Abstract:
An LED driver circuit and a method prevent LED turn-off flash when input power is lost to the driver circuit. The driver circuit includes a DC-DC converter that provides an LED drive voltage to an LED load. A voltage drop sensing circuit detects the loss of input power and discharges a filter capacitor that provides operating power to a controller in a DC-DC converter. The controller turns off to halt the operation of the DC-DC converter before the voltage provided to the LED load decreases to a turn-off threshold of the LED load. The DC-DC converter cannot recharge a load capacitor across the LED load. Thus, once the LEDs in the LED load turn off, the LEDs remain off until the input power is restored.

Description:
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of the following patent application which is hereby incorporated by reference: U.S. Provisional Patent App. No. 62/203,200 filed Aug. 10, 2015, entitled “Circuit and Method for Eliminating Power-Off Flash for LED Drivers.” 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to electronic driver circuits for power light-emitting diodes (LEDs). An LED operates in response to a direct current flowing through the device from the anode to the cathode. Above a current threshold, the LED will begin emitting light at an intensity determined by the magnitude of the current. In a typical lighting application, a plurality of LEDs are connected in series so that a common current flows through the LEDs to cause each of the LEDs to illuminate with substantially the same intensities to provide a uniform lighting effect. The current through the LEDs is provided by an electronic LED driver circuit that provides an output voltage sufficient to cause the current to flow through the series connected LEDs. The LED driver circuit controls the current to a magnitude selected to provide the desired illumination intensity. The magnitude may be controlled by a dimmer circuit to allow the magnitude to be changed to thereby control the illumination intensity produced by the LEDs. 
     In a typical fixed (e.g., non-portable) application, an electronic LED driver circuit receives AC power from a conventional AC supply (e.g., by hard wiring an LED lighting fixture to the electrical wiring of a building or by plugging an LED lighting fixture into a conventional outlet). The LED driver circuit converts the AC power to DC power, and the DC power is connected to the LEDs. 
     An exemplary LED driver circuit  100  is illustrated in  FIG. 1 . The driver circuit receives AC power from an AC source  110 . The AC power is coupled to the inputs  122 ,  124  of a full-wave bridge rectifier  120 . The bridge rectifier includes a first bridge diode  130 , a second bridge diode  132 , a third bridge diode  134  and a fourth bridge diode  136 . The four bridge diodes operate in a conventional manner to convert the AC input power to a DC output voltage. The DC output voltage is provided between a first (+) output  140  and a second (−) output  142  of the bridge rectifier. The voltage on the first (+) output of the bridge rectifier is identified as V BRIDGE , and is provided on a V BRIDGE  bus  144 . The second (−) output of the bridge rectifier is connected to a common DC ground reference of the circuit  146 . 
     The positive DC output voltage (V BRIDGE ) on the first (+) output  140  of the bridge rectifier  120  is connected to a first terminal of a bridge load resistor  150 . A second terminal of bridge load resistor  150  is connected to the DC ground reference  146 . The bridge load resistor  150  operates as a discharge resistor to discharge various capacitors in the circuit when AC power is no longer applied to the inputs  122 ,  124  of the full-wave bridge rectifier  120 . 
     The positive DC output voltage (V BRIDGE ) is also connected to an input  162  of a power factor correction (PFC) circuit  160 . The PFC circuit has an output terminal  164  that provides a DC voltage (V RAIL ) on a voltage bus  166 . A voltage rail (V RAIL ) filter capacitor  168  is connected between the voltage bus and the ground reference. The V RAIL  voltage bus is connected to the voltage input of a DC-DC converter stage  170 . The DC-DC converter stage  170  provides an output voltage (V LED ) to an LED load  172 . Although represented as a single load, the LED load may include a plurality of LEDs connected in series or connected in a series/parallel combination. The V LED  output voltage causes current to flow through the LEDs to illuminate the LEDs in a conventional manner. 
     The DC-DC converter stage causes harmonics on the V RAIL  voltage bus  166 . If the DC-DC converter were connected directly to the output of the full-wave bridge rectifier  120 , the harmonics would reduce the power factor of AC power coupled to the inputs  122 ,  124  of the full-wave bridge rectifier. The PFC circuit  160  isolates the V RAIL  voltage bus from the V BRIDGE  voltage bus. The PFC circuit operates in a conventional manner to cause the overall load between the first and second outputs of the bridge rectifier to have a greater effective power factor (e.g., a power factor closer to an ideal power factor of 1). The PFC circuit may comprise passive components or active devices. For example, in one embodiment, the PFC circuit may be a conventional power factor control circuit based on the STMicroelectronics L6562 Transition-Mode PFC Controller. 
