Patent Application
20080164828
Not Applicable.
Not Applicable.
This invention relates generally to electronic circuits and, more particularly, to electronic circuits used to drive a light emitting diode (LED) load.
A variety of electronic circuits are used to drive diode loads and, more particularly, to control electrical current through strings of series connected light-emitting diodes (LEDs), which, in some embodiments, form an LED display. It is known that individual LEDs have a variation in forward voltage drop from unit to unit. Therefore, the strings of series connected LEDs can have a variation in forward voltage drop.
Strings of series connected LEDs can be coupled to a common boost switching regulator at one end of the strings, the boost switching regulator configured to provide a high enough voltage to supply each of the strings of LEDs. The other end of each of the strings of series connected LEDs can be coupled to a respective current sink, configured to sink a relatively constant current through each of the strings of series connected LEDs.
It will be appreciated that the voltage generated by the common boost switching regulator must be a high enough voltage to supply the one series connected string of LEDs having the greatest total voltage drop, plus an overhead voltage needed by the respective current sink. In other words, if four series connected strings of LEDs have voltage drops of 30V, 30V, 30V, and 31 volts, and each respective current sink requires at least one volt in order to operate, then the common boost switching regulator must supply at least 32 volts.
While it is possible to provide a fixed voltage boost switching regulator that can supply enough voltage for all possible series strings of LEDs, such a boost switching regulator would generate unnecessarily high power dissipation when driving strings of series connected LEDs having less voltage drop. Therefore, in some LED driver circuits, the voltage drops through each of the strings of series connected LEDs are sensed and the common boost switching regulator is controlled to generate an output voltage only high enough to drive the series connected LED string having the highest voltage drop.
While the above-described electronic technique can result in a reduction of power dissipation, the above-described electronic technique can also suffer a high power dissipation if one of the series connected strings of LEDs becomes open circuit, i.e., fails. In this situation, a high voltage drop would be sensed and the common boost switching regulator would be controlled to increased its output voltage as high as it is able, resulting in a higher power dissipation associated with the remaining strings of series connected LEDs.
In accordance with one aspect of the present invention, an electronic circuit includes a current regulator having a current sense node, wherein the current regulator is configured to pass a predetermined current through the current regulator. The electronic circuit also includes an open-circuit detection circuit having an input node and an output node. The input node of the open-circuit detection circuit is coupled to the current sense node of the current regulator. The open-circuit detection circuit is configured to provide an output signal at the output node of the open-circuit detection circuit indicative of a current flowing through the current regulator being below a predetermined current threshold. The electronic circuit also includes a switch having an input node, an output node, and a control node. The input node of the switch is coupled to the current sense node of the current regulator, a selected one of the input node or the output node of the switch is coupled to the input node of the open-circuit detection circuit, and the control node of the switch is coupled to the output node of the open-circuit detection circuit. The open-circuit detection circuit is configured to open the switch in response to the current flowing through the current regulator being below the predetermined current threshold.
Also described is an open circuit protection method for an LED driver circuit comprising a boost switching regulator, which includes drawing a predetermined current through an LED and through a current regulator, detecting a current passing through the current regulator to determine whether the current is less than a predetermined current threshold, and disconnecting the LED from the LED driver circuit in response to a determination that the current passing through the current regulator is less than the predetermined current threshold.
With the above arrangements, open circuit protection is provided by which an open load element or elements coupled to the current regulator is detected and decoupled from the electronic circuit, as may include a boost switching regulator for driving the load, via opening of a respective switch. This arrangement is particularly advantageous in embodiments in which the electronic circuit includes a boost switching regulator that receives, as a feedback, the signal at one or more of the current regulator current sense nodes, since otherwise, an open circuit load condition could cause the switching regulator output voltage to increase excessively as may damage other components and cause significant power dissipation. Advantageously, the open circuit load is not disabled, but simply decoupled from the circuit, thereby permitting the faulty load to again be coupled to the circuit if the fault is temporary.
In some embodiments, the open-circuit detection circuit is configured to open the switch in response to both the signal at the current sense node of the current regulator being below the first predetermined current threshold and the occurrence of another condition, such as an over-voltage condition and/or an over-temperature condition. This arrangement prevents false, perhaps transient, indications of the current regulator current being below the first predetermined current threshold from opening the switch.
