Patent Publication Number: US-11026310-B2

Title: LED driver circuit and LED driving method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 15/626,307 filed Jun. 19, 2017 titled “LED DRIVER CIRCUIT AND LED DRIVING METHOD” (now U.S. Pat. No. 10,362,643). U.S. application Ser. No. 15/626,307 claimed the benefit of U.S. Prov. App. No. 62/366,688 filed Jul. 26, 2016, and also claimed the benefit of U.S. Prov. App. No. 62/359,324 filed Jul. 7, 2016. All the noted applications are incorporated by reference herein as if reproduced in in full below. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to electrical circuits, and more particularly but not exclusively to electrical circuits for light emitting diodes. 
     BACKGROUND 
     A light emitting diode (LED) string may be configured with a plurality of serially-connected LED elements, and may be directly connected to and driven by an input alternating current (AC) source (“AC input”), such as from a wall AC outlet. In that configuration, which is known as Direct AC Drive topology, the AC input may be passed through a dimmer, then rectified by a rectifier circuit, and then supplied as an AC line to the LED elements. The waveform of the AC input that has passed through the dimmer and the rectifier circuit is referred to herein as the “input voltage.” The number of LED elements to be conducted, i.e., turned on, in the LED string can be controlled according to the input voltage. 
     The AC input provides input current that flows through the dimmer and the rectifier circuit. The input current should not be lower than a holding current in order to keep the dimmer turned on. When the input current decreases below the holding current, the dimmer turns off, thereby causing the AC input to be separated from the AC line. Accordingly, when the dimmer is turned off, the input voltage fluctuates, resulting in flicker and difficulty of predicting the input voltage. 
     When the input current is lower than a certain level, a bleeding operation is initiated in order to regulate the input current. During the bleeding operation, a bleeding circuit causes current to flow from the AC line through a metal oxide semiconductor field effect transistor (MOSFET) of the bleeding circuit. The bleeding operation increases power consumption and generates heat that causes the temperature of the MOSFET to rise. 
     SUMMARY 
     In one embodiment, a light emitting diode (LED) driver circuit includes an LED string and a conducting state detection circuit. The conducting state detection circuit detects a conducting state of the LED string, and generates a discharge control signal upon sensing that the LED string is in a non-conducting state. A current source generates a discharge current according to the discharge control signal when the LED string is in the non-conducting state. The LED driver circuit may include a passive bleeder to provide current compensation by internal regulator operation. The LED driver circuit may include an LED spike current suppression circuit to suppress LED spike current that can occur when the input voltage increases above a threshold. The LED driver circuit may include a bias supply circuit that has an input capacitor to provide a bias voltage. 
     These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of an LED driver circuit in accordance with an embodiment of the present invention. 
         FIG. 2  shows waveforms of signals of the LED driver circuit of  FIG. 1  in accordance with an embodiment of the present invention. 
         FIG. 3  shows further details of the LED conducting state detection circuit and the current source of  FIG. 1 , in accordance with an embodiment of the present invention. 
         FIG. 4  shows waveforms of signals of the LED driver circuit of  FIG. 3  in accordance with an embodiment of the present invention. 
         FIG. 5  shows a schematic diagram of an LED driver circuit in accordance with another embodiment of the present invention. 
         FIG. 6  shows further details of an LED current controller in accordance with an embodiment of the present invention. 
         FIG. 7  shows waveforms of signals of the LED driver circuit of  FIG. 6  in accordance with an embodiment of the present invention. 
         FIG. 8  shows a schematic diagram of an LED driver circuit in accordance with another embodiment of the present invention. 
         FIG. 9  shows a schematic diagram of a regulator of an LED current controller in accordance with an embodiment of the present invention. 
         FIG. 10  shows a schematic diagram of an input threshold voltage generator in accordance with an embodiment of the present invention. 
         FIG. 11  shows waveforms of signals of an LED driver circuit with the regulator of  FIG. 9 , in accordance with an embodiment of the present invention. 
         FIG. 12  shows a zoom-in view of waveforms of signals in the example of  FIG. 11  in accordance with an embodiment of the present invention. 
         FIG. 13  shows a schematic diagram of an LED driver circuit in accordance with another embodiment of the present invention. 
         FIG. 14  shows a schematic diagram of a bias supply circuit in accordance with an embodiment of the present invention. 