     As shown in  FIG. 1 , the DC-DC converter stage  170  includes an integrated circuit controller (IC CTRL)  180 . In the illustrated embodiment, the controller may be an L6384 High Voltage Half-Bridge Driver, which is commercially available from STMicroelectronics. The controller drives a first semiconductor switching element (e.g., a MOSFET)  182  and a second switching element (e.g., a MOSFET  184 , which are connected in series between the V RAIL  voltage bus  166  and the circuit ground reference  146 . The first and second switching elements are connected at a common node  186 . The controller has an input (IN) that receives a periodic signal (f) from a signal source  188 , which may be a fixed frequency signal source or a variable frequency signal source. The controller drives the two switching elements in response to the periodic signal. When the first switching element is turned on, the common node is pulled up to the voltage V RAIL . When the second switching element is turned on, the common node is pulled down to ground. The two switching elements are operated in a conventional manner at a selected frequency and with selected duty cycles to produce a switched DC voltage at the common node that alternates between V RAIL  and ground. 
     The common node  186  between the two switching elements  182 ,  184  is connected to a first input  192  of a power tank circuit  190 . A second input  194  of the power tank is connected to the ground reference  146 . The power tank circuit has an input portion  196  and an output portion  198 . The combination of the power tank circuit, the controller  180  and the switching elements  184 ,  186  operate as a resonant DC-DC converter to convert the V RAIL  voltage on the bus  166  to the V LED  voltage applied to the LED load  172 . 
     The input portion  196  of the power tank circuit  190  includes a resonant inductor  200  and a resonant capacitor  202 , which are connected in series between the common node  186  and the ground reference  146 . The inductance of the resonant inductor and the capacitance of the resonant capacitor are selected to resonate at the switching frequency of the controller  180  such that the switched DC voltage on the common node  180  causes an AC voltage with a DC offset component to be produced across the resonant circuit capacitor. The switching frequency of the controller is variable to adjust the magnitude of the AC voltage across the resonant circuit capacitor. For example, when the switching frequency is reduced below the resonant frequency or increased above the resonant frequency, the voltage across the capacitor decreases. Accordingly, by adjusting the switching frequency, the voltage can be selectively reduced to reduce the current through the LED load  172  and thereby cause the light produced by the LED load to be dimmed. 
     A DC blocking capacitor  210  and the primary winding  214  of a transformer  212  are connected in series across the resonant circuit capacitor  202  to cause only the AC component of the voltage across the resonant circuit capacitor to be coupled to the primary winding. The DC blocking capacitor  210  prevents DC current from passing through the primary winding. 
     In the output portion  198  of the power tank circuit  190 , the transformer  212  has a center-tapped secondary winding  216 . The secondary winding has a first winding half  218  and a second winding half  220 . The first winding half is connected between a common node  222  and a first output terminal  224 . The second winding half is connected between the common node and a second output terminal  226 . The first output terminal is connected to the anode of a first rectifying diode  232  in a full-wave rectifier  230 . The second output terminal is connected to the anode of a second rectifying diode  234  in the full-wave rectifier. The cathodes of the two rectifying diodes are connected together at a rectifier output node  236 . The rectifier output node is coupled to a first (+) output  240  of the power tank circuit  190 . The common node of the secondary winding is connected to a second (−) output  242  of the power tank circuit. 
     The voltage produced between the first (+) output  240  and the second (−) output  242  of the power tank circuit  190  is applied across a load capacitor  250  and across the LED load  172 . The voltage is identified as V LED . The load capacitor filters out the high frequency ripple of the V LED  voltage. Although shown outside the power tank circuit, the load capacitor may also be considered to be part of the power tank circuit. 
     A charge pump circuit  260  is connected to the common node  186  between the two switching elements  182 ,  184 . The charge pump circuit includes a charge pump input capacitor  262  having a first terminal connected to the common node. The second terminal of the charge pump input capacitor is connected to the anode of a first charge pump diode  264  and to the cathode of a second charge pump diode  266 . The anode of the second charge pump diode is connected to the DC ground reference  146 . The cathode of the first charge pump diode is connected to a first terminal of a V CC  filter capacitor  270  and to the power input (V CC ) of the integrated circuit controller  180  at a node  272  identified as V CC . A second terminal of the V CC  filter capacitor is connected to the DC ground reference  146 . The second charge pump diode is a Zener diode having a voltage rating selected to clamp the voltage applied to the V CC  node via the first charge pump diode during the positive going (+dv/dt) half of each switching cycle at the common node. The second charge pump diode also provides a discharge path for the charge pump input capacitor during the negative going (−dv/dt) half of each switching cycle at the common node. 
     The V CC  node  272  is also connected to a first terminal of a power input resistor  280 . A second terminal of the power input resistor is connected to the V BRIDGE  bus  144 . As described below, the power input resistor  280  operates as a passive voltage supply circuit that receives power from the V BRIDGE  bus and that provides a first charging voltage via one or more passive components. 
     The power input resistor  280  and the charge pump circuit  260  both supply power to the V CC  node  272  and thus to the V CC  power input of the controller  180 . Upon initial startup, the V CC  filter capacitor  270  is charged from the V BRIDGE  bus  144  through the power input resistor. Thus, the power input resistor (the passive voltage source) operates as a first charging voltage source to the V CC  filter capacitor. The resistance of the power input resistor is selected to charge the capacitor at a relatively slow rate. For example, in one embodiment, the resistor has a resistance of approximately 150 ohms, and the V CC  filter capacitor has a capacitance of approximately 2.2 microfarads. 
