1. Statement of the Technical Field
The inventive arrangements relate to various devices including a primary battery and a hold-up battery. More particularly, the inventive arrangements relate to a time limit circuit for a hold-up battery.
2. Description of the Related Art
Various electronic devices (such as handheld transceivers, GPS equipment, and portable computers) require a relatively large amount of power for operation. As such, a primary battery having high voltage and current characteristics is typically provided for powering such a device during its normal operation. It is also common practice with many such electronic devices to make use of a volatile data storage circuit. Such volatile data storage circuits include volatile random access memory (RAM) circuits that will lose any data stored therein if power to the device is interrupted for any reason. Such information can include cryptographic key information, positioning information system descrambling information, and/or system timing information. The loss of this information can be undesirable in many instances. Accordingly, a relatively small sized hold-up battery is typically provided within the electronic device for powering RAM and certain other types of internal components when a primary battery is in a discharged state, and/or a primary battery is temporarily removed so that it can be replaced. In this regard, it should be appreciated that a hold-up battery can be provided for powering RAM and any other necessary circuits whenever a device is turned off such that the device retains certain information indefinitely.
Still, there are certain problems that exist in connection with the use of hold-up batteries. For example, such batteries are typically small in size and can only hold a limited amount of charge. It should also be appreciated that the amount of voltage and current required to power volatile RAM and any other necessary circuits for a period of time can be relatively large as compared to the amount of charge held by one or more hold-up batteries. Accordingly, it can be desirable to limit the duration of time during which a hold-up battery will supply power to a RAM and any other necessary circuits. For example, an hour is often sufficient time to return to a service location and obtain a replacement for the primary battery. After such time period, a time limit circuit can automatically disconnect the hold-up battery. This will naturally result in the undesirable loss of data. However, the time limit circuit is necessary in order to prevent the hold-up battery from becoming discharged too rapidly. In this regard, it should be noted that customer requirements can dictate that such hold-up batteries provide at least one year of useful life before they become discharged or otherwise require replacement.
Time limit circuits as described herein are useful for preserving the life of a hold-up battery. However, it must be appreciated that any such time limit circuit must also be powered by the hold-up battery. Powering such a time limit circuit places further current demands on the hold-up battery. Accordingly, it is important for any time limit circuit used for this purpose provide a very low current draw. In this regard, microprocessors and other timer circuits can be unsuitable for such purposes because they place significant current demands on a hold-up battery. Also, conventional circuits that rely on an RC time constant for timing purposes are generally not well suited for these hold-up battery timing tasks. That is because current leakage in the various components comprising such circuits can make it difficult to achieve time-limits that are on the order of about one (1) hour.
In view of the forgoing, there remains a need for a circuit that can limit the amount of time a hold-up battery supplies power to certain internal components when a primary battery is in a discharged state, and/or a primary battery is temporarily removed so that it can be replaced.