In accordance with another aspect of the present invention, an electronic circuit includes a switch having an input node, an output node, and a control node. The electronic circuit also includes a current-passing circuit having first and second nodes. The first node of the current-passing circuit is coupled to the input node of the switch and the second node of the current-passing circuit is coupled to the output node of the switch. The electronic circuit also includes a boost switching regulator having an input node and an output node. The input node of the boost switching regulator is coupled to the output node of the switch and the output node of the boost switching regulator is configured to couple to a diode load. The electronic circuit also includes a resistor having first and second nodes. The first node of the resistor is coupled to the control node of the switch and the second node of the resistor is coupled to the output node of the boost switching regulator.
With this arrangement, a simple short circuit protection scheme is provided whereby the switch is closed during normal operation and open when a short circuit condition occurs. The current-passing circuit allows the electronic circuit to start up and to resume operation following removal of a short circuit condition by allowing a small current to pass. This arrangement is possible because the diode load draws very little current until the switching regulator output voltage reaches a sufficient voltage.
In accordance with another aspect of the present invention, an electronic circuit for dimming a light emitting diode having an anode and a cathode includes a current regulator having a current node and a control node. The current node of the current regulator is configured to couple to the light emitting diode. The electronic circuit also includes a boost switching regulator having an input node, an output node, and a control node. The output node of the boost switching regulator is configured to couple to a selected one of the anode of the light emitting diode or to the current regulator. The boost switching regulator is enabled to switch or is disabled from switching in response to an input signal at the control node of the boost switching regulator. The electronic circuit also includes a pulse width modulation circuit having an output node and a control node. The output node of the pulse width modulation circuit is coupled to the control node of the current regulator. The pulse width modulation circuit is configured to generate an AC output signal at the output node of the pulse width modulation circuit, which enables and disables the current regulator at a predetermined frequency and at a selected duty cycle in response to a respective selected input signal at the input node of the pulse width modulation circuit. The duty cycle is selected in accordance with a selected brightness of the light emitting diode. Substantially simultaneously with the current regulator being disabled, the input signal at the control node of the switching regulator is indicative of the boost switching regulator being disabled.
With this arrangement, the output voltage of the switching regulator is held substantially constant during intervals when the current regulators are disabled by the pulse width modulation. Audible noise that may otherwise be generated due to voltage swings on ceramic capacitors is reduced or eliminated.
In some embodiments, a capacitor is coupled to the control node of the boost switching regulator through a switch that is controlled by the pulse width modulation circuit such that the switch is closed when the current regulator is enabled and open when the current regulator is disabled. With this arrangement, the voltage at the control input to the switching regulator is held substantially constant, at its previous voltage level, during intervals in which the current regulators are disabled by the pulse width modulation circuit, thereby resulting in rapid stabilization of the boost switching regulator control loop during each pulse width modulation cycle.
The foregoing features of the invention, as well as the invention itself may be more fully understood from the following detailed description of the drawings, in which:
Before describing the present invention, some introductory concepts and terminology are explained. As used herein, the term “boost switching regulator” is used to describe a known type of switching regulator that provides an output voltage higher than an input voltage to the boost switching regulator. While a certain particular circuit topology of boost switching regulator is shown herein, it should be understood that a boost switching regulator can be formed in a variety of circuit configurations.
As used herein, the term “current regulator” is used to describe a circuit or a circuit component that can regulate a current passing through the circuit or circuit component to a predetermined, i.e., regulated, current. A current regulator can be a “current sink,” which can input a current, or a “current source,” which can output a regulated current. A current regulator has a “current node” at which a current is output in the case of a current source, or at which a current is input in the case of a current sink. For a current sink, the current node is also referred to herein as an “input node” since it inputs a current. For a current source, the current node is also referred to herein as an “output node” since it outputs a current. The current node can be the same node or a different node from a “current sense node,” used to sense a current flowing through the current regulator. The current sense node is described more fully below in conjunction with
Referring to
In some embodiments, the boost switching regulator 12 includes an inductor 18 having first and second nodes. The first node of the inductor 12 is coupled to the input node 14 of the boost switching regulator 12. The boost switching regulator 12 also includes a diode 20 having an anode and a cathode. The anode is coupled to the second node of the inductor 14. The boost switching regulator 12 also includes a capacitor 22 coupled to the cathode. The boost switching regulator 12 can include, or is otherwise coupled to, a switching circuit 32 having a switching node 24 coupled to the second node of the inductor 18. In some embodiments, an input capacitor 16 can be coupled to the input node 14 of the boost switching regulator 12.
