Current control circuit for micro-fluid ejection device heaters

1. A method for controlling the current through at least on heater element in a microfluid ejection device with a compensation circuit comprising:

powering a plurality of heater elements with a power circuit;

exciting the heater elements by providing current therethrough;

coupling at least one logic switch to at least one heater element for selectively applying and controlling power from the power circuit to the at least one heater element;

sensing the heater current through at least one heater element;

inducing a signal from the heater current, the signal having a magnitude which corresponds to the difference between the magnitude of the current and a reference value;

applying the signal to the switch to regulate the heater current and thereby change the magnitude of the difference between the current and reference value;

modifying the signal to reflect the changing magnitude of the heater current;

regulating the heater current to the reference value by applying the modified signal to the switch; and

limiting the heater current through at least one resistor by applying a bias signal.

2. The method as melted in claim 1 , wherein the bias signal is fixed during the manufacture of the device.

3. The method as recited in claim 1 , wherein the bias signal limits the heater current through only one heater element.

4. The method as recited in claim 3 , wherein the bias signal limits the heater current through each heater element in the micro-fluid ejection device.

5. The method as recited in claim 3 , wherein the bias signal limits the heater current through each heater element in a primitive.

6. The method as recited in claim 1 , further comprising limiting the induced signal to at least one switch by applying a bias signal.

7. The method as recited in claim 6 , wherein the bias signal is set during the manufacture of the device.

8. The method as recited in claim 7 wherein the bias signal limits the induced signal through only one switch.

9. The method as recited in claim 6 , wherein the bias signal limits the induced signal through each switch in the micro-fluid ejection device.

10. The method as recited in claim 6 , wherein the bias signal limits the induced signal through each switch, in a primitive.

11. The method as recited in claim 1 , wherein the reference value is constant.

12. The method as recited in claim 1 , wherein the reference value changes to maintain constant power through at least one heater element.

13. The method as recited in claim 12 , wherein the change of the reference value approximately replicates the change in current necessary to maintain constant power.

14. A method for regulating current through at least one heater element in a micro-fluid ejection device comprising:

coupling a logic switch to at least one heater element;

operating the logic switch to produce current through the heater element; and

regulating the current by:

inducing a signal from the heater current, the signal having a magnitude which corresponds to the difference between the magnitude of the current and a reference value;

changing the reference value to approximate the rate of change of the current necessary to maintain constant power through the element using the equation

P=I 2 *R;

applying the signal to the switch to regulate the heater current to the reference value; and

modifying the signal to reflect the changing difference between the current and the reference value;

applying the changing signal to the switch; and

limiting the heater current through at least one resistor by applying a bias signal.

15. A micro-fluid ejection device comprising:

a plurality of heater elements operable to eject fluid;

a power circuit operable to provide power to at least one heater element;

at least one logic switch connected to at least one heater element for selectively applying and controlling power from the power circuit to the at least one heater element; and

at least one compensation circuit comprising:

a sensing resistor coupled to at least one heater element;

an offset circuit for establishing a reference value;

a differential amplifier having about unity gain and being coupled to the sensing resistor and offset circuit, the differential amplifier responsive to produce a signal having a magnitude corresponding to the difference in heater current and the reference value;

a current mirror circuit coupled to the differential amplifier; and

a source follower circuit coupled to the current mirror circuit and the logic switch whereby the current mirror translates the signal from the differential amplifier to the source follower to control the switch with the translated, signal, the signal operative to reduce the magnitude of the difference between the heater current and reference value;

wherein the signal is applied to the switch; and

wherein the signal changes to reflect the reducing magnitude of the difference between the heater current and reference value and drives the current to the reference value.

16. The device as recited in claim 15 , further comprising bias ports for setting a heater current limit and induced signal limit.

17. The device as recited in claim 16 , wherein the bias level is set during the manufacture of the device.

18. The device as recited in claim 15 , wherein the reference valve changes to maintain constant power through at least one heater element.

19. The device as recited in claim 18 , wherein the reference value changes to approximate the rate of change of the current necessary to maintain constant power through the element.