     The V CC  filter capacitor  270  continues to charge through the power input resistor  280  until the voltage on the V CC  node  272  reaches a threshold voltage sufficient to initiate the operation of the controller  180 . When the threshold voltage is reached, the controller begins to operate in a conventional manner as described above to switch the two switching elements  182 ,  184  to generate the switched DC voltage on the common node  186 . The switched DC voltage is coupled through the charge pump capacitor  262  and the first charge pump diode  264  to provide a second charging voltage source to charge the V CC  filter capacitor. Together, the two switching elements and the charge pump  260  operate as an active voltage source to charge the VCC filter capacitor when the DC-DC converter  170  is operating. The power provided to the V CC  filter capacitor via the charge pump is not dissipated by a dropping resistor or other resistive element. Thus, during normal operation, the voltage on the V CC  node to maintain the charge on the V CC  filter capacitor is provided primarily by current provided by the charge pump. 
     The above-described LED drive circuit  100  provides current to the LED load  172  as long as the AC source  110  continues to provide input power. When the input power is lost (e.g., the AC source turns off or is disconnected), the voltage on the V RAIL  bus  166  is maintained by the V RAIL  capacitor  168  as the V RAIL  filter capacitor starts to discharge slowly. The V RAIL  filter capacitor discharges at a rate determined by the capacitance of the V RAIL  filter capacitor and the current provided to the LED load via the DC-DC converter  170 . A greater load (e.g., more current flowing through the LED load) causes the V RAIL  filter capacitor to discharge faster; and a lower load causes the V RAIL  filter capacitor discharged slower. Thus, when the LED load is operating in a dimmed condition with a lower current flowing through the LED load, the V RAIL  filter capacitor discharges slowly such that the controller  180  continues to operate for a substantial time (e.g., up to at least a few hundred milliseconds). 
     The foregoing effect is illustrated in  FIG. 2 . In the uppermost waveform of  FIG. 2 , the voltage V RAIL  is shown as being substantially constant from a time t 0  until a time t 1 . At the time t 1 , the AC input power is lost (e.g., by turning off a wall switch or the like). Although the AC source  110  is no longer providing a voltage to the input of the PFC  160  via the full-wave bridge rectifier  120 , the voltage V RAIL  does not decrease immediately. Rather, the voltage decreases slowly as the V RAIL  filter capacitor  168  discharges through the LED load  172 . Although the discharge is illustrated as a straight line, it should be understood that the discharge may be nonlinear (e.g., exponential). For simplification, nonlinear voltage and current values are represented herein as straight line segments. 
     During the initial discharge of the V RAIL  filter capacitor  168 , the controller  180  and the switching elements  182 ,  184  continue to operate to provide an AC voltage at the common node  186 . The charge pump  260  continues to operate to provide V CC  to the power input of the controller. The power tank  190  continues to provide the voltage V LED  on the rectifier output node  236  connected to the LED load  172 ; however, as shown in the middle waveform in  FIG. 2 , the voltage V LED  also decreases in response to the decreasing voltage V RAIL . The decreasing voltage V LED  causes the current flowing through the LED load to decrease as represented by a current I LED  shown in the lowermost waveform in  FIG. 2 . The decreasing current causes the light produced by the LED load to gradually dim. 
     As further shown in  FIG. 2 , the voltage V RAIL  continues to decrease as the V RAIL  filter capacitor  166  continues to discharge. The voltage V LED  also continues to decrease in response to the decreasing voltage V RAIL ; however, at a time t 2 , the voltage V LED  decreases below a threshold voltage V LEDTH  and is no longer sufficient to maintain current flow through the series-connected LEDs in the LED load  172 . Thus, the LEDs in the LED load turn off at the time t 2 , and current no longer flows through the LEDs as illustrated the current I LED  decreasing to zero at the time t 2 . 
     Although the foregoing operation would not be an issue if the LEDs in the LED load  172  remained off, the controller  180  continues to switch the switching elements  182 ,  184  and thus continues to maintain an AC voltage on the common node  186  at the input to the power tank circuit  190 . The power tank circuit continues to provide the DC voltage V LED  to the load capacitor  240  across the LED load. Because the LEDs in the LED load are no longer conducting and thus present no load to the power tank circuit, the voltage across the load capacitor starts to increase rapidly. This rapid increase is represented as a sharp voltage spike  290  between the time t 2  and a time t 3 . The voltage spike has a magnitude substantially greater than the threshold voltage V LEDTH  of the series-connected LEDs in the LED load. Thus, a large current—represented by a spike  292  in the current waveform LED in  FIG. 2 —flows through the LED load and causes a visible flash of light from the LEDs in the LED load. The sudden flash of light after the LEDs have apparently turned off is annoying to occupants of an area illuminated by the LEDs and may suggest to the occupants that the LED fixture has failed. 
