Semiconductor circuit arrangement for controlling a controlled device.

Abstract

 The current in a controlled device such as a light emitting diode (LED) 10, driven by a transistor switch Q₁, is stabilized by a current control circuit including a comparator type feedback network Q₃, Q₄, Q₅, Q₆, which stabilizes the voltage at a node 11 located between said switch Q₁ and the series connection of a ballast resistor R and LED 10.

Description

[0001] This invention relates to a semiconductor circuit arrangement for controlling current transmitted through a controlled device.

[0002] In the prior art, the current in a controlled device such as a semiconductor light emitting diode (LED) has been regulated by a control circuit containing an insulated gate field effect transistor (IGFET) driver switch of relatively very large transconductance in series with a ballast resistor. The IGFET driver is typically formed in a semiconductive silicon chip in accordance with standard MOS (metal-oxide-semiconductor) technology. During operation, if the voltage drop across the IGFET driver in its "on" condition is relatively small compared with applied voltage, the brightness of the LED in its "on" condition is somewhat stabilized by the ballast resistor. However, such a control circuit suffers from poor current regulation, whereby the current in the LED during operation can fluctuate by as much as a factor of 3 when the voltage of the external power supply, of typically about 5 or 6 volts, fluctuates by only 20 percent. Although this fluctuation in current can be reduced by using larger voltages for the power supply in conjunction with a larger ballast resistor, such an approach to the current fluctuation problem still suffers from the requirement of a physically relatively large IGFET driver. This device, which consumes an undesirably large amount of semiconductive silicon chip area, is required in order to keep the driver resistance, and hence the driver voltage drop, relatively small (0.5 volt drop) for the desired LED operating current. Moreover, ordinary processing variations in the manufacture of the IGFET driver of the prior art circuit cause corresponding variations in the LED operating current, thereby adversely affecting either the brightness or the lifetime of the LED on account of, respectively, either too little or too much operating current. It would therefore be desirable to have a control circuit for stabilizing the operating current in an LED, which mitagates the shortcomings of the prior art.

[0003] In accordance with the present invention the current in a controlled device such as an LED is stabilized by a semiconductor circuit arrangement which includes transistor switch means having feedback means operable on the control input of the input thereof for controlling the current flowing through the transistor switch means and thus through the controlled device when connected in series therewith. Preferably the feedback means is of comparator form for stabilizing a node of the series circuit in dependence upon a comparison of the node voltage and a reference voltage. By reason of the feedback means, the transistor switch means can operate with a relatively large source-drain voltage, typically of about 5 volts; therefore, for a given operating current in the thereby controlled (LED) device the transistor switch means can now have a relatively high resistance, thereby reducing the required amount of semiconductor chip area therefor.

[0004] In a specific embodiment of the invention, an LED is connected in series with a ballast resistor and the high current path (source-drain) of an IGFET driver switch (Q_i). The node between the IGFET driver and the series connection of the LED and Ballast resistor is connected through a comparator type feedback network back to the low current control (gate) terminal of the IGFET driver. This control terminal of the IGFET driver is also connected through the high current path of an auxiliary control IGFET (Q2) switch to a voltage source, the low current control terminal of this auxiliary IGFET being connected to an input terminal for application thereto of input signals to turn the LED "on" and "off".

[0005] This invention together with its features, objects, and advantages will be better understood from the following exemplary embodiment which is described in conjunction with the accompanying single figure drawing which is a schematic circuit diagram of a semiconductor circuit arrangement for regulating the current in a semiconductor LED in accordance with a specific embodiment of the invention.

[0006] As shown in the drawing, a semiconductor LED 10 has one of its terminals connected to a voltage source V_GG and another of its terminals connected to a ballast resistor R. As one example, the circuit parameters will be described in terms of P-MOS technology . Typically, the source V_GG is approximately -12 volts, and the resistor R is approximately a thousand ohms. The LED is characterized by an operating "on" current of about 10 milliamperes with an operating voltage drop of about 2 to 3 volts. The LED and the resistor R are connected in series with the high current (i.e. source-drain) path of an IGFET driver Q₁ to another voltage source V_SS of about +5 volt. In its "on" state, the driver Q₁ has a resistance advantageously equal to about R/2 or less.

[0007] As further shown in the drawing, the IGFETs Q₃, Q₄, Q₅ and Q₆ are in a comparator feedback network arrangement for stabilizing the voltage at node 11 located between R and Q_l. For this purpose, the node 11 is connected to a low current (i.e. gate) terminal of Q₆ whose high current path connects VGG to a node 13. The node 13 is connected to V_SS through the high current path of Q₃ whose gate terminal is grounded (V=O). The gate terminal of the driver Q_I is connected to a node 12 which is connected through Q₅ to V_GG and through Q₄ to the node 13. The IGFET Q₅ is in a diode configuration; that is, the drain and gate terminals of Q₅ are shorted together, so that Q₅ behaves as a diode which tends to conduct current only in the direction toward the source V_GG. On the other hand, the gate terminal of Q₄ is connected to ground serving as a reference potential.