The integrated circuit 30 includes current regulators 74a-74d, each having an input node 74aa-74da, respectively. In the illustrative embodiment, the current regulators 74a-74d are current sinks. However, in other embodiments described below in conjunction with
The current flowing into the input nodes 74aa-74da of the current sinks 74a-74d being below the predetermined current threshold can be identified in a variety of ways. For example, in one particular arrangement, voltages, for example, voltages at the input nodes 74aa-74ad of the current sinks 74a-74d are indicative of the current flowing into the input nodes 74aa-74da. With these arrangements, the voltages can be compared with a first predetermined voltage threshold 62 in order to identify if any one of the voltages is below the first predetermined voltage threshold 62 (indicative of the current flowing through the current regulator being below a predetermined current threshold), a condition that can occur if one of the series connected strings of diode 28a-28d becomes open circuit. With these arrangements, it will be appreciated that the current nodes 74aa-74da can also be current sense nodes. This technique is depicted in figures and discussion below.
In other particular embodiments, voltages, for example, voltages at other respective current sense nodes (not shown) of the current sinks 74a-74d are indicative of the current flowing into the input nodes 74aa-74da. With these arrangements, the voltages can be compared with the first predetermined voltage threshold 62 in order to identify if any one of the voltages is below the first predetermined voltage threshold 62 (indicative of the current flowing through the current regulator being below a predetermined current threshold).
In yet other particular embodiments, control signals generated by other circuitry (not shown) at respective control nodes 74ab-74bd of the current sinks 74a-74d are indicative of the current flowing into the current nodes 74aa-74da. In some arrangement, the control nodes 74ab-74bd can be associated with gates of respective FETs (not shown), in which case the control signals can be control voltages. In other arrangements, the control nodes are associated with bases of junction transistors (not shown), in which case the control signals can be control currents. With the FET arrangements, the control voltages can be compared with the first predetermined voltage threshold 62 in order to identify if any one of the voltages is below the first predetermined voltage threshold 62. With the junction transistor arrangements, the control currents can be converted to voltages and can also be compared with the first predetermined voltage threshold 62 in order to identify if any one of the voltages is below the first predetermined voltage threshold 62. In both arrangements, the sensed level is indicative of the current flowing through the current regulator being below a predetermined current threshold.
In all of the above arrangements, the current regulators 74a-74d, shown as current sinks 74a-74d have a respective node, which can be sensed (referred to herein as a “current sense” node), in order to measure a current flowing through the current regulator. The current sense node can be the same node or a different node from the “current node,” which supplies or which draws current.
The integrated circuit 30 also includes switches 56a-56d having input nodes (unlabeled for clarity), output nodes (unlabeled for clarity), and control nodes 56aa-56da, respectively. The input nodes of the switches 56a-56d are coupled to the input nodes 74aa-74ad of the current sinks 74a-74d, respectively. The output nodes of the switches 56a-56d are coupled to the input nodes of the open-circuit detection circuit 58 (i.e., to the open LED detect comparators 60). The control nodes 56aa-56da of the switches 56a-56d, respectively, are coupled to the output nodes of the open-circuit detection circuit 58 (i.e., to the latching circuit 64). The open-circuit detection circuit 58 is configured to open one or more of the switches 56a-56d in response to the voltage at the input nodes 74aa-74da of a respective one or more of the current sinks 74a-74d being below the first predetermined voltage threshold 62 (i.e., the current flowing through a current regulator 74a-74d being below the predetermined current threshold). Thus, when the voltage at one or more of the input nodes 74aa-74da is less than the first predetermined voltage threshold 62, the respective input node(s) of the open-circuit detection circuit 58 are decoupled from the input nodes 74aa-74da of the current sinks 74a-74d, respectively.
In some embodiments, the open-circuit detection circuit 58 includes the open LED detect comparators 60 having input nodes and output nodes. The input nodes of the open detect comparators 60 are coupled to the input nodes of the open-circuit detection circuit 58. The open detect comparators 60 are configured to compare voltages appearing at respective input nodes with the first predetermined voltage threshold 62. In some embodiments, the open-circuit detection circuit 58 also includes first logic gates 60a-60d, each having a respective input node and a respective output node. In the illustrative embodiment, the first logic gates 66a-66d are AND gates, however, in other embodiments, other logic gate structures can also be used. The input nodes of the first logic gates 66a-66d are coupled to the output node of the open LED detect comparators 60. In some embodiments, the open-circuit detection circuit 58 also includes the latching circuit 64 having input nodes and output nodes. The input nodes of the latching circuit 64 are coupled to the output nodes of the first logic gates 66a-66d, and the output nodes of the latching circuit 64 are coupled to the output nodes of the open-circuit detection circuit 58.