     SUMMARY OF THE INVENTION 
     The invention disclosed herein provides a solution to the above-described problem of the post-turn-off flash of the LEDs. One aspect of the solution is a method that turns off the controller in the DC-DC converter to discontinue switching the switching elements shortly after the AC source is disconnected. Turning off the controller terminates the operation of the power tank circuit immediately to thereby terminate the generation of the voltage V LED  applied to the LED load. Accordingly, the power tank circuit no longer charges the load capacitor across the LED load after the LEDs in the LED load are turned off. This precludes the generation of the voltage spike and the resulting current spike that cause the LED flash. 
     Another aspect of the invention is an LED driver circuit and a method that prevent LED turn-off flash when input power is lost to the driver circuit. The driver circuit includes a DC-DC converter that provides an LED drive voltage to an LED load. A voltage drop sensing circuit detects the loss of input power and discharges a filter capacitor that provides operating power to a controller in a DC-DC converter. The controller turns off to halt the operation of the DC-DC converter before the voltage provided to the LED load decreases to a turn-off threshold of the LED load. The DC-DC converter cannot recharge a load capacitor across the LED load. Thus, once the LEDs in the LED load turn off, the LEDs remain off until the input power is restored. 
     In accordance with an aspect of the embodiment disclosed herein, a drive circuit provides a DC voltage to a plurality of light-emitting diodes (LEDs). The drive circuit includes a rectifier that converts an applied AC voltage to a rectified DC voltage. A passive voltage circuit receives the rectified DC voltage and produces a first charging voltage. A power factor correction circuit receives the rectified DC voltage and generates a rail DC voltage. A switching DC-DC converter receives the rail DC voltage and converts the rail DC voltage to an LED drive voltage and to a second charging voltage. The DC-DC converter includes a controller, at least first and second semiconductor switches, and a resonant tank circuit. The semiconductor switches are selectively switched by the controller to produce a switched DC voltage. The resonant tank circuit is responsive to the switched DC voltage to produce the LED drive voltage. The controller has a power input terminal. The controller is operable to switch the semiconductor switches only when a voltage on the power input terminal is at least as great as a controller threshold voltage. A filter capacitor is coupled to provide a controller supply voltage to the power input terminal of the controller. The filter capacitor receives the first charging voltage when the applied AC voltage is initially applied to the rectifier. The first charging voltage charges the capacitor to the controller threshold voltage. The capacitor receives the second charging voltage when the controller is operable after the capacitor charges to the controller threshold voltage. A voltage drop sensing circuit is coupled to receive the first charging voltage. The voltage drop sensing circuit senses when the first charging voltage decreases upon loss of the applied AC voltage. The voltage drop sensing circuit responsive to the decreasing first charging voltage to discharge the filter capacitor below the controller threshold voltage to halt the operation of the controller and thereby cease producing the LED drive voltage. 
     In certain embodiments, the passive voltage circuit may be a power input resistor. 
     In certain embodiments, the power input resistor in the passive voltage circuit includes a first terminal and a second terminal. The first terminal is connected to the rectifier. The second terminal is coupled to the filter capacitor and coupled to the voltage drop sensing circuit. 
     In certain embodiments, the power input resistor in the passive voltage circuit includes a first terminal and a second terminal. The first terminal is connected to the rectifier. The second terminal is connected to the voltage drop sensing circuit. The passive voltage circuit further includes a Zener diode and a forward-biased diode connected in series between the second terminal of the power input resistor and the filter capacitor. 
     In certain embodiments, the power input resistor in the passive voltage circuit includes a first terminal and a second terminal. The first terminal is connected to the rectifier. The second terminal further connected to the voltage drop sensing circuit. The passive voltage circuit further includes a forward-biased diode and resistor connected in series between the second terminal of the power input resistor and the filter capacitor. 
     In certain embodiments, the voltage drop sensing circuit comprises a discharge resistor and a discharge transistor. The discharge resistor and the discharge transistor are connected in series across the filter capacitor. The discharge transistor is responsive to the decreasing first charging voltage to turn on the discharge transistor and to discharge the filter capacitor via the discharge resistor. 
     In certain embodiments, the voltage drop sensing circuit further includes a voltage sensing capacitor connected to the control terminal of the discharge transistor. The voltage sensing capacitor has a capacitance less than the capacitance of the filter capacitor. The voltage sensing capacitor discharges faster than the filter capacitor upon loss of the applied AC voltage to turn on the discharge transistor and increase the discharge rate of the filter capacitor. 
     In certain embodiments, a capacitor is coupled to the output of the power factor correction circuit. The capacitor maintains the DC rail voltage on the output of the power factor correction circuit at a slowly decreasing level for a selected time after the loss of the applied AC voltage to enable the DC-DC converter to continue generating the LED drive voltage. The LED drive voltage decreases in response to the decreasing level of the DC rail voltage. The voltage drop sensing circuit is operable to halt the operation of the controller before the LED drive voltage decreases to a threshold voltage for operating the plurality of LEDs. 