[0008] The node 12 is further connected to V_SS through the high current path of Q₂. The gate of Q₂ is connected to an input signal source 20 which provides signals for turning Q₂ "on" and "off". As more fully explained below, when Q₂ is "on", then Q₁ is "off" and hence the LED 10 is also "off"; and when Q₂ is "off", then Q₁ is "on" and hence the LED 10 is also "on". Thus, the feedback arrangement acts as a signal inverter as well as a current stabilizer.

[0009] For optimum operation, the transconductance ratios ^{B21 B31} B₄, B₅ and ^B6 of the ^{IGFETs Q2}_, Q₃, ^Q₄, Q₅ and Q₆, respectively, should satisfy the following: B₅ should be much less than B₃; B₃ should be much less than either B₄ and B₆; and B₄ and B₆ should be much less than B₂. By "much less than" is meant less than by preferably a factor of 10, but in any event at least by a factor of 2 or 5. For example, by way of an illustrative example only, suitable approximate values for the B's are: B₅ = 2 x 10^-6 mho/V; B₃ = _{15 x} 10^-6 mho/V; B₄ - B₆ = 100 x 10 mho/V; and B₂ = 250 x 10^-6 mho/V. Moreover, the transistor Q₁ is advantageously characterized by moderately high B₁: for a 10 milliamp LED current, a suitable approximate value is B₁ = 250 x 10^-6 mho/volt. In the absence of the comparator feedback circuit, the required transconductance of the IGFET driver would be about 1,200 x 10^-6 mho/volt. Operation of the circuit shown in the drawing can be understood from the following considerations. Starting from a condition in which the LED and the driver Q₁ are both "off" in the presence of a signal from the source 20 sufficient to maintain Q₂ in its "on" state, it will first be shown that this condition is stable; and it will then be shown that a signal applied that is sufficient to switch and maintain Q₂ in its "off" state will also switch and maintain both the driver Q₁ and the LED "on" in a stabilized current condition. In order to explain this operation, it is to be noted that when at first the input signal maintains Q₂ in its "on" state, then the driver Q_l will thus be in its "off" state and hence the LED will also be in its "off" state. Under these conditions, the node 12 tends to remain at essentially the potential V_SS by virtue of the connection of this node to the source V_SS through the relatively high B IGFET Q₂. This connection is through the transistor of the highest B in the comparator circuit (Q,, Q₅ and Q₆ in particular). Thus, the node 12 remains in a stable condition at essentially V_SS (the substrate of all transistors is connected to V_SS as is ordinarily true in P-MOS integrated circuits). Accordingly, the voltage on the node 12 maintains the IGFET Q₁ in its "off" state, thereby maintaining the LED 10 in its "off" state also. Meanwhile, since the node 11 is essentially at potential at V_GG due to the path through R and the LED to the source V_GG, the transistor Q6 is in its "on" state; so that the node 13 is essentially at potential V_GG (except for a threshold of Q₆ which, with the backgate bias effect, is about -5 or -6 volts). This is true even though Q₃ is also "on" because of the high B₆ of Q₆ as compared with the low B₃ of Q₃. On the other hand, since node 13 is at essentially V_GG while the node 12 is at V_SS, Q₄ is "on"; but this "on" condition of Q₄ combined with the "on" conditions of Q₅ and Q₆ is not sufficient to pull the node 12 away from V_SS, since Q₂ has the highest transconductance B of all. Thus, the node 12 remains stably at V_SS, thereby keeping Q₁ in its "off" state and hence the LED stably remains in its "off" state also.

[0010] When the input signal applied by the source 20 to the gate of Q₂ is then switched to a value sufficient to turn Q₂ "off", the potential of the node 12 tends toward V_GG but without reaching it because the driver Q₁ turns "on" before this node 12 reaches ground. As soon as the driver Q₁ turns "on", however, the LED turns "on" also and the node 11, between Q₁ and R, goes from the potential V_GG toward the potential V_SS, since the "on" resistance of the driver is advantageously made sufficiently small compared with R, typically about R/2. As the node 11 goes toward V_SS, the transistor Q₆ allows the node 13 to go toward V_SS by virtue of the "on" state of Q₃. But when this node 13 reaches ground plus the threshold of Q₄, then Q₄ turns "on", in the opposite direction with node 13 as its source and node 12 as its drain, thereby preventing the node 12 from going any further toward V_GG. In this way, the node 12 is kept at a potential suitable for maintaining the driver Q₁ and the LED in their "on" states. In effect, the transistor arrangement of Q,, Q₄, Q₅ and Q₆ acts as a feedback comparator for stabilizing, against fluctuations of either polarity, the voltage at node 11 essentially at the voltage applied to the gate of Q₄, whenever the signal input turns Q2 "off". Thus, the LED remains "on" until the input signal is thereafter switched to a value sufficient to turn the transistor Q₂ back to its "on" state.

