The present invention relates, in general, to electronics, and more particularly, to methods of forming semiconductor devices and structure.
In the past, the semiconductor industry utilized various methods and structures to form driver circuits for light emitting diodes (LEDs). Some of the LED driver circuits were designed to received an analog control signal and generate an analog drive signal that linearly varied the current through the LED. One example of such a control circuit that was available from Fairchild Semiconductor Corp. of South Portland Me. was referred to by the part number FAN5611. In some applications, it was desirable to have other methods of controlling the current through the LED.
Accordingly, it is desirable to have an LED controller that can vary the current through an LED in more than one method in response to a control signal.
For simplicity and clarity of the illustration, elements in the figures are not necessarily to scale, and the same reference numbers in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-Channel devices, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action.
Controller 20 includes a voltage input 21 and a voltage return 22 that typically are connected to respective terminals 12 and 11 to receive power for operating controller 20. Controller 20 also has a control input 24, a current output 26, and a sense input 27. In some embodiments, controller 20 may also have an enable input 23 that is used for enabling and disabling the operation of controller 20. Controller 20 may include an enable circuit 30, a control circuit 39, and a pass element, such as an output transistor 60. Control circuit 39 generally includes a limiter circuit 40, a amplifier 53, a voltage divider formed by resistors 51 and 52, a buffer resistor 41, and a control switch or transistor 56. As will be seen further hereinafter, limiter circuit 40 is configured to detect the control signal reaching a threshold value of the control signal and responsively inhibit increasing the value of current 15, thereby keeping the value of current 15 substantially constant, for values of the control signal that are greater than the threshold value.
Enable input 23 receives an enable signal that goes high to enable the operation of controller 20. The high from input 23 enables transistor 31 which pulls the gate of transistors 33 and 37 low to enable transistor 33 and disable transistor 37. Enabling transistor 33 couples bias circuit 34 to receive power and begin supplying bias currents to the other elements of controller 20. The bias currents generated by bias circuit 34 are used to supply an operating bias current to enable the operation of the other elements of controller 20, such as circuit 39.
Limiter circuit 40 is configured to detect the control signal reaching the threshold value (Vth) and responsively inhibit increasing the value of current 15, thereby keeping the value of current 15 substantially constant at a corresponding threshold current (Ith) formed by the threshold value of the control signal. Circuit 40 maintains current 15 substantially constant for values of the control signal that are greater than the threshold value (Vth) of the control signal. The exemplary embodiment of limiter circuit 40 illustrated in
Because of the dual control functionality of input 24, controller 20 may be used to control current 15 responsively to an analog signal applied to input 24 or responsively to a digital signal, such as a PWM signal, applied to input 24. For example, an analog signal that varies between the value of return 22 and the threshold voltage of the control signal can be used to control current 15 in an analog manner responsively to values of the control signal. Thus, the analog value of the control signal controls the value of current 15 and the brightness of LED 13 in an analog manner. Also, a digital signal that varies between the value of return 22 and a value that is greater than the threshold value of the control signal can be used to control current 15 in a digital manner. At the low value of the digital control signal, current 15 may be substantially zero and at the high value of the control signal, current 15 will be at a maximum value, thus, the brightness of LED 13 will vary in a digital manner between substantially no light and a maximum amount of light. The duty cycle of the digital control signal may be used to control the average value of current 15 and brightness of LED 13. As can be seen, a pulse width modulated (PWM) signal can be used to digitally vary current 15 and the light intensity of LED 13. Such control functionality facilitates obtaining similar values of current 15, thus light intensity of LED 13, for both analog control signals and PWM control signals. For example, an analog signal that is approximately half way between the value of return 22 and the threshold value provides a current 15 value and corresponding light intensity that is approximately one-half of the maximum value. A similar light intensity may be obtained by a PWM control signal that has an approximately fifty percent duty cycle.
In order to implement this functionality for controller 20, input 24 is connected to a first terminal of resistor 41 which has a second terminal connected to node 42. A first terminal of resistor 51 is connected to node 42 and a second terminal and is commonly connected to the inverting input of amplifier 53 and a first terminal of resistor 52. A second terminal of resistor 52 is connected to return 22. A first terminal of resistor 44 is connected to node 42 and a second terminal is commonly connected to the non-inverting input of amplifier 47 and to a first terminal of resistor 45. A second terminal of resistor 45 is connected to return 22. The inverting input of amplifier 47 is connected to the output of reference 49. The output of amplifier 47 is connected to a gate of transistor 43 which has a drain connected to node 42 and a source connected to return 22. The non-inverting input of amplifier 53 is connected to input 27 and to the source of transistor 60. The output of amplifier 53 is connected to a gate of transistor 56. A drain of transistor 56 is commonly connected to a gate of transistor 60 and the output of bias circuit 34 through a resistor 36. A source of transistor 56 is connected to return 22. A drain of transistor 60 is connected to output 26. Although transistor 43 is illustrated coupled to the reference signal from return 22, it will be appreciated that transistor 43 may be coupled to any reference signal that has a value that is less than the value of the third signal corresponding to the threshold value of the control signal. Those skilled in the art will appreciate that transistor 60 may be external to controller 20 in some embodiments. Additionally, it will be appreciated by those skilled in the art that controller 20 may be used to control any other current operated device in addition to LED 13.
In view of all of the above, it is evident that a novel device and method is disclosed. Included, among other features, is forming a controller that keeps the output current constant as the control signal increases past a threshold value.
While the subject matter of the invention is described with specific preferred embodiments, it is evident that many alternatives and variations will be apparent to those skilled in the semiconductor arts. For example, although transistor 43 is illustrated coupled to the reference signal from return 22, it will be appreciated that transistor 43 may be coupled to any reference voltage that has a value that is less than the threshold value. Additionally, limiter circuit 40 may have other implementations as long as the limiter circuit inhibits increasing current 15 after the control signal reaches the threshold value. Additionally, the word “connected” is used throughout for clarity of the description, however, it is intended to have the same meaning as the word “coupled”. Accordingly, “connected” should be interpreted as including either a direct connection or an indirect connection.