     In accordance with another aspect of the embodiment disclosed herein, a drive circuit provides a DC voltage to a plurality of light-emitting diodes (LEDs) in response to an applied input voltage. The drive circuit includes a first charging voltage circuit responsive to the applied input voltage to generate a first charging voltage. A rail voltage circuit is responsive to the applied input voltage to generate a rail voltage. A switching DC-DC converter is responsive to the rail DC voltage to generate an LED drive voltage and a second charging voltage. The DC-DC converter includes a controller having a power input terminal. The DC-DC converter is operable only when a voltage on the power input terminal of the controller is at least as great as a controller threshold voltage. A filter capacitor is coupled to provide a controller supply voltage to the power input terminal of the controller. The filter capacitor receives the first charging voltage when the applied input voltage is active. The first charging voltage charges the filter capacitor to the controller threshold voltage. The filter capacitor receives the second charging voltage when the controller is operable after the filter capacitor charges to the controller threshold voltage. A voltage drop sensing circuit is coupled to receive the first charging voltage. The voltage drop sensing circuit senses when the first charging voltage decreases upon loss of the applied input voltage. The voltage drop sensing circuit is responsive to the decreasing first charging voltage to discharge the filter capacitor below the controller threshold voltage to halt the operation of the controller and thereby cease producing the LED drive voltage. 
     In certain embodiments, a capacitor is connected to the rail voltage circuit. The capacitor maintains the DC rail voltage at a slowly decreasing level for a selected time after the loss of the applied input voltage to enable the DC-DC converter to continue generating the LED drive voltage. The LED drive voltage decreases in response to the decreasing level of the DC rail voltage. The voltage drop sensing circuit is operable to halt the operation of the DC-DC converter before the LED drive voltage decreases to a threshold voltage for operating the plurality of LEDs. 
     In accordance with another aspect of the embodiment disclosed herein, a method prevents power-off flash in a light-emitting diode (LED) drive circuit. The method includes generating a switched DC voltage from an applied input voltage with a switching DC-DC converter. The switching DC-DC converter is controlled by a switching controller having a power input terminal. The method further includes generating an LED drive voltage from the switched DC voltage; and generating a first capacitor charging voltage responsive to the applied input voltage. The method further includes generating a second capacitor charging voltage responsive to the switched DC voltage. The method further includes applying the first capacitor charging voltage and the second capacitor charging voltage to a controller power input capacitor to charge the controller power input capacitor and provide a DC supply voltage to the switching controller. The method further includes sensing a loss of the applied input voltage and discharging the controller input capacitor to disable the switching controller before the LED drive voltage decreases to a voltage level below an operational threshold voltage of the plurality of LEDs. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing an electronic LED drive circuit as conventionally known in the art. 
         FIG. 2  illustrates waveforms of a rail voltage V RAIL  of  FIG. 1 , the voltage V LED  across the LED load connected to the LED drive circuit of  FIG. 1 , and the current I LED  through the LED load connected to the LED drive circuit of  FIG. 1 . 
         FIG. 3  illustrates a circuit diagram showing an LED drive circuit with a voltage drop sensing circuit that prevents power-off flash of the LEDs driven by the drive circuit. 
         FIG. 4  illustrates waveforms of the rail voltage V RAIL , the LED load voltage V LED , the LED load current I LED , the sensed voltage V SENSE , and the V CC  voltage in the LED drive circuit of  FIG. 3 . 
         FIG. 5  illustrates a circuit diagram showing an LED drive circuit similar to the LED drive circuit of  FIG. 3  but with a modified voltage drop sensing circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary solution to the problem disclosed in  FIGS. 1 and 2  is illustrated by the improved LED driver circuit  300  in  FIG. 3 . In  FIG. 3 , elements corresponding to elements in  FIG. 1  are identified with corresponding element numbers and are not described in detail below. The power tank circuit  190  and the charge pump circuit  260  are represented as blocks. The components within the two circuits are illustrated in  FIG. 1  and are described above. 
     Unlike the driver circuit  100  in  FIG. 1 , the driver circuit  300  in  FIG. 3  does not connect the power input resistor  280  directly to the V CC  node  272  and the power input terminal of the controller  180 . Instead, the power input resistor is connected to a voltage sensing node  310 , which has a sense voltage (V SENSE ) thereon. 
     The voltage sensing node  310  is also connected to the cathode of a Zener diode  312 . In the illustrated embodiment, the Zener diode has a Zener voltage of approximately 10 volts. The anode of the Zener diode is connected to the anode of an isolation diode  314 . The cathode of the isolation diode is connected to the V CC  node  272 . Thus, the passive voltage source in  FIG. 3  includes three passive components: the power input resistor; the Zener diode; and the isolation diode. The V CC  node is connected to the power input terminal (V CC ) of the controller. The V CC  node is also connected to the first terminal of the V CC  filter capacitor  270  and to the output of the charge pump  260  as described above in connection with  FIG. 1 . 