[0011] Although the invention has been described in detail in terms of a specific embodiment, various modifications can be made without departing from the scope thereof. For example, N-MOS technology can be used instead of P-MOS; that is, all the transistors Q₁-Q₆ can be integrated in a P-type semiconductor chip with N+ type source and drain regions, with suitable modifications in V_SS and VGG. Moreover, other types of transistors than IGFETs can be used, such as J-FETs or bipolar transistors. Also, a unidirectional current inhibiting diode element of conductance B₅ in the forward direction can be used instead of the transistor Q₅. Moreover, the voltages applied to gate electrode of Q₄ and of Q₃ can both be other than ground, in order to stabilize the voltage at node 11 during operation at a corresponding voltage other than essentially ground potential. In any event, however, it is preferred that the voltage difference (V_SS - V_GG) be at least three or more times the voltage drop across the LED in its "on" state, and that the voltage at node 11 be stabilized to a value that is sufficiently different from V_SS to enable the use of a relatively small sized driver Q₁ of relatively high resistance, thereby to conserve semiconductor chip area.

Claims

1. Semiconductor circuit arrangement comprising transistor switch means for controlling a controlled device when connected in series therewith, in response to an input signal applied to a control input of said switch means, characterised by feedback means (Q_;, Q₄, Q₅, Q₆,) operable on the control input of the transistor switch means (Q₁) for controlling the current flowing through the transistor switch means.

2. Circuit arrangement as claimed in claim 1, in which the feedback means is of comparator form and is operable on the control input of the transistor means in dependence upon a comparison of a voltage afforded by said transistor means (at node 11) and a reference voltage (ground).

3. Circuit arrangement as claimed in claim 1 or claim 2, including a controlled device (₁0) in series with a ballast resistor (R) and a first transistor driver switch (Qq), the driver switch controlling the controlled device in response to an input signal such that both the driver switch and the controlled device turn "off" in response to one input signal and turn "on" in response to another input signal and a comparator feedback control network including first (12) and second (11) feedback terminals, the first feedback terminal (12) connected to a low current carrying control input terminal of the first transistor driver switch (Q₁) and the second feedback terminal (11) connected to a high current carrying terminal of the first transistor driver switch.

4. Circuit arrangement as claimed in claim 3, comprising a second transistor (Q₂) having a low current carrying terminal for connection thereto of the input signal and having a pair of high current carrying terminals, one of the said high current carrying terminals of the second transistor being connected to the said low current carrying terminal of the first transistor (Q₁), third (Q₃), fourth (Q₄), and sixth (Q₆) transistors each having a low current carrying terminal and a pair of high current carrying terminals, means for connecting mutually together one of the high current carrying terminals of each of the third, fourth, and sixth transistors, a fifth (Q₅) unidirectional current inhibiting device connected between the other high current carrying terminals of the fourth and sixth transistors, and means for connecting the said other high current carrying terminal of the fourth transistor to said one of the high current carrying terminals of the second transistor.

5. Circuit arrangement as claimed in claim 4, in which the low current carrying terminals of the third and fourth transistors are connected to terminals for the application thereto of reference potentials, and in which the other high current carrying terminals of the second and third transistors are connected to terminals for connection thereto of a first voltage source, and the other high current carrying terminal of the sixth transistor is connected to a terminal for connection thereto of a second, different voltage source.

6. Circuit arrangement as claimed in claim 5, in which the first, second, third, fourth and sixth transistors are insulated gate field effect transistors and in which the fifth current inhibiting device is a field effect transistor whose gate terminal is shorted to its drain terminal.

7. Circuit arrangement as claimed in claim 6, in which the controlled device is a light emitting diode.

8. Circuit arrangement as claimed in any of claims 4 to 7, in which the first, second, third, fourth, fifth and sixth transistors are formed on a semiconductive silicon chip.

9. Circuit arrangement as claimed in any of claims 4 to 8, in which the second, third, fourth, fifth and sixth transistors respectively have transconductances B₂, B₃, B₄, ^B₅, and B₆, respectively which satisfy the following conditions: B₅ is less than B₃; B₃ is less than either B₄ or B₆; B₄ and B₆ are each less than B₂.

10. Circuit arrangement as claimed in claim 9, in which B₅ is less than B₃ by a factor of at least 5; B₄ and B₆ are each less than B₂ by a factor of at least 2.

Drawing