Accurate voltage comparator with voltage-to-current converters for both reference and input voltages

1. A voltage sensor circuit comprising:

a source-input node having a source-input voltage that is varied by a voltage source, the voltage sensor circuit sensing the source-input voltage of the source-input node;

a first current source, responsive to the source-input voltage, for generating a first current that varies with variations in the source-input voltage;

a first resistor, coupled to the first current source and receiving the first current, for generating a compare-input voltage on a compare-input node in response to the first current, the compare-input voltage varying with variations in the first current;

a second current source for generating a second current that is insensitive to variations in the source-input voltage;

a second resistor, coupled to the second current source and receiving the second current, for generating a reference voltage on a reference node in response to the second current, the reference voltage varying with variations in the second current;

a stable node, coupled to the first current source and coupled to the second current source, the stable node having a stable voltage that is relatively insensitive to changes in a supply voltage;

a comparator coupled to the compare-input node and the reference node, for comparing the compare-input voltage to the reference voltage and generating an output voltage at an output node that indicates when the compare-input voltage is above the reference voltage;

wherein the first current source comprises:

a first mirror transistor having a channel that conducts the first current between the stable node and the compare-input node in response to a first gate node;

a first setting transistor, with a gate connected to the first gate node, having a channel that conducts a first setting current between the stable node and the first gate node;

a first sensing transistor having a channel that conducts a portion of the first setting current from the first gate node, the first sensing transistor having a gate connected to the source-input voltage;

wherein the second current source comprises:

a second mirror transistor having a channel that conducts the second current between the stable node and the reference node in response to a second gate node;

a second setting transistor, with a gate connected to the second gate node, having a channel that conducts a second setting current between the stable node and the second gate node; and

a second sensing transistor having a channel that conducts a portion of the second setting current from the second gate node, the second sensing transistor having a gate connected to a fixed voltage.

2. The voltage sensor circuit of claim 1 wherein the fixed voltage applied to the gate of the second sensing transistor is the stable voltage.

3. The voltage sensor circuit of claim 2 wherein the first and second mirror transistors and the first and second setting transistors are p-channel transistors;

wherein the first and second sensing transistors are n-channel transistors.

4. The voltage sensor circuit of claim 3 wherein a cross-over voltage of the source-input voltage that causes the output voltage to change states varies less than +/−4% over a temperature range from −40 to +85 degrees C.

5. The voltage sensor circuit of claim 1 further comprising:

a voltage generator for generating the stable voltage on the stable node that is independent of the supply voltage to the comparator.

6. The voltage sensor circuit of claim 5 wherein the voltage generator is a band-gap voltage generator.

7. The voltage sensor circuit of claim 6 wherein the comparator is powered by the supply voltage.

8. A temperature-insensitive voltage sensor comprising:

an input voltage from a varying voltage source;

compare means, having a first input and a second input, for comparing voltages on the first and second inputs to generate an output;

first resistor means, receiving a first current, for generating a compare voltage on the first input of the compare means;

first mirror transistor means, having a gate connected to a first gate node, for generating the first current from a stable node to the first input of the compare means and to the first resistor means;

first current-source transistor means, having a gate and a drain connected to the first gate node and a source connected to the stable node, for generating a first gate voltage on the first gate node;

first sensing transistor means, having a gate driven by the input voltage, for varying a first sink current from the first gate node in response to the input voltage;

second resistor means, receiving a second current, for generating a reference voltage on the second input of the compare means;

second mirror transistor means, having a gate connected to a second gate node, for generating the second current from the stable node to the second input of the compare means and to the second resistor means;

second current-source transistor means, having a gate and a drain connected to the second gate node and a source connected to the stable node, for generating a second gate voltage on the second gate node; and

second sensing transistor means, having a gate driven by a constant voltage, for generating a second sink current from the second gate node.

9. The temperature-insensitive voltage sensor of claim 8 further comprising:

stable-voltage generator means for generating a stable voltage on the stable node, the stable voltage being insensitive to a supply voltage to the compare means.

10. The temperature-insensitive voltage sensor of claim 9 wherein the first and second sensing transistor means are n-channel transistors having grounded sources;

wherein the first and second mirror transistor means are p-channel transistors having sources connected to the stable node;

wherein the first and second current-source transistor means are p-channel transistors having sources connected to the stable node and each having a drain shorted to a gate.

11. The temperature-insensitive voltage sensor of claim 10 wherein the constant voltage to the gate of the second sensing transistor means is the stable voltage.