only basic functions built in. This allows the electronic devices to be reduced in size and weight.
The expansion unit operates from electric power supplied from the electronic device into which it is plugged. On the other hand, the electronic device is configured to apply electric power to the expansion unit only when its main power supply is turned on.
The expansion unit can operate by itself on power supplied from the electronic device. Even in employing only the function that the expansion unit provides, the user is required to turn on the main power supply of the electronic device.
In other words, the supply of power to the expansion unit results in the supply of power to the entire electronic device and consequently in wasteful power discharge of the battery.