Leading-Edge Phase-Cut Dimmer Detector

Abstract

Circuits and methods for detecting the presence of a leading-edge phase-cut dimmer. The dimmer detector comprises an edge detector, a pulse stretcher and a filter. The edge detector detects whether an input signal has a rapidly rising edge and generates an output signal pulse if a rapidly rising edge is detected. If the edge detector outputs a signal pulse, the pulse stretcher generates a stretched pulse having a duration that is longer than the signal pulse received from the edge detector. The filter produces a dimmer detect signal that indicates whether a leading-edge phase-cut dimmer is detected. If the pulse stretcher output signal comprises at least a predetermined number of stretched pulses within a predetermined amount of time, the dimmer signal signals the presence of a leading-edge phase-cut dimmer.

What is claimed is: 
     
         1 . A circuit for detecting a leading-edge phase-cut dimmer, comprising:
 an edge detector operable to receive an input signal, detect whether the input signal has a rapidly rising edge, and generate an edge detector output signal, wherein the edge detector output signal comprises a signal pulse if a rapidly rising edge is detected in the input signal;   a pulse stretcher operable to receive the edge detector output signal and generate a pulse stretcher output signal, wherein if the edge detector output signal comprises a signal pulse, the pulse stretcher output signal comprises a stretched pulse having a duration that is longer than the signal pulse received from the edge detector; and   a filter operable to receive the pulse stretcher output signal and generate a filter output signal, wherein if the pulse stretcher output signal comprises at least a predetermined number of stretched pulses within a predetermined amount of time, the filter output signal indicates the presence of a leading-edge phase-cut dimmer.   
     
     
         2 . The circuit of  claim 1 , wherein the edge detector comprises:
 a comparator configured to receive the input signal at a first input of the comparator, the output of the comparator representing the edge detector output signal;   an RC circuit comprising a resistor having a first end coupled to the input signal and a second end coupled to a capacitor; and   an offset voltage source coupled to the junction of the resistor and the capacitor of the RC circuit, operable to add a voltage offset to the signal present at the junction of the resistor and the capacitor, and providing the resulting offset signal to the second input of the comparator.   
     
     
         3 . The circuit of  claim 1 , wherein the pulse stretcher comprises a capacitor whose value determines the pulse width of any stretched pulse generated by the pulse stretcher. 
     
     
         4 . The circuit of  claim 1 , wherein the pulse stretcher comprises:
 an R-S flip-flop coupled to receive the edge detector output signal at its S input;   a switch coupled to receive the output of the R-S flip-flop at a first terminal, wherein a voltage at said first terminal controls current flow from a second terminal of the switch to a third terminal of the switch;   a current source supplying a fixed current to the second terminal of the switch;   a first comparator coupled to receive a signal present at the third terminal of the switch, compare it to a first reference voltage level, and output a logical “1” if the signal present at the third terminal of the switch is greater than the reference voltage, the output of the comparator being coupled to the R input of the R-S flip-flop; and   a second comparator coupled to receive the signal present at the third terminal of the switch, compare it to a second reference voltage level lower than the first reference voltage level, and output a logical “1” if the signal present at the third terminal of the switch is greater than the reference voltage, the output of the second comparator representing the output of the pulse stretcher.   
     
     
         5 . The circuit of  claim 4 , wherein the pulse stretcher further comprises a capacitor coupled to the third terminal of the switch. 
     
     
         6 . The circuit of  claim 1 , wherein the filter comprises a capacitor whose value in part determines the number of stretched pulses that must be received from the pulse stretcher before the presence of a leading-edge phase-cut dimmer is declared. 
     
     
         7 . The circuit of  claim 1 , wherein the filter comprises:
 a switch coupled to receive the pulse stretcher output signal at a first terminal, wherein a voltage at said first terminal controls current flow from a second terminal of the switch to a third terminal of the switch;   a first current source supplying a fixed current to the first terminal of the switch;   a second current source connected to the third terminal of the switch and providing a fixed current flowing away from said third terminal, wherein a magnitude of the second current source is lower than a magnitude of the first current source; and   a comparator coupled to receive the signal present at the third terminal of the switch, compare it with a reference voltage level, and output a logical “1” if the signal present at the third terminal of the switch is greater than the reference voltage.   
     
     
         8 . The circuit of  claim 7 , wherein the filter further comprises a capacitor coupled to the third terminal of the switch. 
     
     
         9 . The circuit of  claim 1 , wherein the circuit further comprises a latch operable to receive the filter output signal and to latch a filter output signal that indicates the presence of a leading-edge phase-cut dimmer. 
     
     
         10 . A method for detecting a leading-edge phase-cut dimmer, comprising:
 receiving an input signal;   detecting whether the input signal has a rapidly rising edge;   if a rapidly rising edge is detected in the input signal, generating a signal pulse;   stretching the signal pulse to generate a stretched pulse having a duration that is longer than the duration of the signal pulse; and   if at least a predetermined number of stretched pulses are generated within a predetermined amount of time, generating a signal indicating the presence of a leading-edge phase-cut dimmer.   
     