In some arrangements, the latching circuit 64 can be reset via an enable port (not shown), which can be activated in a number of ways, including, but not limited to activation upon recycling power to the integrated circuit 30, or at any time the series connected strings of LEDs 28a-28d are turned off, for example, during a standby mode.
In some embodiments, the integrated circuit 30 also includes a minimum select circuit 54 having input nodes and an output node 52. The input nodes of the minimum select circuit 54 are coupled to the output nodes of the switches 54a-54d. The minimum select circuit 54 is configured to provide a signal at the output node 52 of the minimum select circuit 54 indicative of a signal (e.g., a voltage) at the output nodes of the switches 56a-56d. In particular, the signal at the output node 52 of the minimum select circuit 54 is indicative of a lowest one of the signals (e.g., voltages) at the output nodes of the switches 54a-54d, and therefore, a lowest voltage at the input nodes 74aa-74da of the current sinks 74a-74d, which can be indicative of a lowest current flowing through a current sink, i.e., an open-circuit condition. Therefore, the signal at the output node 52 of the minimum select circuit 54 is indicative of a largest voltage drop of the series connected LED strings 28a-28d. One of ordinary skill in the art will be able to design the minimum select circuit 54 having internal comparators and switches, so that an analog output voltage at the output node 52 is indicative of a lowest analog voltage at the input nodes of the minimum select circuit 54.
In some embodiments, the integrated circuit 30 also includes an error amplifier 48 coupled to receive the output signal from the output node 52 of the minimum select circuit 54 and to compare the output signal from the minimum select circuit 54 with a second predetermined voltage threshold 50.
In some embodiments, the integrated circuit 30 also includes the switching circuit 32 having the switching node 24 and a control node 46. The control node 46 of the switching circuit is coupled to the output node of the error amplifier 48, and therefore, to the output node 52 of the minimum select circuit 54. A duty cycle of the switching circuit 32 is responsive to the signal at the output node 52 of the minimum select circuit 54.
In operation, it should be apparent that, if one or more of the series connected LED strings 28a-28d fails, i.e., becomes open circuit, the open circuit condition can be detected by the open-circuit detection circuit 58, resulting in an opening of an associated one or more of the switches 56a-56d. The opening of one or more of the switches 56a-56d results in a respective one or more of the voltages at the input nodes 74aa-74da of the current sinks 74a-74d not being considered by the minimum select circuit 54, and therefore, not being involved in control of the duty cycle of the switching circuit 32 or of the voltage at the output node 26 of the boost switching regulator 12.
In some embodiments, the integrated circuit 30 also includes an over-voltage detection circuit 38 having an input node 40 and an output node 44. The output node 44 of the over-voltage protection circuit 38 is coupled to another input node 72 of the open-circuit detection circuit 58. The over-voltage detection circuit 38 is configured to provide an output signal at the output node 44 of the over-voltage protection circuit 38 indicative of a voltage at the input node 40 of the over-voltage detection circuit 38 being above a third predetermined voltage threshold 42. With this arrangement, the output signals at the output nodes of the open-circuit detection circuit 58 are indicative of the voltages at the input nodes 74aa-74da of the current sinks 74a-74d, respectively, being below the first predetermined voltage threshold 62 and are also indicative of the voltage at the input node 40 of the over-voltage detection circuit 38 being above the third predetermined voltage threshold 42.
With the over-voltage detection circuit 38, in operation, it should be apparent that, if one or more of the series connected LED strings 28a-28d fails, i.e., becomes open circuit, the open circuit condition can be detected by the open-circuit detection circuit 58, resulting in an opening of an associated one or more of the switches 56a-56d, but only when the voltage at the output node 26 of the boost switching regulator 12 rises to the third predetermined voltage threshold 42. The opening of one or more of the switches 56a-56d results in a respective one or more of the voltages at the input nodes 74aa-74da of the current sinks 74a-74d, respectively, not being considered by the minimum select circuit 54, and therefore, not being involved in control of the duty cycle of the switching circuit 32 or of the voltage at the output node 26 of the boost switching regulator 12.
In some embodiments, the integrated circuit 30 also includes a temperature detection circuit 68 having an output node coupled to a yet another input node 70 of the open-circuit detection circuit 58. The temperature detection circuit 68 is configured to provide an output signal at the output node of the temperature detection circuit indicative of a temperature of the electronic circuit 10 (e.g., the integrated circuit 30) being above a predetermined temperature threshold. With this arrangement, the output signal at the output node of the open-circuit detection circuit 58 is indicative of the voltages at the input nodes 74aa-74da of the current sinks 74a-74d, respectively, being below the first predetermined voltage threshold 62 and also indicative of the temperature of the electronic circuit 10 being above the predetermined temperature threshold.