     Unlike the previously described driver circuit  100 , the LED driver circuit  300  in  FIG. 3  further includes a voltage drop sensing circuit  320 . The voltage drop sensing circuit has an input terminal  322  connected to the voltage sensing (V SENSE ) node  310  and has an output terminal connected to the V CC  node  272 . The structure and the operation of the voltage drop sensing circuit are described in more detail below. 
     During power up and during normal operation, the LED driver circuit  300  in  FIG. 3  operates in a similar manner to the LED driver circuit  100  in  FIG. 1 . When power from the AC source  110  is initially applied to the driver circuit in  FIG. 3 , the voltage V BRIDGE  on the V BRIDGE  bus  144  is applied to the V CC  node  272  via the power input resistor  280 , the Zener diode  312  and the isolation diode  314 . The Zener diode and the isolation diode cause the voltage at the V CC  node provided from the V SENSE  node  310  to be approximately 10.7 volts below the voltage V BRIDGE . The initial voltage on the V CC  node is sufficient to charge the V CC  filter capacitor  270  to a sufficient voltage level to cause the controller  180  to begin operating. Thus, the two switching elements  182 ,  184  begin switching to supply the AC voltage to the power tank circuit  190  and to the charge pump circuit  260  as described above. The charge pump provides current to further charge the V CC  filter capacitor. When the V CC  filter capacitor is fully charged, the voltage provided by the charge pump circuit is greater than the voltage provided by the power input resistor via the Zener diode and the isolation diode. Thus, the isolation diode is reverse-biased when the controller is operating. While the AC source is connected, the LED driver circuit of  FIG. 3  operates to provide power to the LED load via the power tank circuit  190  as described above. 
     The voltage drop sensing circuit  320  operates to prevent the LED flash problem described above. The voltage drop sensing circuit includes a discharge resistor  340 , which has a first terminal connected to the V CC  node  272  via the output terminal  324 . The discharge resistor has second terminal connected to the emitter terminal of a discharge transistor  342 , which is a PNP bipolar transistor in the illustrated embodiment. The collector of the discharge transistor is connected to the ground reference  146 . 
     The anode of a base clamping diode  344  is connected to the base of the discharge transistor  342 . The cathode of the base clamping diode is connected to the emitter of the discharge transistor. The base clamping diode prevents the voltage on the base of the discharge transistor from exceeding the voltage on the emitter of the discharge transistor by more than one forward diode drop. 
     The anode of a base diode  346  is also connected to the base of the discharge transistor  342 . The cathode of the base diode is connected to a first terminal of a voltage sensing capacitor  350  and to the first terminal of a bleeder resistor  352 . The commonly connected cathode and first terminals are connected to the input terminal  322  of the voltage drop sensing circuit  320  are thus connected to the V SENSE  node  310 . The respective second terminals of the voltage sensing capacitor and the bleeder resistor are connected to the ground reference. 
     The voltage drop sensing circuit  320  does not affect the operation of the LED driver circuit  300  when the AC power is initially applied and while the LED driver circuit continues to operate with the AC power connected. When AC power is initially applied to the LED driver circuit, the voltage on the V BRIDGE  bus  144  is applied to the voltage sensing node  310  via the power input resistor  280 . Accordingly, the voltage is applied to the respective first terminals of the bleeder resistor  352  and the voltage sensing capacitor  350  via the input  332  of the voltage drop sensing circuit. The resistance of the bleeder resistor is substantially greater than the resistance of the power input resistor. Thus, substantially all of the V BRIDGE  voltage is applied across the voltage sensing capacitor as the V SENSE  voltage. The capacitance of the voltage sensing capacitor is relatively small compared to the capacitance of the V CC  filter capacitor  270 . Thus, the voltage sensing capacitor charges very quickly while the V CC  filter capacitor charges slowly on initial power up such that the voltage on the voltage sensing capacitor is initially greater than the voltage on the V CC  filter capacitor. The higher voltage on the voltage sensing capacitor prevents the emitter-base junction of the discharge transistor  342  from being forward biased. Thus, the discharge transistor remains off during initial power on of the LED driver circuit. 
     After the LED driver circuit  300  is powered up, the voltage sensing capacitor  350  remains charged to the V SENSE  voltage determined by the voltage divider formed by the power input resistor  280  and the bleeder resistor  352 . The voltage is slightly less than the V BRIDGE  voltage. The V CC  filter capacitor  270  is charged to a voltage less than the V RAIL  voltage, which is less than the V BRIDGE  voltage. Accordingly, the emitter-base junction of the discharge transistor  342  remains reverse biased during normal operation. 
