Electrostatic discharge protection device for high speed transmission lines
A semiconductor device for coupling a transient voltage at an input node to a reference node, the device having a bipolar transistor adapted to couple its collector to an input node and its emitter to the reference node and a driver device adapted to be coupled between the input node and the base terminal of the transistor such that the driver device is responsive to a transient voltage at the input node to turn on the transistor, thereby shunting the transient voltage to the reference node. Preferably, the input node is coupled to a high speed data transmission line that operates below 5v and the reference node is coupled to ground and the transistor is an NPN transistor. The driver may preferably be a gate-drain connected MOS transistor with its gate-drain terminal coupled to the collector terminal of the transistor and its source terminal coupled to the base terminal of the transistor. Alternatively, the driver may be a light emitting diode (LED) or any other diode with a different material (band-gap) and die size than the LED, connected to the bipolar transistor to create a low voltage clamping device.
This is a continuation of a PCT international application PCT/US03/28377 filed on Sep. 11, 2003, which claims priority to and benefit of U.S. Provisional Patent Application Ser. No. 60/409,568, filed on Sep. 11, 2002.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to a circuit for protecting transmission lines against transients, such as those caused by Electrostatic Discharge (ESD); more particularly, the invention relates to a semiconductor device having low capacitance and low leakage and which is useful for the protection of high speed data transmission lines which typically operating at voltages below 5V.
2. Description of the Related Art
With the increase in performance and component density of digital electronic circuits, which is due in part to improved sub-micron masking processes, there is a corresponding need to reduce power supply voltages to these circuits in order to reduce the internal power dissipation of the circuitry. A further advantage of the lower voltage power supply and corresponding low power consumption is the possibility of operating these circuits at higher clock speeds a heretofore possible.
Conventional logic voltage levels have generally decreased from the typical 5V range to 1.5V or lower. It is predicted that this logic voltage level will further decrease to the sub-one volt level in the next few years. Similarly, it is anticipated that the masking process will decrease from 1.5 microns to less than a few hundredths of a micron in the next few years.
A disadvantage of this increased scale of integration is that semiconductor devices, in particular metal oxide semiconductor (MOS) transistors, are now more susceptible to damage by over-voltage transient pulses. These pulses are generally caused by electrostatic discharge pulses or positive or negative voltage spikes generated during switching or just caused by the inductive effect of a transmission line impedance. A further cause of damage is as a result of accidental connection to the wrong voltage supply levels as, for example, by connecting a 5V supply voltage to a 2.5V or a 1.5V input on a device.
Many solutions to this problem have been proposed, such as the use of a single P-N (transient voltage suppressor) TVS/Zener device or clamp. However, these devices are ineffective in these low voltage applications since it is difficult to achieve low leakage levels at supply voltages below 5V. Another solution is to use steering diode line termination circuitry. However, these are effective to minimize the overshoot and undershoot switching events that could be absorbed in part by the power supply, but under an extremely fast transient event, as in an ESD, the circuitry could be damaged.
Currently some semiconductor component manufacturers, like SEMTECH Corp, offer very low leakage TVS/Zener devices, such as their part No. SLVU2.8, which is capable of handling ESD and transient protection levels intended for 3.3V systems. These devices are based on Low-voltage Punch-Through concept as described in U.S. Pat. No. 6,015,999.
A disadvantage of the above described devices is that they must be coupled in series with a rectifier diode in order to achieve capacitance levels in the order of 10 pF or below, so as to exhibit a low insertion loss.
These low capacitance values are necessary in order that the protection devices can be used to protect against ESD and transient events on high speed transmission data line ports, such as 10/100/1000Ethernet, USB1.1 and 2.0 and FireWire-IEEE 1394 or similar.
Other device solutions are described in U.S. Pat. Nos. 6,442,008; 6,163,446; 6,492,859; 6,268,639; 6,320,735; 6,015,999 and 6,268,990. However, for various reasons these devices do not exhibit the low leakage and low capacitance required for low voltage applications.
Accordingly there is a need for a protection device that exhibits low capacitance, low leakage at low voltages and that can preferably be used to protect high speed transmission lines.
BRIEF SUMMARY OF THE INVENTIONAn advantage of the present invention is to provide transient protection device for low voltage circuits.