With the temperature detection circuit 68, in operation, it should be apparent that, if one or more of the series connected LED strings 28a-28d fails, i.e., becomes open circuit, the open circuit condition can be detected by the open-circuit detection circuit 58, resulting in an opening of an associated one or more of the switches 56a-56d, but only when the temperature detected by the temperature detection circuit 68 is above the predetermined temperature threshold and only when the voltage at the output node 26 of the boost switching regulator 12 rises to the third predetermined voltage threshold 42. The opening of one or more of the switches 56a-56d results in a respective one or more of the voltages at the input nodes 74aa-74da of the current sinks 74a-74d, respectively, not being considered by the minimum select circuit 54, and therefore, not being involved in control of the duty cycle of the switching circuit 32 or of the voltage at the output node 26 of the boost switching regulator 12. In some other arrangements, the integrated circuit 30 has either the temperature detection circuit 68 or the over-voltage detection circuit 38, but not both.
In some alternate arrangements, the input nodes of the open-circuit detection circuit 58 (i.e., the open LED detect comparators 60) are instead coupled to the input nodes of the switches 56a-56d, e.g., to the current sense nodes 74aa-74ad, respectively.
In some alternate embodiments, some portions of the circuitry shown within the integrated circuit 30 are not within the integrated circuit 30. Partitioning of circuitry between integrated and discrete can be made in any way.
While the current regulators 28a-28d are shown to be current sinks 28a-28d in the illustrative electronic circuit 10, as described more fully blow in conjunction with
Referring now to
With this particular arrangement, in operation, it should be apparent that, if one or more of the series connected LED strings 28a-28d fails, i.e., becomes open circuit, the open circuit condition can be detected by the open-circuit detection circuit 58, resulting in an opening of an associated one or more of the switches 56a-56d, but only after a delay provided by the delay module 106. Therefore, transient behavior, which can result from electrical noise or the like, is avoided. As described above, the opening of one or more of the switches 56a-56d after the delay results in a respective one or more of the voltages at the input nodes 74aa-74da of the current sinks 74a-74d, respectively, not being considered by the minimum select circuit 54, and therefore, not being involved in control of the duty cycle of the switching circuit 32 or of the voltage at the output node 26 of the boost switching regulator 12. In some arrangements, the integrated circuit 10 also has the temperature detection circuit 68 (
Referring now to
Unlike the arrangement of
Referring now to
The boost switching regulator includes current limiting provisions not shown in the boost switching regulator 12 of
In the electronic circuit 120, however, the boost switching regulator also includes a switch element 122, here a field effect transistor, coupled between an input voltage source and the first node of the inductor 18, a current passing circuit 124, here a resistor, coupled in parallel with the switch element 122, and a resistor 126 coupled between the output node 26 of the boost switching regulator 12 and a control node of the switch element 122. In some embodiments, the current passing element 124 (resistor) can represent leakage though the switch element 122, and is not a separate component.
In operation, if a short circuit, or a higher than desired load current, appears at the output node 26 of the boost switching regulator, the output voltage at the output node 26 becomes less than the input voltage (Vbat), and the control node of the switch element 122 is pulled downward in voltage, tending to open the switching element 122 and tending to turn off the switching regulator. It will be appreciated however, that a switching signal provided at the SW node can continue to operate, though the switching regulator no longer provides current when in the short circuit condition. A diode 128 provides a voltage clamp with the resistor 126 to protect the switch element 122.
In other arrangements, rather than connecting the control node of the switch element 122 to the resistor 126 as shown, the control node can be coupled to a selected one of the cathodes of one of the diodes in one of the strings of series connected diodes 28a-28d. By selecting an appropriate one of the cathodes, the switch element 122 will turn back on upon removal of the short circuit condition in much the same way as described below.
When the open circuit or high load condition is removed, the current passing circuit 124 allows a current to flow past the switch element 122 resulting in an increasing voltage at the cathode of the diode 20. The voltage can increase at the cathode in this way, in particular because the switching regulator is coupled only to series connected diodes, which do not draw current until their threshold voltage is reached. When the voltage at the cathode of the diode 20 reaches a sufficient voltage, the switching element closes (or turns on) by way of the resistor 126. Because the switching provided at the SW node of the integrated circuit 30 continues, the switching regulator can resume normal operation once the short circuit or high load condition is removed. Operation upon removal of a short circuit condition is also indicative of operation at startup.
