The present invention relates in general to integrated circuits and in particular to programmable low battery detect circuitry.
Many of today""s battery operated consumer electronic devices provide internal circuitry for detecting the reduction of the battery charge below a certain threshold. The low battery detect circuit is commonly set to trigger some type of user alert mechanism once the battery charge drops below the threshold level. A typical low battery detect circuit uses an internal reference voltage that is set to the desired threshold level. The battery voltage level is continually compared against the reference voltage level, with the output of the compare circuitry signaling the state of the battery.
The threshold level at which a low battery is detected depends on the type of battery used, the voltage level it operates at, and the application circuitry it powers. Even within the same general field of application, however, what constitutes low battery varies from one specific application to another. A low battery detect circuit that is designed with a single fixed threshold voltage must therefore be redesigned for the same application if, for example, the next generation devices operate at lower power supply voltage levels. A fixed low battery threshold level has similar drawbacks in case of general purpose microcontrollers that may be used in different applications. Because in certain applications it is common for a device to operate with either one battery at a first voltage level, or two batteries at twice the first voltage level, certain types of microcontrollers may provide for two different low battery threshold levels. However, the application range for even these types of microcontrollers remains limited.
There is a need for programmable low battery detect circuits that provide a low battery threshold level that can be programmably adjusted within a range of voltage levels.
The present invention provides low battery detect circuitry having user programmable low battery threshold level. Broadly, the present invention utilizes a comparator that compares a stable reference voltage against a battery-supplied power source voltage as divided down by a digitally programmable resistive divider chain. In a preferred embodiment, the resistive divider chain is made up of multiple identical resistive elements that are programmed in or out of the resistive chain by a corresponding multiple of field effect transistors (FETs). By allowing the user to digitally adjust the resistive divider ratio by fine increments, the low battery level can be programmed to any level between a given range.
Accordingly, in one embodiment, the present invention provides a low battery detect circuit including a comparator coupled between a power supply node and ground, and having a first input, a second input, and an output, a reference voltage generating circuit having an output coupled to the first input of the comparator, a programmable resistive divider circuit coupled to the second input of the comparator, wherein the programmable resistive divider circuit includes a plurality of resistors coupled between the power supply node and ground, a plurality of transistors coupled to a subset of the plurality of resistors, and control logic having a plurality of outputs respectively coupled to the plurality of transistors.
In a preferred embodiment, the reference voltage generating circuit is a band-gap reference circuit, the plurality of resistors are identical in value, and the plurality of transistors are of field effect type. The low battery detect circuit further includes storage circuitry coupled to inputs of the control logic for storing programming information.
A better understanding of the nature and advantages of the programmable low battery detect circuit of the present invention may be gained with reference to the detailed description and the drawings below.