     
         11 . The method of  claim 10 , further comprising:
 if a signal indicating the presence of a leading-edge phase-cut dimmer is generated, providing a constant current load to an LED lighting device; and   if a signal indicating the presence of a leading-edge phase-cut dimmer is not generated, providing a high-power-factor load to an LED lighting device.   
     
     
         12 . A circuit for detecting a leading-edge phase-cut dimmer, comprising:
 a light-emitting diode driven by an LED driver signal;   an edge detector operable to receive the LED driver signal, detect whether the LED driver signal has a rapidly rising edge, and provide an edge detector output signal, wherein the edge detector output signal comprises a signal pulse if a rapidly rising edge is detected in the LED driver signal;   a pulse stretcher operable to receive the edge detector output signal and provide a pulse stretcher output signal, wherein if the edge detector output signal comprises a signal pulse, the pulse stretcher output signal comprises a stretched pulse having a duration that is longer than the signal pulse received from the edge detector;   a filter operable to receive the pulse stretcher output signal and generate an indicator signal, wherein if the pulse stretcher output signal comprises at least a predetermined number of stretched pulses within a predetermined amount of time, the indicator signal indicates the presence of a leading-edge phase-cut dimmer in a circuit generating the LED driver signal; and   a loading circuit configured to receive the indicator signal and, if the indicator signal indicates the presence of a leading-edge phase-cut dimmer, provide a constant current load to the light-emitting diode, and, if the indicator signal indicates the absence of a leading-edge phase-cut dimmer, provide a high power factor load to the light-emitting diode.   
     
     
         13 . The circuit of  claim 12 , wherein the edge detector comprises:
 a comparator configured to receive the LED driver signal at a first input of the comparator, the output of the comparator representing the edge detector output signal;   an RC circuit comprising a resistor having a first end coupled to the LED driver signal and a second end coupled to a capacitor; and   an offset voltage source coupled to the junction of the resistor and the capacitor of the RC circuit, operable to add a voltage offset to the signal present at the junction of the resistor and the capacitor, and providing the resulting offset signal to the second input of the comparator.   
     
     
         14 . The circuit of  claim 12 , wherein the pulse stretcher comprises a capacitor whose value determines the pulse width of any stretched pulse generated by the pulse stretcher. 
     
     
         15 . The circuit of  claim 12 , wherein the pulse stretcher comprises:
 an R-S flip-flop coupled to receive the edge detector output signal at its S input;   a switch coupled to receive the output of the R-S flip-flop at a first terminal, wherein a voltage at said first terminal controls current flow from a second terminal of the switch to a third terminal of the switch;   a current source supplying a fixed current to the second terminal of the switch;   a first comparator coupled to receive a signal present at the third terminal of the switch, compare it to a first reference voltage level, and output a logical “1” if the signal present at the third terminal of the switch is greater than the reference voltage, the output of the comparator being coupled to the R input of the R-S flip-flop; and   a second comparator coupled to receive the signal present at the third terminal of the switch, compare it to a second reference voltage level lower than the first reference voltage level, and output a logical “1” if the signal present at the third terminal of the switch is greater than the reference voltage, the output of the second comparator representing the output of the pulse stretcher.   
     
     
         16 . The circuit of  claim 15 , wherein the pulse stretcher further comprises a capacitor coupled to the third terminal of the switch. 
     
     
         17 . The circuit of  claim 12 , wherein the filter comprises a capacitor whose value in part determines the number of stretched pulses that must be received from the pulse stretcher before the presence of a leading-edge phase-cut dimmer is declared. 
     
     
         18 . The circuit of  claim 12 , wherein the filter comprises:
 a switch coupled to receive the pulse stretcher output signal at a first terminal, wherein a voltage at said first terminal controls current flow from a second terminal of the switch to a third terminal of the switch;   a first current source supplying a fixed current to the first terminal of the switch;   a second current source connected to the third terminal of the switch and providing a fixed current flowing away from said third terminal, wherein a magnitude of the second current source is lower than a magnitude of the first current source; and   a comparator coupled to receive the signal present at the third terminal of the switch, compare it with a reference voltage level, and output a logical “1” if the signal present at the third terminal of the switch is greater than the reference voltage.   
     
     
         19 . The circuit of  claim 18 , wherein the filter further comprises a capacitor coupled to the third terminal of the switch. 
     
     
         20 . The circuit of  claim 12 , wherein the loading circuit comprises a multiplexer configured to receive the indicator signal at its select input, wherein if the indicator signal indicates the presence of a leading-edge phase-cut dimmer, the multiplexer outputs a first signal that causes a constant current load to be provided to the light-emitting diode, and, if the indicator signal indicates the absence of a leading-edge phase-cut dimmer, the multiplexer outputs a second signal that causes a high power factor load to be provided to the light-emitting diode.