CMOS low battery voltage detector

• US 6,445,305 B2
• Filed: 02/01/2001
• Issued: 09/03/2002
• Est. Priority Date: 02/09/2000
• Status: Expired due to Term

• Current Assignee: Mitel Semiconductor AB
• Original Assignee: Mitel Semiconductor AB
• Inventors: Andersson, Olle, Ohlsson, Tony
• Primary Examiner(s): TRIEU, VAN THANH
• Application Number:
  US09/773,655
• Publication Number:
  US 20010026226A1
• Time in Patent Office: 579 Days
• Field of Search: 340/636, 340/635, 340/663, 323/299, 323/267, 323/271, 323/349, 323/354, 320/107, 320/132, 320/136, 307/16
• US Class Current: 340/636.15
• CPC Class Codes: G01R 19/16542 for batteries charge condition monitoring in G01R31/36
• CPC Class Codes: G01R 19/16552 in I.C. power supplies

Abstract

A system and method for detecting a low battery voltage supplied to a battery operated integrated circuit. A stable reference voltage provided by a bangap reference is compared with the battery voltage. A switched capacitor circuit is used instead of the more conventional resistor combination to supply a scaled representation of the battery voltage. Power requirements are reduced by combining the bandgap reference and the comparator into a single component.

Claims

1. 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;

a 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) such that

2. A detector as defined in claim 1 wherein said stable voltage reference is a bandgap reference circuit.

3. A detector as defined in claim 2 wherein said bandgap reference circuit comprises a pair of bipolar transistors supplied with a first current density in said first state and a second greater current density in said second state.

4. A detector as defined in claim 1 wherein said first operational state is a precharge phase and said second operational state is an evaluation phase.

5. A detector as defined in claim 1 fabricated on an integrated circuit (IC) employing CMOS technology.

6. A detector as defined in claim 5 wherein said comparator is an operational amplifier.

7. A detector as defined in claim 6 wherein offset voltages due to first and second inputs to said operational amplifier are cancelled by said switched capacitor circuit.

8. A detector as defined in claim 1 wherein capacitors in said switched capacitor circuit define a weighting factor for establishing said threshold.

9. A detector as defined in claim 1 wherein said bandgap reference circuit comprises three bipolar transistors with a first switchable current source is supplied to one transistor and a second current source is supplied to the other two transistors.

10. 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;

11. A detector as defined in claim 10 having three capacitors (C₁, C₂ and C₃) wherein K₁ and K₂ are determined by said capacitors according to the ratios K₁ =C₁ +C₂/C₃ and K₂=C₁/C_3.

12. 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

13. The method as defined in claim 12 wherein said switching means are MOS devices.

14. The method of claim 12 wherein said stable voltage reference is a bandgap cell having a value of approximately 1.2 volts and Vmin is approximately 1.2/K .