         FIG. 15  shows waveforms of signals of the bias supply circuit of  FIG. 14  in accordance with an embodiment of the present invention. 
     
    
    
     The use of the same reference label in different drawings indicates the same or like components. 
     DETAILED DESCRIPTION 
     In the present disclosure, numerous specific details are provided, such as examples of circuits, components, and methods, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. 
       FIG. 1  shows a schematic diagram of an LED driver circuit  1  in accordance with an embodiment of the present invention. In the example of  FIG. 1 , the LED driver circuit  1  includes a rectifier circuit  3 , an LED string  2 , an LED current controller  4 , a current source  5 , a conducting state detection circuit  6 , a dimmer  7 , and a capacitor C 1 . 
     The capacitor C 1  is connected to an AC line to which the input voltage Vin is supplied via the rectifier circuit  3 . The capacitor C 1  filters the ripples of the input voltage Vin. 
     The dimmer  7  is connected between the AC input ACin and the rectifier circuit  3 . The dimmer  7  may be a phase-cut dimmer that passes only the AC input ACin belonging to a certain range of phases, which is referred to herein as “phase angle”. The AC input ACin passed through the dimmer  7  is rectified by the rectifier circuit  3  to generate the input voltage Vin. The waveform of the input voltage Vin may have only a portion corresponding to the phase angle. 
     The LED string  2  includes a plurality of LED elements LED 1 -LEDn that are connected in series with one another. The input voltage Vin is provided to the LED string  2 . While  FIG. 1  illustrates a plurality of LED elements, the LED string  2  may be configured with only one LED element. 
     The controller  4  controls current flowing through a plurality of channels CH_ 1 -CH_n. Each of the plurality of channels CH_ 1 -CH_n is positioned between a corresponding LED element of the plurality of LED elements LED 1 -LEDn and a corresponding regulator of a plurality of regulators  40 _ 1 - 40 _ n . In one embodiment, each of the plurality of regulators  40 _ 1 - 40 _ n  comprises a linear regulator. 
     In the example of  FIG. 1 , one sense resistor RCS is connected to the controller  4 , and current flowing through each of the plurality of channels CH_ 1 -CH_n is controlled according to one sense voltage VCS that is developed on the sense resistor. Current may flow through one of the plurality of channels CH_ 1 -CH_n. Accordingly, the current ILED flowing to the LED string  2  may flow through any one of the plurality of channels CH_ 1 -CH_n to the sense resistor RCS to develop the sense voltage VCS. 
     The controller  4  includes the plurality of regulators  40 _ 1 - 40 _ n  connected to the plurality of channels CH_ 1 -CH_n, respectively. One of the plurality of regulators  40 _ 1 - 40 _ n  may be enabled according to the input voltage Vin and control current flowing through a corresponding channel. In one embodiment, each of the plurality of regulators  40 _ 1 - 40 _ n  may control different amounts of currents. 
     Each of the plurality of regulators  40 _ 1 - 40 _ n  is connected to a corresponding channel of the plurality of channels CH_ 1 -CH_n, receives a corresponding control voltage of the plurality of control voltages VC_ 1 -VC_n, and controls the sense voltage VCS with the control voltage to thereby control the current of the corresponding channel. Each of the plurality of control voltages VC_ 1 -VC_n is the voltage that determines the current flowing through a corresponding channel and may be set according to the corresponding channel. In one embodiment, the control voltages VC_ 1 -VC_n have different values with VC_n being the highest and VC_ 1  being the lowest. For example, each control voltage VC may be a fixed voltage with a relationship VC_ 1 &lt;VC_ 2 &lt; . . . &lt;VC_n. That is, the control voltage VC_ 1  corresponding to the regulator  40 _ 1  is lower than the control voltage VC_ 2  corresponding to the regulator  40 _ 2 , the control voltage VC_ 2  corresponding to the regulator  40 _ 2  is lower than the control voltage VC_ 3  corresponding to the regulator  40 _ 3 , etc. 
     For convenience of explanation, it is assumed hereinbelow that when the control voltage level corresponding to the LED element LED 1  is 1, the control voltage level that corresponds to the LED element LEDn is n. 