     When the AC source  110  is disabled or is no longer connected to the inputs  122 ,  124  of the LED driver circuit  300 , the voltage drop sensing circuit  320  operates to prevent the LED driver circuit from causing the LED flash described above. The operation of the voltage drop sensing circuit is illustrated by waveforms in  FIG. 4 . An upper waveform in  FIG. 4  represents a timing diagram illustrating the reduction in a rail voltage V RAIL  after the AC source to the LED drive circuit of  FIG. 3  is disconnected. A second waveform in  FIG. 4  represents a voltage V LED  across the LED load connected to the LED drive circuit of  FIG. 3 . A third waveform in  FIG. 4  represents a current I LED  through the LED load connected to the LED drive circuit of  FIG. 3 . A fourth waveform in  FIG. 4  represents the V SENSE  voltage across the voltage sensing capacitor  350  and thus represents the voltage on the voltage sensing node  310 . A fifth waveform in  FIG. 4  represents the voltage on the V CC  node  272  corresponding to the voltage across the V CC  filter capacitor  270 . 
     The five waveforms in  FIG. 4  represent the normal operation of the LED driver circuit  300  from a time t 0  to a time t 1  when the AC power from the AC source  110  continues to be applied to the inputs  122 ,  124  of the full-wave bridge rectifier  120 . At the time t 1 , the AC power is disconnected or otherwise disabled such that the V RAIL  voltage begins to decrease as the V RAIL  filter capacitor  168  discharges. The decreasing V RAIL  voltage causes corresponding decreases in the V LED  voltage and the I LED  current. As further shown in  FIG. 4 , the V CC  voltage on the V CC  node  272  initially remains substantially constant because the charge pump  260  continues to provide charging current to the V CC  filter capacitor  270  as the controller  180  continues to operate despite the decreasing V RAIL  voltage. 
     If the V CC  voltage across the V CC  filter capacitor  270  were allowed to remain at the initial level as the V RAIL  voltage decreases, the controller  180  would continue to switch the two switching elements  182 ,  184 , and the LED flash would occur as before; however, in the embodiment of  FIG. 3 , the voltage drop sensing circuit  320  prevents the LED flash. When the AC source  110  is disconnected or otherwise disabled, the V BRIDGE  voltage on the V BRIDGE  voltage bus  144  decreases rapidly and no longer provides current through the power input resistor  280  to maintain the charge across the voltage sensing capacitor  350 . The voltage sensing capacitor begins to discharge through the bleeder resistor  352 , the power input resistor  280  and the bridge load resistor  150 . The capacitance of the voltage sensing capacitor is much less than the capacitance of the V CC  filter capacitor. Thus, the discharge rate of the voltage sensing capacitor is much greater than the discharge rate of the V CC  filter capacitor as illustrated by the steep decrease in the V SENSE  voltage in  FIG. 4  between the time t 1  and a time t a . As discussed above, the decreasing straight line actually represents a first portion of an exponential discharge. 
     At the time t a , the V SENSE  voltage on the voltage sensing capacitor  350  drops below the V CC  voltage (e.g., by the total of a forward emitter-base drop and a forward diode drop) such that the discharge transistor  342  starts conducting and the emitter of the discharge transistor is pulled down to a voltage near the zero volts on the ground reference  146 . The V SENSE  voltage on the voltage sensing capacitor continues to exponentially discharge through the bleeder resistor  352 , the power input resistor  280  and the bridge load resistor  150  as represented by a second straight line segment. 
     When the discharge transistor conducts, the V CC  filter capacitor  270  is discharged rapidly through the discharge resistor  340  as illustrated by a steep discharge portion of the V CC  waveform in  FIG. 4  between the time t a  and a time t b . When the V CC  filter capacitor discharges below the operating voltage threshold of the controller  180 , the controller will no longer switch the two switching elements  182 ,  184  to produce an AC voltage on the common node  186 . Thus, the power tank circuit  190  no longer provides a DC voltage to maintain the charge on the load capacitor  240 . The V LED  voltage on the load capacitor will continue to decrease as the voltage is discharged through the LED load  172 . 
     When the V LED  voltage on the load capacitor  240  reaches the threshold voltage for the series-connected LEDs in the LED load  172  at a time t 2 , the LEDs will discontinue conducting, which causes the I LED  current to quickly drop to zero. Although the load on the output of the power tank circuit  190  is reduced, the reduction in the load does not cause the voltage across the load capacitor to temporarily increase because the controller and the two switching elements are no longer operating drive to produce an AC voltage at the input to the power tank circuit. Accordingly, the charge pump circuit  260  is not able to replenish the charge on the load capacitor. As a result the V LED  voltage continues to slowly discharge without producing a voltage spike to cause the LED flash described above. 
     As described above, the LED drive voltage (V LED ) drifts downward as the V RAIL  filter capacitor  168  and the load capacitor  250  discharge in the embodiment of  FIG. 3 . The capacitance of the voltage sensing capacitor  350  and the resistance of the bleeder resistor  352  are selected such that the discharge transistor  342  is turned on well before the V LED  voltage decreases to the threshold voltage of the LED load  172 . The V CC  filter capacitor  270  is discharged rapidly via the discharge resistor  340  such that the controller  180  is disabled while the V LED  voltage is still above the threshold voltage of the LED load. Thus, when the V LED  voltage reaches the threshold voltage and the LEDs in the LED load turn off, the disabled controller cannot cause switching of the two switching elements  182 ,  184  to increase the V LED  voltage regardless of the voltage remaining on the V RAIL  filter capacitor. 