Referring now to
With the arrangements of
Referring now to
The electronic circuit 160 includes an integrated circuit 162 having a pulse width modulation circuit 164. The pulse width modulation circuit 164 has an output node 168 and a control node 166. The output node 168 of the pulse width modulation circuit 164 is coupled to control nodes 74ab-74db of the current sinks 74a-74d. The pulse width modulation circuit 164 is configured to generate an AC output signal at the output node 168 of the pulse width modulation circuit 164, which enables and disables the current sinks 74a-74d at a predetermined frequency and at a selected duty cycle in response to a respective selected input signal at the input node 166 of the pulse width modulation circuit 164. The duty cycle is selected in accordance with a selected or desired brightness of the series connected light emitting diodes 28a-28d.
In some particular arrangements (not shown), the pulse width modulation circuit is not within the integrated circuit 162, but instead communicates the AC signal to the control nodes 74ab-74db by way of a link, for example, a single-wire or multi-wire serial interface, e.g., RS-232, CAN, SMBus, SPI, or I2C.
When the current sinks 74a-74d are disabled, the minimum select circuit 164 detects the condition of all of the current sinks 74a-74d being disabled, and, substantially simultaneously with the current sinks 74a-74d being disabled, the input signal at the control node 46 of the switching regulator (i.e., of the switching circuit 32) is indicative of the boost switching regulator being disabled. Therefore, at substantially the same time that the current sinks 74a-74d are disabled, the switching circuit 32 stops switching. However, in this particular condition, the output voltage of the switching regulator is held at or near its value prior to the disabled condition by way of the capacitor 22. In other embodiments, the boost switching regulator is disabled within two microseconds to ten milliseconds of the current sinks 74a-74d being disabled.
In some embodiments, the electronic circuit 160 can also include a switch 172 having an input node, an output node, and a control node 172a, wherein the output node of the switch is coupled to the control node 46 of the boost switching regulator (i.e., of the switching circuit 32). The control node 172a of the switch 172 is coupled to the pulse width modulation circuit 164. The switch 172 is closed when the current sinks 74a-74d are enabled and open when the current sinks 74a-74d are disabled. The electronic circuit 160 can also include a capacitor 170 coupled to the input node of the switch 172. The capacitor 170 holds a voltage when the switch 172 is open, corresponding to a voltage of the control node 46 of the switching regulator (i.e., of the switching circuit 32) when the switch 172 is closed. With this arrangement, when the switch 172 is opened as the current sinks 74a-74d are disabled, the capacitor 170 can hold the control voltage of the switching circuit 32 accordingly. Therefore, when the switch 172 is again closed as the current sinks 74a-74d are again enabled, the control voltage at the control node 46 is at a value substantially equal to its previous voltage.
With this arrangement, the output voltage at the output node 26 of the switching regulator is held substantially constant during intervals when the current sinks 74a-74d are disabled by the pulse width modulation circuit 164. In this way, audible noise that may otherwise be generated due to voltage swings on ceramic capacitors is reduced or eliminated.
The error amplifier 48 can be a transconductance amplifier, which provides a current in response to a voltage at the output node 52 of the minimum select circuit.
The on-off frequency of the signal at the output node 168 of the pulse width modulation circuit 164 can be predetermined to provide an illumination of the series connected strings of LEDs 28a-28d, without apparent flicker. For example, the frequency can be a predetermined value between about twenty and one thousand Hertz. However, these limits may be extended based on the specific applications retirements. The on-off duty cycle of the signal at the output node 168 of the pulse width modulation circuit 164 can be selected according to the input signal provided at the input node 166 to the pulse width modulation circuit 164, and according to a selected (or user-desired) brightness of the series connected strings of LEDs 28a-28d. The duty cycle can be selected anywhere in a range of zero to one hundred percent. The duty cycle can be selected from time to time by a user. In some arrangements, for example, the signal at the input node 166, and the resulting brightness of the series connected strings of LEDs 28a-28d, is selected with a keyboard control on a laptop computer.
It should be understood that the various embodiments shown herein can be combined together into one electronic circuit or they can be separately used in different embodiments.
All references cited herein are hereby incorporated herein by reference in their entirety.
Having described preferred embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. It is felt therefore that these embodiments should not be limited to disclosed embodiments, but rather should be limited only by the spirit and scope of the appended claims.