     In the example of  FIG. 1 , the number of LED elements that will conduct is determined according to the input voltage Vin and, among the channels connected to the conducting LED elements, a regulator of a channel in the highest position is enabled. The current of the corresponding channel is controlled by the enabled regulator based on the corresponding control voltage. Referring to the LED string illustrated in  FIG. 1 , the channel is in the higher position as the position of the LED element is closer to the right-hand side. 
     An example operation of the controller  4  when the input voltage Vin increases is now explained. As the input voltage Vin increases, the first LED element LED 1  conducts so that current flows through the channel CH_ 1 . The regulator  40 _ 1  controls the current ILED so that the level of the sense voltage VCS follows the level “1”. 
     According to the increasing input voltage Vin, the second LED element LED 2  conducts so that the current ILED flows through the channel CH_ 2 . The regulator  40 _ 2  controls the current ILED so that the level of the sense voltage VCS follows the level “2”. At this time, the level of the sense voltage VCS is higher than “1”, and the regulator  40 _ 1  is disabled. 
     When the input voltage Vin increases to a certain level, the (n)th LED element LEDn conducts so that the current ILED flows through the channel CH_n. The regulator  40 _ n  controls the current ILED so that the level of the sense voltage VCS follows the level “n”. At this time, the level of the sense voltage VCS is higher than “n−1”, and the regulator  40 _ n −1 is disabled. 
     Next, an example operation of the controller  4  as the input voltage Vin decreases is explained. According to the decreasing input voltage Vin, the (n)th LED element LEDn is turned off so that current does not flow through the channel CH_n and the regulator  40 _ n  is disabled. Accordingly, the regulator  40 _ n −1 is enabled and the current ILED flows through the channel CH_n−1. 
     According to the decreasing input voltage Vin, the LED elements are turned off in the order of the (n−1)th LED element, the (n−2)th LED element, ( . . . ), the second LED element LED 2 , and the first LED element LED 1  and, accordingly, the regulators are enabled in the order of the regulator  40 _ n −2, the regulator  40 _ n −3, ( . . . ), the regulator  40 _ 2 , and the regulator  40 _ 1 . The current ILED flows through the corresponding channels in the order of the channel CH_n−2, the channel CH_n−3, ( . . . ), the channel CH_ 2 , and the channel CH_ 1 . 
     In the example of  FIG. 1 , the plurality of regulators  40 _ 1 - 40 _ n  of the controller  4  share the sense resistor RCS, and the plurality of control voltages VC_ 1 -VC_n are set differently such that the regulators to be enabled among the plurality of regulators  40 _ 1 - 40 _ n  are changed in accordance with a change in the input voltage Vin, and the target value of the corresponding channel current is also changed. 
     The operation of the controller  4  has been described above by way of example; the present disclosure is not limited thereto. 
     The input voltage Vin decreases when the dimmer  7  turns off according to a decrease in the input current Iin. Due to the decrease in the input voltage Vin, the plurality of LED elements LED 1 -LEDn transition to a non-conducting state. Hereinafter, the “LED non-conducting state” refers to a state in which all of the plurality of LED elements LED 1 -LEDn are not conducting so that the current ILED does not flow to the LED string  2 . In the LED non-conducting state, current does not flow through any of the plurality of channels CH_ 1 -CH_n. 
     In response to sensing the LED non-conducting state, the LED driver circuit  1  controls the input voltage Vin by discharging the capacitor C 1 . The LED driver circuit  1  may sense the LED non-conducting state from the sense voltage VCS, from a voltage at an end of one of the plurality of LED elements LED 1 -LEDn, or from an output of a regulator amplifier of the controller  4 . 
     The LED conducting state detection circuit  6  receives the sense voltage VCS or a voltage at an end of one of the plurality of LED elements LED 1 -LEDn, and when the sense voltage VCS or the voltage at an end of one of the plurality of LED elements LED 1 -LEDn decreases below a discharge threshold voltage, controls the discharge current IS flowing to the current source  5 . The LED conducting state detection circuit  6  may also control the discharge current IS flowing to the current source  5  when the LED non-conducting state is detected from an output of a regulator amplifier of the controller  4 . 
     Because the current ILED does not flow in the LED non-conducting state, the sense voltage VCS is not generated and may be lower than the discharge threshold voltage. Also, the input voltage Vin may be decreased in the LED non-conducting state such that the voltage at an end of one of the plurality of LED elements LED 1 -LEDn may be lower than the discharge threshold voltage. 