       FIG. 5  illustrates a second embodiment of an LED drive circuit  500 , which is similar to the LED drive circuit  300  of  FIG. 3 . The LED drive circuit of  FIG. 5  includes a modified voltage drop sensing circuit  510  having fewer components than the voltage drop sensing circuit  320  of  FIG. 3 . The LED drive circuit of  FIG. 5  also has fewer components providing power to the controller  180 . Other than as described below, the elements in  FIG. 5  correspond to the elements in  FIG. 3  and are numbered accordingly. 
     The voltage drop sensing circuit  510  includes an input terminal  512  and an output terminal  514 . The output terminal is connected to the V CC  node  272  and thus is connected to the first terminal of the V CC  filter capacitor  270  as previously described. The input terminal of the voltage drop sensing circuit of  FIG. 5  is connected directly to the second terminal of the power input resistor  280  via the V SENSE  node  310  as in  FIG. 3 . 
     The LED drive circuit  500  does not include the Zener diode  312  and the isolation diode  314  shown in  FIG. 3  to provide power to the V CC  node  272  from the power input resistor  280 . Rather, power from the power input resistor is provided to the V CC  node via the base clamping diode  344  and the discharge resistor  340  in the voltage drop sensing circuit. Thus, the passive voltage source in  FIG. 5  comprises the power input resistor, the base clamping diode and the discharge resistor. 
     The discharge resistor  340  in the voltage drop sensing circuit  510  of  FIG. 5  has the first terminal connected to the output terminal  514  and has the second terminal connected to the emitter of the discharge transistor  342  as previously described. The collector of the discharge transistor is connected to the ground reference  146 . Unlike the discharge transistor in the previously described embodiment, the base of the discharge transistor in  FIG. 5  is connected directly to the input terminal  512  and to the first terminal of the voltage sensing capacitor  350 . The second terminal of the voltage sensing capacitor is connected to the ground reference. The bleeder resistor  352  ( FIG. 3 ) is not included in the embodiment of  FIG. 5  to reduce parts count and to reduce power dissipation. 
     In the voltage drop sensing circuit  510  of  FIG. 5 , the base clamping diode  344  has the anode connected to the base of the discharge transistor  342  and has the cathode connected to the emitter of the discharge transistor as in the embodiment of  FIG. 3 . The anode of the base clamping diode in  FIG. 5  is connected directly to the input terminal  512  of the voltage drop sensing circuit. 
     In the embodiment of  FIG. 5 , the base clamping diode  344  is in the supply path to the V CC  filter capacitor  270  when the LED drive circuit  500  is initially powered on. Current flows from the V BRIDGE  bus  144  through the power input resistor  280  to the V SENSE  node  310 . Current is conducted from the V SENSE  node through the base clamping diode and through the discharge resistor  340  to the V CC  node  272  to charge the V CC  filter capacitor  270 . The V CC  filter capacitor is charged via the charging path until the voltage on the V CC  node reaches the threshold voltage for operation of the controller  180 . After the controller starts to operate to switch the two switching elements  182 ,  184 , power is provided to the V CC  node via the charge pump circuit  260  to maintain the charge on the V CC  filter capacitor as described above. 
     The voltage drop sensing circuit  510  of  FIG. 5  operates in a similar manner to the previously described voltage drop sensing circuit  310  of  FIG. 3 . The voltage sensing capacitor  350  remains charged while the V BRIDGE  voltage on the V BRIDGE  bus  144  is maintained at a high voltage level by the rectified output of the full-wave bridge rectifier  120 . When the AC source  110  is disconnected or otherwise disabled, the V BRIDGE  voltage drops rapidly as described above. The V SENSE  voltage on the V SENSE  node  310  is initially maintained by the voltage sensing capacitor  350 ; however, the voltage sensing capacitor begins to discharge via the power input resistor  280  and the bridge load resistor  150 . The base clamping diode  344  is reverse-biased, which precludes the V CC  node  272  from providing current to maintain the charge on the voltage sensing capacitor. Accordingly, the voltage on the base of the discharge transistor  244  drops to cause the emitter-base junction of the discharge transistor to become forward biased. The discharge transistor conducts to start discharging the V CC  filter capacitor  270  via the discharge resistor  340 . Thus, the voltage on the V CC  node drops rapidly while the voltage on the V RAIL  bus  166  remains at a relatively higher voltage and decreases at a slower rate. 
     When the voltage on the V CC  node  272  drops below the operational threshold voltage of the controller  180 , the controller ceases operation and no longer switches the two switching elements  182 ,  184  to produce the AC voltage on the common node  186 . The power tank circuit  190  ceases operation, and the V LED  voltage on the rectifier output node  236  continues to drop as the load capacitor  240  discharges through the LED load  172 . Since the controller remains off, the V LED  voltage does not spike when the LEDs within the LED load no longer conduct. 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of the present invention of a new and useful “Circuit and Method for Eliminating Power-Off Flash for LED Drivers,” it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.