1. Field of the Invention
The present invention generally relates to a semiconductor integrated circuit device, and more particularly relates to a semiconductor integrated circuit device including a digital circuit and an analog circuit.
2. Description of the Related Art
Currently, lithium ion batteries are commonly used in portable devices such as digital cameras. One problem in using a lithium ion battery is that it is generally difficult to measure the remaining battery power based on its voltage. For this reason, the remaining battery power of a lithium ion battery is calculated, for example, by measuring and totaling the amounts of charge-and-discharge currents of the lithium ion battery with, for example, a microprocessor (patent document 1).
For example, fuel gauge ICs are used for measuring remaining battery power as described above. A fuel gauge IC includes, for example, analog circuits such as a high-resolution A/D converter and digital circuits such as a CPU for totaling the measured amounts of currents and a timer and is provided as a one-chip semiconductor integrated circuit device.
In the digital circuits, noise associated with, for example, charge-and-discharge currents, through currents, and harmonic components is generated in synchronization with clock signals. The noise generated in the digital circuits is transmitted within the one-chip semiconductor integrated circuit device to the analog circuits and, for example, reduces the resolution of A/D conversion.
Meanwhile, along with the downsizing of battery packs, fuel gauge ICs have also become smaller. Downsizing fuel gauge ICs increases the influence of noise and makes it difficult to provide a circuit or an electronic part used for noise reduction as an internal component. This is a problem not only for fuel gauge ICs, but also for any semiconductor integrated circuit device including an analog circuit and a digital circuit.
[Patent document 1] Japanese Patent Application Publication No. 2001-174534
For the above reason, in a conventional semiconductor integrated circuit device including both an analog circuit and a digital circuit, an external component is normally used for noise protection.