     Upon sensing the LED non-conducting state, the LED conducting state detection circuit  6  may generate a discharge control signal DCS to control the current flowing through the current source  5 . 
     The current source  5  generates the discharge current IS according to the discharge control signal DCS, and the capacitor C 1  is discharged in accordance with the discharge current IS. It is thus possible to control the input voltage Vin in the LED non-conducting state, without the power consumption associated with regulating the input current Iin to the holding current. 
       FIG. 2  shows waveforms of signals of the LED driver circuit of  FIG. 1  in accordance with an embodiment of the present invention.  FIG. 2  shows, from top to bottom, the input voltage Vin, the sense voltage VCS, and possible waveforms of the discharge current IS. 
     In the example of  FIG. 2 , the LED non-conducting state is sensed using the sense voltage VCS for illustration purposes only. For example, an anode voltage or a cathode voltage of the LED element LED 1  may be used to sense the LED non-conducting state in other embodiments. In the example of  FIG. 2 , the waveforms (a)-(h) illustrate different ways of generating the discharge current IS by way of example. 
     As illustrated in  FIG. 2 , according to the decrease in the input voltage Vin, the LED non-conducting state occurs at time point T 1 , and the sense voltage VCS reaches zero voltage that is lower than the discharge threshold voltage at the time point T 1 . 
     As shown in (a) and (b) of  FIG. 2 , the LED conducting state detection circuit  6  may be synchronized at time point T 1  to generate a discharge control signal DCS to increase the discharge current IS. Alternatively, as shown in (c) and (d) of  FIG. 2 , the LED conducting state detection circuit  6  may be synchronized to generate the discharge control signal DCS to increase the discharge current IS at time point T 2 , which is delayed from time point T 1  by a delay period TD. 
     As shown in (e), (f), (g) and (h) of  FIG. 2 , the discharge current IS may flow with a certain offset level before time point T 1 . This is only to illustrate that the offset level discharge current IS can flow for a duration other than in the LED non-conducting state. 
     As shown in (e) and (f) of  FIG. 2 , the LED conducting state detection circuit  6  may be synchronized at time point T 2  to generate a discharge control signal DCS to increase the discharge current IS. 
     As shown in (g) and (h) of  FIG. 2 , the LED conducting state detection circuit  6  may block the discharge current IS at time point T 1 , and be synchronized at time point T 2  to generate a discharge control signal DCS to increase the discharge current IS. 
     The current source  5  may generate the discharge current IS into a variety of waveforms like those illustrated in  FIG. 2 ( a )-( h )  according to the discharge control signal DCS. In the LED non-conducting state, the input voltage Vin is decreased by the discharge current IS. In the LED non-conducting state, the waveform of the decreasing input voltage Vin may be controlled according to the waveform of the discharge current IS. 
     The LED conducting state detection circuit  6  and the current source  5  are now described with reference to the LED driver circuit  1  shown in  FIG. 3 . 
       FIG. 3  shows further details of the LED conducting state detection circuit  6  and the current source  5  in accordance with an embodiment of the present invention. Components illustrated in  FIG. 3  that overlap with those illustrated in  FIG. 1  are indicated with the same reference numerals and not redundantly described below. Although  FIG. 3  depicts the LED conducting state detection circuit  6  using a cathode voltage of the LED element LED 1  as an example, the present disclosure is not limited thereto. In the example of  FIG. 3 , the cathode voltage of the LED element LED 1  is the input voltage Vin subtracted by a forward voltage of the LED element LED 1 , and thus varies according to the input voltage Vin. 
     In the example of  FIG. 3 , the LED conducting state detection circuit  6  includes three resistors R 1 -R 3 , a transistor  61 , a capacitor C 2 , and a Zener diode  62 . The transistor  61  may be implemented as an NPN bipolar junction transistor (BJT), although it may be implemented using other types of transistors. 
     The resistor R 1  and the resistor R 2  are connected in series between the cathode of the LED element LED 1  and the ground, and resistive-divide the cathode voltage of the LED element LED 1 . The resistive-divided voltage is supplied to the base of the transistor  61 . 
