Circuit and method for coil current control
Circuits and methods to control a current in a coil are disclosed. The circuit and methods provide over-dwell protection and soft shut-down functionality to safely discharge the coil. The safe discharge of the coil is facilitated by a soft-start ramp signal that reduces the coil current gradually by controlling a switching device according. A profile of the soft-start ramp signal over time determines the gradual reduction. The profile of the soft-start ramp signal can be adjusted to set (i) an over-dwell period of the coil current, after which the coil current is shut down, and (ii) a soft shut-down period, over which the coil current is gradually reduced.
1. A soft-start ramp generator, comprising:
a capacitor; and
a voltage controlled current source configured to generate an output current according to a plurality of operating characteristics based on a voltage across the capacitor, wherein the output current generated by the voltage controlled current source is configured to charge the capacitor such that when the capacitor is charged, the voltage controlled current source generates the output current according to each of the plurality of operating characteristics.
2. The soft-start ramp generator according to claim 1 , wherein the plurality of operating characteristics includes:
a first operating characteristic wherein the output current is a first current while the voltage across the capacitor is charged form a zero voltage to a first voltage.
3. The soft-start ramp generator according to claim 2 , wherein the plurality of operating characteristics includes:
a second operating characteristic wherein the output current is increased linearly from the first current to a second current while the voltage across the capacitor is charged from the first voltage to a second voltage.
4. The soft-start ramp generator according to claim 3 , wherein the plurality of operating characteristics includes:
a third operating characteristic wherein the output current is the second current while the voltage across the capacitor is charged from the second voltage to a maximum voltage.
5. The soft-start ramp generator according to claim 4 , wherein the voltage controlled current source generates the output current according to the first operating characteristic, the second operating characteristic, and the third operating characteristic in succession as the capacitor is charged.
6. The soft-start ramp generator according to claim 5 , wherein the voltage across the capacitor corresponds to a soft-start ramp signal that includes:
a substantially constant portion corresponding to the first operating characteristic;
a soft-start portion corresponding to the second operating characteristic; and
a linear portion corresponding to the third operating characteristic.
7. The soft-start ramp generator according to claim 6 , wherein the substantially constant portion begins at a maximum voltage of the soft-start ramp signal.
8. The soft-start ramp generator according to claim 6 , wherein a duration of the substantially constant portion is less than five percent (5%) of an overall duration of the soft-start ramp signal.
9. The soft-start ramp generator according to claim 6 , wherein the soft-start portion decreases from a voltage at an end of the substantially constant portion according to a rate of decrease that changes over a duration of the soft-start portion.
10. The soft-start ramp generator according to claim 9 , wherein the rate of decrease at a start of the soft-start portion is less than the rate of decrease at an end of the soft-start portion.
11. The soft-start ramp generator according to claim 9 , wherein the rate of decrease corresponds to a slope of the second operating characteristic.
12. The soft-start ramp generator according to claim 6 , wherein the linear portion decreases linearly from a voltage at an end of the soft-start portion to zero volts.
13. The soft-start ramp generator according to claim 1 , wherein:
the capacitor is coupled between the voltage controlled current source and a source voltage; and
the soft-start ramp generator is configured to output a soft-start ramp signal at an output node between the voltage controlled current source and the capacitor.
14. A method for generating a soft-start ramp signal, the method comprising:
charging a voltage across a capacitor with an output current from an output of a voltage controlled current source;
applying the voltage across the capacitor to an input of the voltage controlled current source so that the voltage controlled current source is configured to generate the output current according to a plurality of operating characteristics as the voltage across the capacitor is charged; and
generating the soft-start ramp signal based on the voltage across the capacitor.
15. The method according to claim 14 , wherein the plurality of operating characteristics includes:
a first operating characteristic wherein the output current is a first current while the voltage across the capacitor is charged from a zero voltage to a first voltage;
a second operating characteristic wherein the output current is increased linearly from the first current to a second current while the voltage across the capacitor is charged from the first voltage to a second voltage; and
a third operating characteristic wherein the output current is the second current while the voltage across the capacitor is charged from the second voltage to a maximum voltage.
16. The method according to claim 15 , wherein the voltage controlled current source is configured to generate the output current according to the first operating characteristic, the second operating characteristic, and the third operating characteristic in succession as the capacitor is charged.
17. The method according to claim 15 , wherein the soft-start ramp signal includes:
a substantially constant portion corresponding to the first operating characteristic that begins at a maximum voltage of the soft-start ramp signal;
a soft-start portion corresponding to the second operating characteristic that decreases from a voltage at an end of the substantially constant portion according to a rate of decrease that changes over a duration of the soft-start portion; and
a linear portion corresponding to the third operating characteristic that decreases linearly from a voltage at an end of the soft-start portion to zero volts.
18. The method according to claim 17 , wherein a duration of the substantially constant portion is less than five percent (5%) of an overall duration of the soft-start ramp signal.
19. The method according to claim 17 , wherein the rate of decrease at a start of the soft-start portion is less than the rate of decrease at an end of the soft-start portion.
20. The method according to claim 19 , wherein the rate of decrease corresponds to a slope of the second operating characteristic.