Methods and apparatus for an ignition system

1. An igniter circuit configured to operate according to a clock signal and control an ignition coil, comprising:

a first counter responsive to the clock signal and configured to count for a first predetermined period and generate a first count output;

a second counter responsive to the clock signal and configured to count and generate a second count output; wherein the second counter starts counting at a predetermined instance during the first predetermined period;

a switch element comprising:

a first terminal, wherein the first terminal is a gate terminal;

a second terminal coupled to the ignition coil; and

a third terminal;

a current limiter circuit comprising:

an input terminal coupled to an output terminal of the second counter; and

an output terminal coupled to the first terminal of the switch element;

wherein the current limiter circuit is configured to generate an output based on the second count output; and

a shutdown controller comprising:

an output terminal coupled to an input terminal of the second counter and the switch element; and

an input terminal coupled to the first terminal of the switch element;

wherein the shutdown controller is configured to detect a change to the current limiter circuit output.

2. The igniter circuit according to claim 1 , wherein:

the second terminal of the switch element is a collector terminal;

the third terminal of the switch element is an emitter terminal and is coupled to a current sense circuit; and

wherein the switch element:

is responsive to the current limiter circuit output; and

has a saturation current responsive to the ignition coil.

3. The igniter circuit according to claim 2 , wherein an initial count value of the second counter corresponds to a reference current value that is greater than the saturation current of the switch element.

4. The igniter circuit according to claim 2 , wherein the shutdown controller is configured to:

detect a decrease in a gate voltage at the gate terminal of the switch element; and

pause the second counter when the decrease is detected.

5. The igniter circuit according to claim 4 , wherein the second counter resumes counting at the end of the first predetermined period.

6. The igniter circuit according to claim 1 , wherein a total count of the second counter during a soft shutdown period is a function of a supply voltage level.

7. The igniter circuit according to claim 1 , wherein the first counter:

starts counting when a control signal is enabled; and

ends at a predetermined count value.

8. The igniter circuit according to claim 1 , wherein the first predetermined period is independent of both a supply voltage level and a temperature of the ignition coil.

9. The igniter circuit according to claim 1 , wherein the first counter and the shutdown controller are configured to selectively operate the second counter according to the first count output.

10. A method for controlling an ignition system, comprising:

controlling an igniter circuit adapted to couple to an ignition coil, the igniter circuit configured to:

count a first predetermined period with a first counter;

count with a second counter, wherein the second counter starts counting at a predetermined instance during the first predetermined period;

generate a reference current according to an output value of the second counter;

generate an output voltage according to the reference current and a coil current;

control an insulated-gate bipolar transistor according to the output voltage;

detect a decrease in the output voltage; and

pause operation of the second counter when the decrease in the output voltage is detected.

11. The method according to claim 10 , wherein generating the reference current comprises: generating a linearly decreasing reference current from a maximum value to a saturated value.

12. The method according to claim 11 , wherein generating the reference current further comprises generating a linearly decreasing reference current from the saturated value to zero, wherein the reference current reaches zero no later than an end of the first predetermined period.

13. The method according to claim 10 , further comprising resuming operation of the second counter when the first predetermined period ends.

14. An ignition system, comprising:

an ignition coil; and

an igniter circuit coupled to the ignition coil and comprising an insulated-gate bipolar transistor (IGBT), wherein the igniter circuit is configured to:

generate a first count output for a first predetermined period;

generate a second count output;

generate a linearly decreasing reference current according to the second count output;

apply a gate voltage to the IGBT, wherein the gate voltage is based on the linearly decreasing reference current and an ignition coil current; and

detect a decrease in the gate voltage.

15. The ignition system according to claim 14 , wherein the igniter circuit starts to generate the second count output at a predetermined instance during the first predetermined period.

16. The ignition system according to claim 14 wherein the first predetermined period is independent of both a supply voltage level and a temperature of the ignition coil.

17. The ignition system according to claim 14 , wherein the igniter circuit is further configured to:

pause the second count output when the gate voltage starts to decrease; and

resume the second count output when the first predetermined period ends.

18. The ignition system according to claim 14 , wherein a rate of change of the second count output is based on a clock frequency.

19. The ignition system according to claim 14 , wherein an initial second count output corresponds to a reference current value that is greater than a saturation current of the IGBT.