     A collector of the transistor  61  is connected to a node N 1 , and an emitter of the transistor  61  is connected to the ground. The resistor R 3  is connected between the voltage source VS and the node N 1 . In the example of  FIG. 3 , the voltage source VS is the means for charging the capacitor C 2 . In other configurations, the current source  5  may be used to charge the capacitor C 2 . 
     The capacitor C 2  and the Zener diode  62  are connected in parallel between the node N 1  and the ground. The voltage on the capacitor C 2  controls the voltage of the node N 1  as the capacitor C 2  is charged or discharged according to a switching operation of the transistor  61 . The Zener diode  62  may clamp the voltage of the node N 1  to the Zener voltage. The discharge control signal DCS follows the voltage of the node N 1 . 
     When the cathode voltage of the LED element LED 1  is decreased according to a decrease in the input voltage Vin and reaches the discharge threshold voltage, the base voltage of the transistor  61  is decreased and thereby turns off the transistor  61 . Accordingly, the capacitor C 2  is charged with the current flowing from the voltage source VS through the resistor R 3  such that the voltage of the node N 1  is increased. The voltage of the node N 1  may be increased to the Zener voltage. That is, the discharge control signal DCS may be increased starting from a time point when the transistor  61  is turned off according to the decrease in the input voltage Vin, and clamped to the Zener voltage of the Zener diode  62 . 
     The transistor  61  turns on when the cathode voltage of the LED element LED 1  is higher than the discharge threshold voltage. In that case, current flows to the ground through the transistor  61 , and the capacitor C 2  is not charged. 
     In the example of  FIG. 3 , the current source  5  includes a transistor  51  and a resistor R 4 . The transistor  51  may be implemented as an n-channel type MOSFET, although it may be implemented as other types of transistors. 
     The discharge control signal DCS is supplied to a gate of the transistor  51 , and a drain of the transistor  51  is connected to the AC line (see also  FIG. 1 ). A source of the transistor  51  is connected to one end of the resistor R 4 . The flow of the discharge current IS through the transistor  51  may be controlled according to the discharge control signal DCS. 
       FIG. 4  shows waveforms of signals of the LED driver circuit  1  of  FIG. 3  in accordance with an embodiment of the present invention.  FIG. 4  shows, from top to bottom, the input voltage Vin, the cathode voltage VLED 1  of the LED element LED 1 , the discharge current IS, the discharge control signal DCS, and the input current Iin. 
     The cathode voltage illustrated in  FIG. 4  is the cathode voltage of the LED element LED 1 , which is indicated by VLED 1 . The input voltage Vin illustrated in  FIG. 4  is an input voltage generated by a leading edge dimmer  7 . The waveforms are illustrated in  FIG. 4  only by way of example to describe the exemplary embodiments, and the present disclosure is not limited thereto. 
     In the example of  FIG. 4 , the dimmer  7  is turned on at time point T 10  so that the input voltage Vin is generated and firing of the input current Iin is generated. A number of LED elements according to the level of the input voltage Vin conduct and the current ILED flows to the LED string  2 . During the ON period of the dimmer  7 , the input current Iin follows the current ILED and, accordingly, the input current Iin may also change according to the number of LED elements that conduct according to the level of the input voltage Vin. 
     When the input voltage Vin reaches its peak and then decreases due to the decrease in the input current Iin, the dimmer  7  turns off at time point T 11 . While it is illustrated by way of example that the cathode voltage VLED 1  becomes zero voltage and the input current Iin does not flow at time point T 11 , this is only an example provided to explain the embodiments, and the present disclosure is not limited hereto. 
     The cathode voltage VLED 1  is lower than the discharge threshold voltage at time point T 11 , and the LED conducting state detection circuit  6  generates the discharge control signal DCS. The current source  5  supplies the discharge current IS according to the discharge control signal DCS. The discharge current IS may be increased to the index waveform according to voltage-current characteristic of the transistor  51 . 
     While  FIG. 4  illustrates that the discharge current IS starts to flow without delay upon sensing LED non-conducting state, a delay may occur between time point T 11  and the time point of generating the discharge current IS as noted above with reference to  FIG. 2 . Furthermore, the discharge current IS may flow with an offset level before time point T 11 . 
