EP0000844B1

EP0000844B1 - Semiconductor circuit arrangement for controlling a controlled device.

Info

Publication number: EP0000844B1
Application number: EP78300269A
Authority: EP
Inventors: Howard Clayton Kirsch
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1977-08-11
Filing date: 1978-08-08
Publication date: 1983-03-23
Also published as: JPS5430456A; EP0000844A1; DE2862207D1; US4160934A

Description

This invention relates to a semiconductor circuit arrangement for controlling current transmitted through a controlled device.
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 mitigates the shortcomings of this prior art.
In U.S. patent No. 3,903,454 there is disclosed a semiconductor circuit arrangement comprising first transistor switch means for controlling a controlled device when connected in series therewith in response to an input signal, and comparator feedback means operable on the control input of the first transistor switch means for controlling the current flowing through said transistor switch means in dependence upon a comparison of a voltage afforded by said transistor switch means and a reference potential, said comparator feedback means comprising first and second feedback terminals and second and third transistors each having a pair of high current carrying terminals and a low current carrying terminal, the first feedback terminal being connected to the control input of the first transistor switch means and the second feedback terminal being connected to a high current carrying terminal of said first transistor switch means, one of the high current carrying terminals of the second transistor being connected to one of the high current carrying terminals of the third transistor, the low current carrying terminal of the third transistor being connected to the second feedback terminal, and the other of the high current carrying terminals of the second transistor being connected to the first feedback terminal.
In accordance with the present invention the input signal to said arrangement is applied to the control input of said switch means, and the low current carrying terminal of the second transistor is connected to a terminal for the application thereto of a fixed reference potential (ground), the second feedback terminal being adapted to have the controlled device connected to it. In this way 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.
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.
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 0₁ to another voltage source V_ss of about +5 volt. In its "on" state, the driver 0₁ has a resistance advantageously equal to about R/2 or less.
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,. For this purpose, the node 11 is connected to a low current (i.e. gate) terminal of 0₆ whose high current path connects V_GG 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=0). The gate terminal of the driver Q₁ 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.
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 Q, 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.
For optimum operation, the transconductance ratios B₂, B₃, 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× 10^-6 mho/V; B₃=15×10^-6 mho/V; B₄=B₆=100×10^-6 mho/V; and B₂=250×10^-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=250×10^-6 mho/volt. In the absence of the comparator feedback circuit, the required transconductance of the IGFET driver would be about 1,200×10^-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 Q, 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 Q, 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, 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 (Q₃, 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 Q, in its "off" state and hence the LED stably remains in its "off" state also.
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 Q, 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 0₄, 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 Q, and the LED in their "on" states. In effect, the transistor arrangement of 0₃, Q₄, Q_l 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.
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 V_GG. 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 0₁ of relatively high resistance, thereby to conserve semiconductor chip area.

Claims

1. Semiconductor circuit arrangement comprising first transistor switch means (Q₁) for controlling a controlled device (10) when connected in series therewith in response to an input signal, and comparator feedback means operable on the control input of the first transistor switch means for controlling the current flowing through said transistor switch means in dependence upon a comparison of a voltage afforded by said transistor switch means and a reference potential, said comparator feedback means comprising first (12) and second feedback (11) terminals and second (Q₄) and third (Q₆) transistors each having a pair of high current carrying terminals and a low current carrying terminal, the first feedback terminal (12) being connected to the control input of the first transistor switch means (Q₁) and the second feedback terminal being connected to a high current carrying terminal of said first transistor switch means, one of the high current carrying terminals of the third transistor (Q₄) being connected to one of the high current carrying terminals of ghe third transistor (Q₆), the low current carrying terminal of the third transistor being connected to the second feedback terminal, and the other of the high current carrying terminals of the second transistor being connected to the first feedback terminal, characterised in that the input signal to said arrangement is applied to the control input of said switch means (Q₁), and in that the low current carrying terminal of the second transistor (Q₄) is connected to a terminal for the application thereto of a fixed reference potential (ground), said second feedback terminal (11) being adapted to have said controlled device (10) connected to it.

2. Circuit arrangement as claimed in claim 1, comprising a fourth transistor (Q₂) having a low current carrying terminal by means of which the input signal is applied to said switch means (0₁) and having a pair of high current carrying terminals, one of the said high current carrying terminals of the fourth transistor (Q₂) being connected to the low current carrying terminal of said first transistor (Q,), a fifth transistor (Q₃) having a low current carrying terminal and a pair of high current carrying terminals, one of the high current carrying terminals of the fifth transistor (Q₃) being connected to the interconnected ones of the high current carrying terminals of the second (Q₄) and third (Qg) transistors, and a unidirectional current inhibiting device (Q₅) connected between the other high current carrying terminals of the second (Q₄) and third (Q₆) transistors.

3. Circuit arrangement as claimed in claim 2, in which the low current carrying terminal of the fifth (Q₃) transistor is connected to a terminal for the application thereto of a reference potential, and in which the other high current carrying terminals of the fourth (Q₂) and fifth (0₃) transistors are connected to terminals for connection thereto of a first voltage source, and the other high current carrying terminal of the third (Q₆) transistor is connected to a terminal for connection thereto of a second, different voltage source to which the series connected controlled device (10) and ballast resistor (R) are connected.

4. Circuit arrangement as claimed in claim 2 or claim 3, in which the first (Q₁), second (Q₄), third (Q₆), fourth (Q₂) and fifth (Q₃) transistors are field effect transistors and the current inhibiting device (0₅) is a sixth field effect transistor (Q_s) the gate terminal of which is shorted to its drain terminal, and in which the second, third, fourth, fifth and sixth transistors respectively have transconductances B₄, B₆, B₂, B₃ and B₅, 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.

5. Circuit arrangement as claimed in claim 4, 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.