Light emitting diode control circuit with hysteretic control and low-side output current sensing

1. A light emitting diode (LED) control circuit comprising:

a metal oxide semiconductor (MOS) transistor, the MOS transistor having a first terminal that is connected to an input voltage of the LED control circuit;

a sense resistor having a first end connected to a second terminal of the MOS transistor and a second end that is connected to ground;

a controller integrated circuit (IC) that is configured to control a switching operation of the MOS transistor by hysteretic control, to sense a current sense voltage that is developed on the sense resistor by an output current, to turn off the MOS transistor when the current sense voltage reaches a first threshold voltage, to generate a sawtooth voltage, and to turn on the MOS transistor when the sawtooth voltage reaches a second threshold voltage.

2. The LED control circuit of claim 1 , wherein the controller IC comprises:

a first pin that receives the current sense voltage;

a first comparator that is configured to compare the current sense voltage to the first threshold voltage to generate a first comparator output voltage for turning off the MOS transistor.

3. The LED control circuit of claim 1 , wherein the controller IC comprises a sawtooth generator that is configured to generate the sawtooth voltage, the sawtooth generator comprising:

a current source; and

a capacitor that is charged by the current source to generate the sawtooth voltage during an off-time of the MOS transistor.

4. The LED control circuit of claim 3 , wherein the sawtooth generator further comprises:

a switch that is configured to connect the current source to the capacitor when the MOS transistor is turned off.

5. The LED control circuit of claim 3 , wherein the sawtooth voltage is reset when the MOS transistor is turned on.

6. The LED control circuit of claim 1 , wherein the controller IC comprises:

a transconductance amplifier that is configured to generate the second threshold voltage by comparing a reference voltage to an on-time voltage that is indicative of an on-time of the MOS transistor.

7. The LED control circuit of claim 6 , wherein the controller IC further comprises:

a second comparator that is configured to compare the sawtooth voltage to the second threshold voltage to generate a second comparator output voltage for turning on the MOS transistor.

8. The LED control circuit of claim 1 , wherein the controller IC further comprises:

a second pin that is connected to a gate terminal of the MOS transistor; and

a gate driver for driving the gate terminal of the MOS transistor through the second pin.

9. A controller integrated circuit (IC) for controlling a switching operation of a switch of a light-emitting diode (LED) control circuit, the controller IC comprising:

a turn off circuit that is configured to receive a current sense voltage from a sense resistor that is connected between a terminal of the switch and ground, and to turn off the switch when the current sense voltage reaches a first threshold voltage, the current sense voltage being indicative of an output current of the LED control circuit; and

a turn on circuit that is configured to generate a second threshold voltage based on an on-time of the switch, and to turn on the switch when a control voltage that is increasing during an off-time of the switch reaches the second threshold voltage.

10. The controller IC of claim 9 , wherein the turn off circuit comprises:

a first comparator that is configured to compare the current sense voltage to the first threshold voltage to generate a first comparator output voltage for turning off the switch.

11. The controller IC of claim 9 , wherein the control voltage is a sawtooth voltage that is generated by a sawtooth generator.

12. The controller IC of claim 11 , wherein the sawtooth generator comprises:

a current source; and

a capacitor that is charged by the current source during the off-time of the switch.

13. The controller IC of claim 11 , wherein the turn on circuit comprises:

an operational transconductance amplifier (OTA) that is configured to generate the second threshold voltage by comparing a reference voltage to an on-time voltage that is indicative of the on-time of the switch.

14. The controller IC of claim 13 , wherein the turn on circuit further comprises:

a second comparator that is configured to compare the sawtooth voltage to the second threshold voltage to generate a second comparator output voltage for turning on the switch.

15. The controller IC of claim 9 , wherein the switch comprises a metal oxide semiconductor field effect transistor (MOSFET).

16. A method of operating an LED control circuit comprising:

generating a turn-on threshold that is indicative of an on-time of a switch;

sensing a current sense voltage during the on-time of the switch, the current sense voltage being developed by an output current on a sense resistor during the on-time of the switch, the current sense voltage being indicative of the output current;

turning off the switch when the current sense voltage reaches a turn-off threshold to start an off-time of the switch;

increasing a control signal during the off-time of the switch; and

turning on the switch when the control signal reaches the turn-on threshold.

17. The method of claim 16 , wherein the control signal comprises a sawtooth voltage.

18. The method of claim 17 , wherein increasing the control signal during the off-time of the switch comprises:

charging a capacitor during the off-time of the switch to generate the sawtooth voltage.

19. The method of claim 16 , wherein the switch is a metal oxide semiconductor (MOS) transistor.

20. The method of claim 19 , wherein sensing the current sense voltage during the off-time of the switch comprises:

sensing the current sense voltage from the sense resistor that is connected between a terminal of the MOS transistor and ground.