     Continuing with the example of  FIG. 4 , the capacitor C 1  is discharged by the discharge current IS and the falling waveform of the input voltage Vin is controlled according to the waveform of the discharge current IS. When the input voltage Vin reaches zero voltage at time point T 12 , the discharge current IS may not flow anymore. The discharge control signal DCS may rise and be clamped to the Zener voltage VZ. The dimmer  7  is turned on again at time point T 13  and the operation from time point T 10  to time point T 12  repeats. 
     Rather than regulating the input current Iin to a holding current, which is the method used in the related art in order to prevent the dimmer from turning off, the LED driver circuit  1  according to an exemplary embodiment controls the input voltage Vin by discharging the capacitor C 1  after the dimmer  7  is turned off. By doing so, problems such as variations in the input voltage Vin, flicker, rising temperature of MOSFET, and so on can be alleviated. 
     Inrush current may be generated when the dimmer  7  is turned on. As illustrated in  FIG. 4 , the input current Iin has a firing due to influence of the inrush current. The LED driver circuit may further include a passive bleeder in order to prevent excessive rising of the input current Iin due to the inrush current. However, when the input voltage Vin starts to fall with a negative slope after the peak, negative current may flow to the passive bleeder, thus resulting in a problem of decreasing input current Iin. 
       FIG. 5  shows a schematic diagram of an LED driver circuit  10  in accordance with an embodiment of the present invention. 
     In the example of  FIG. 5 , the LED driver circuit  10  includes a passive bleeder  11 . The components of the LED driver circuit  10  illustrated in  FIG. 5  that overlap with those illustrated in  FIG. 1  are indicated with the same reference numerals and symbols. 
     In the example of  FIG. 5 , the passive bleeder  11  is connected between a node N 2  to which the input voltage Vin is supplied and a node N 3  where the sense voltage VCS is generated. The passive bleeder  11  includes a resistor RP and a capacitor CP. 
     The bleeder current IBL flows through the resistor RP and the capacitor CP to the node N 3 . When the input voltage Vin starts to decrease, the bleeder current IBL may have a negative value, and the phase of the bleeder current IBL may have 90 degree of difference from the phase of the input current Iin. 
       FIG. 6  shows further details of an LED current controller  4  in accordance with an embodiment of the present invention. The LED driver circuit of  FIG. 6  comprises the passive bleeder  11 , the controller  4 , and the LED string  2 . 
     In the example of  FIG. 6 , the plurality of regulators  40 _ 1 - 40 _ n  may comprise linear regulators. In the example of  FIG. 6 , the plurality of regulators  40 _ 1 - 40 _ n  each includes a transistor  42  connected to a corresponding channel of the plurality of channels CH_ 1 -CH_n, and an amplifier  41  (“regulator amplifier”) for controlling the transistor  42 . A drain of the transistor  42  is connected to the corresponding channel, and a source of the transistor  42  is connected to one end of the sense resistor RCS. An output of the amplifier  41  is connected to a gate of the transistor  42 , an inverting terminal (−) of the amplifier  41  is connected to a source of the transistor  42 , and a non-inverting terminal (+) of the amplifier  41  is inputted with a corresponding control voltage among the plurality of control voltages VC_ 1 -VC_n. 
     The amplifier  41  generates an output based on a difference between a corresponding control voltage inputted to the non-inverting terminal (+) and the sense voltage VCS inputted to the inverting terminal (−), and the transistor  42  controls the current of the corresponding channel according to the output from the amplifier  41 . Then the voltages of the non-inverting terminal (+) and the inverting terminal (−) of the amplifier  41  are regulated to be equal. 
     As can be appreciated, one way of detecting the LED non-conducting state is to monitor the output of the amplifier  41 . When the output of the amplifier  41  is pulled up, it indicates that the input voltage Vin is lower than the forward voltage of the corresponding LED element and the LED current ILED does not flow. Another way of detecting the LED non-conducting state is to monitor the sense voltage VCS. When the sense voltage VCS is lower than a certain level, it indicates that the conduction of the LED string is almost finished. Accordingly, an LED conducting state detection circuit  6  may be configured to detect the LED non-conducting state by monitoring an end (cathode or anode) of an LED element, the output of the amplifier  41 , and/or the sense voltage VCS. 
