CMOS low battery voltage detector

A detector for providing a low battery voltage indication comprising:

• a stable voltage reference and comparator circuit for comparing the battery voltage and the voltage reference;
• switch means to switch between a first operational state and a second operational state;
• a switched capacitor circuit to store charges related to the battery voltage and the reference voltage in each operational state;
• and
• a clock to initiate switching between the first and second states;
• wherein an output from the comparator during the second state indicates whether the battery voltage is below a preset threshold (Vmin) set such that:
• Vmin>VBE+K1ΔVBE−B2BAT
• where:
• Vmin is the threshold value
• VBE is the base to emitter voltage across a first transistor of the bandgap reference;
• ΔVBE is the difference between base to emitter voltages in two bandgap transistors having difference current densities;
• VBAT is the battery voltage;
• K1 is a first scaling factor, and
• K2 is a second scaling factor.

A method of detecting low battery voltage supplied to an integrated circuit comprising:

• providing a stable voltage reference and comparator circuit for comparing battery voltage against the reference voltage;
• providing a capacitor circuit for storage charges associated with the battery voltage and the reference voltage;
• providing switching means to switch between a first phase wherein said capacitors are charged to a first voltage level and a second phase wherein said capacitors are charged to a second level;
• and
• comparing said first and second levels to determine whether said battery voltage is above or below a preset threshold (Vmin) such that
• Vmin=Vref−K*Vbat
• where:
• Vref is the reference voltage
• Vbat is the battery voltage;
• and
• K is a scaling factor.