     Even when the bleeder current IBL is a negative value, the input current Iin may be compensated because the voltage of the node N 3  is controlled to the sense voltage VCS by the enabled regulator among the plurality of regulators  40 _ 1 - 40 _ n.    
     When the passive bleeder  11  is connected between the node N 2  and the ground, the input current Iin decreases as much as a bleeder current IBL when the bleeder current IBL is a negative value. However, in another exemplary embodiment, even when the bleeder current IBL is a negative value, because the voltage of the node N 3  is regulated to the sense voltage VCS, the current ILED flowing through a channel corresponding to the enabled regulator is increased as much as the bleeder current IBL. 
     Because the input current Iin is the sum of the bleeder current IBL and the current ILED, the current ILED may be increased as much as the bleeder current IBL is decreased. Accordingly, the input current Iin can be compensated. 
       FIG. 7  shows waveforms of signals of the LED driver circuit of  FIG. 6  in accordance with an embodiment of the present invention.  FIG. 7  shows, from top to bottom, the input voltage Vin, the sense voltage VCS, the bleeder current IBL, and the input current Iin. 
     As illustrated in  FIG. 7 , the bleeder current IBL is generated as a negative value after time point T 20  of a peak of the input current Vin. Advantageously, because the sense voltage VCS is regulated to the control voltage, the input current Iin is not decreased due to the bleeder current IBL. Without the bleeder current IBL being connected to the node N 3 , the input current Iin may decrease as indicated by the phantom line in the waveform of the input current Iin. 
     The embodiment illustrated in  FIGS. 1 and 3  may be combined with the embodiment illustrated in  FIG. 5  as shown in  FIG. 8 . 
       FIG. 8  shows a schematic diagram of an LED driver circuit  100  in accordance with an embodiment of the present invention. As illustrated in  FIG. 8 , the LED driver circuit  100  may include the current source  5 , the LED conducting state detection circuit  6 , and the passive bleeder  11 . The configuration and operation of the LED driver circuit  100  are as previously described. 
       FIG. 9  shows a schematic diagram of a regulator of an LED current controller in accordance with an embodiment of the present invention. The regulator of  FIG. 9  may be employed as a regulator in an LED current controller  4 . In the example of  FIG. 9 , the regulator is illustrated as a regulator  40 _ 1  of the controller  4 . It is to be noted that the regulator of  FIG. 9  may be employed in other LED current controllers. 
     A regulator  40 _ 1  may include the regulator amplifier  41  and the transistor  42  as previously explained with reference to  FIG. 6 . The drain of the transistor  42  is connected to a cathode of the LED element LED 1  of the LED string  2 . The inverting (−) input of the amplifier  41  and the source of the transistor  42  are connected to the sense voltage VCS that is developed on the sense resistor RCS. The operation of these components are as previously explained. 
     In the example of  FIG. 9 , the regulator  40 _ 1  further includes a spike current suppression circuit comprising a transistor  45 . The gate of the transistor  45  is driven by an input threshold voltage VIN.TH through an inverter. 
       FIG. 10  shows a schematic diagram of an input threshold voltage generator in accordance with an embodiment of the present invention. In the example of  FIG. 10 , a resistive divider comprising resistors R 10  and R 11  scales the input voltage Vin to generate a voltage that is indicative of the input voltage Vin, which is input to an amplifier  46 . The amplifier  46  compares the scaled input voltage Vin to a threshold voltage Vth to generate the input threshold voltage VIN.TH. The input threshold voltage VIN.TH is high when the scaled input voltage Vin is greater than the threshold voltage Vth, and the input threshold voltage VIN.TH is low when the scaled input voltage Vin is lower than the threshold voltage Vth. An optional delay  47  may be added to the output of the amplifier  46 . 
     Referring back to  FIG. 9 , when the input threshold voltage VIN.TH is low, the transistor  45  is on and pulls down the output G( 1 ) of the amplifier  41 . Because the output G( 1 ) is pulled down, LED spike current that may occur when the input voltage VIN increases back up is suppressed. 
       FIG. 11  shows waveforms of signals of an LED driver circuit with the regulator of  FIG. 9 , in accordance with an embodiment of the present invention.  FIG. 11  shows, from top to bottom, the input voltage Vin, the input threshold voltage VIN.TH, the sense voltage VCS, and the output G( 1 ) of the amplifier  41 . Also shown in  FIG. 11  are the cathode voltage VLED 1  of the LED element LED 1  and a threshold voltage Vth. As can be appreciated, the input voltage Vin and/or the threshold voltage Vth may be scaled depending on implementation details. 
     As shown in  FIG. 11 , the input threshold voltage VIN.TH is high when the input voltage Vin is higher than the threshold voltage Vth, and is low when the input voltage Vin is lower than the threshold voltage Vth. The output G( 1 ) decreases as the sense voltage VCS increases, and increases as the sense voltage VCS decreases. The sense voltage VCS drops to zero when the input voltage Vin decreases below the cathode voltage VLED 1  of the LED element LED 1 , entering the LED non-conducting state. A zoom-in view at time T 30  is shown in  FIG. 12 . 
       FIG. 12  shows a zoom-in view of waveforms of signals in the example of  FIG. 11  at time T 30 .  FIG. 12  shows, from top to bottom, the input voltage Vin, the input threshold voltage VIN.TH, the output G( 1 ) of the amplifier  41 , and the sense voltage VCS.  FIG. 12  shows the input voltage Vin as rectangular for ease of illustration. 
     As shown in  FIG. 12 , the input threshold voltage VIN.TH is high when the input voltage Vin increases above the threshold voltage Vth at time T 30 . This turns off the transistor  45 , thereby removing the pull down and allowing the output G( 1 ) to increase and turn on the transistor  42 . Turning on the transistor  42  allows current to flow through the channel CH_ 1  and to the sense resistor RCS, thereby causing the sense voltage VCS to increase. Because of the pull down on the output G( 1 ) when the input voltage Vin is below the threshold voltage Vth, a spike current that may occur at the time T 30  (see dotted area  201 ) is suppressed. 
       FIG. 13  shows a schematic diagram of an LED driver circuit in accordance with an embodiment of the present invention. The LED driver circuit of  FIG. 13  includes the previously described LED string  2 , rectifier circuit  3 , LED current controller  4 , dimmer  7 , and sense resistor RCS. 
     In the example of  FIG. 13 , the LED driver circuit further includes a bias supply circuit  300  for providing a bias voltage BIAS 1  to the controller  4  and, optionally, a bias voltage BIAS 2  to a peripheral device  301 . The bias supply circuit  300  may be a low dropout regulator (LDO) or simply a resistor that is connected to the input voltage Vin. However, that implementation would lead to large power losses as the input voltage Vin becomes higher. 
       FIG. 14  shows a schematic diagram of a bias supply circuit  300  in accordance with an embodiment of the present invention. In the example of  FIG. 14 , the bias supply circuit  300  includes a capacitor C 21 , a diode D 1 , and a diode D 2 . One end of the capacitor C 21  is connected to the input voltage Vin, and the other end of the capacitor C 21  is connected to an anode of the diode D 1  and to a cathode of the diode D 2 . The bias supply circuit  300  may optionally include a resistor R 20  that is in series with the capacitor C 21  to form a passive bleeder. The diode D 2  may optionally be a Zener diode (see  310 ) to limit the bias voltage if the bias voltage is too high due to excessive energy from the capacitor C 21 . The phase-cut dimmer  7  (shown in  FIG. 13 ) is optional and can be added to modulate the input voltage Vin. 
     In the example of  FIG. 14 , the anode of the diode D 2  is connected to ground, and the cathode of the diode D 2  is connected to the anode of the diode D 1  and to the other end of the capacitor C 21 . The anode of the diode D 1  is connected to the cathode of the diode D 2  and to the other end of the capacitor C 21 . The cathode of the diode D 1  provides the bias voltage BIAS to the controller  4 , peripheral device  301 , and/or other circuits. 
     In the example of  FIG. 14  a capacitor current I_C 21  flows to the capacitor C 21  from the input voltage Vin, a first diode current I_D 1  flows from the anode to the cathode of the diode D 1 , and a second diode current I_D 2  flows from the cathode to the anode of the diode D 2 .  FIG. 15  shows the waveforms and timing relationships of these signals in one embodiment.  FIG. 15  shows, from top to bottom, the input voltage Vin, the capacitor current I_C 21 , the first diode current I_D 1 , and the second diode current I_D 2 . 
     LED driver circuits and methods of operating same have been disclosed. While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.