In a preferred embodiment the device may be used as a transient voltage suppressor or ESD protector for high speed data transmission lines.
In accordance with this invention there is provided a semiconductor device for coupling a transient voltage from an input node to a reference node, the device comprising:
- a) a bipolar transistor adapted to couple its collector to the input node and its emitter to the reference node; and
- b) a driver device adapted to be coupled between the input node and the base terminal of the transistor such that the driver device is responsive to a transient voltage at the input node to turn on the transistor, thereby shunting the transient voltage to the reference node.
In a preferred embodiment the input node is coupled to a high speed data transmission line and the reference node is coupled to ground.
It is further preferred that the transmission line operates below 5v and the transistor is an NPN transistor.
The driver may preferably be a gate-drain connected MOS transistor with its gate-drain terminal coupled to the collector terminal of the transistor and its source terminal coupled to the base terminal of the transistor. Alternatively the driver may be a light emitting diode (LED) or any other diode with a different material (band-gap) and die size than the LED, connected to the bipolar transistor to create a low voltage clamping device.
An advantage of the present invention is that it provides a low voltage clamping device having a stable breakdown or “zener” voltage characteristic in applications operating at low voltages while providing low leakage and low capacitance.
BRIEF DESCRIPTION OF THE DRAWINGSEmbodiments of the invention may best be understood by referring to the following description and accompanying drawings wherein:
FIGS. 1(a) and (b) are schematic diagrams of a device configuration according to embodiments of the present invention;
FIGS. 2(a) and (b) are schematic diagrams of a device configuration according to further embodiments of the present invention;
FIGS. 3(a) and (b) are schematic diagrams showing operation of the devices of FIGS. 1(a) and (b);
FIGS. 4(a), (b) and (c) show schematically voltage-current traces for the device of the present invention;
FIGS. 5(a), (b) and (c) show respectively a die layout, a schematic diagram and equivalent circuit for a two device configuration according to the present invention; and
In the following description, numerous specific details are set forth to provide a thorough understanding of the invention. However, it is understood that the invention may be practiced without these specific details. In other instances, well-known structures or and/or processes have not been described or shown in detail in order not to obscure the invention. In the description and drawings, like numerals refer to like structures or processes. The operation of semiconductor devices such as MOS and bipolar transistor devices are well known in the art and will not be described further except where necessary to clarify aspects of the invention.
Referring to
Referring to
Referring to FIGS. 2(a) and (b), third and fourth embodiments 130, 140, which are similar devices to those of FIGS. 1(a) and (b) are shown; however in these embodiments the N-channel MOSFETs 108, 128 are replaced by LEDs 138, 148. In
The operation of the devices of the present invention may be explained by referring to FIGS. 3(a) and 3(b) in which the devices correspond to the embodiments 120, 140 illustrated with respect to FIGS. 1(b) and 2(b), respectively. In general either the N-channel MOSFET 128 or the LED 148 are the driver devices when in combination with the bipolar transistor 126. The MOSFET is connected as described to convert it to a voltage reference, having its drain-to-source voltage fixed and being equivalent to the gate-to source threshold voltage, typically around 2V to 3V. This has a sharp voltage knee characteristic. In the case of the LED, the voltage drop across the device is typically 1.7V for conventional red, green or amber LED's, and 3.6V for Blue and White LED's.
Referring to FIGS. 3(a) and 3(b), it is assumed that the PNP Transistor combinations are connected between a voltage bus represented by +Vs (input node) and ground (reference node). In the drawings, VEB is the emitter-base voltage for the transistor; IC, IB and IE are the corresponding collector, base and emitter currents; VF is the forward voltage and VGS is the gate-source threshold voltage of the MOSFET and IDS is its drain-source current. In normal operation the device is in the off-state, that is
Vs<VEB+VGS(TH) where VEC=VEB+VGS(TH)
or,
Vs<VEB+VF where VEC=VEB+VF
In this off-state condition there is a negligible current flowing from the emitter to collector of the transistor since there is not enough current flowing through the base of the transistor to turn it on. If Vs is increased enough to be equal the emitter-collector voltage, that is
Vs+Delta Vs=VEC
the device starts to act as a TVS/Zener diode with a “Zener voltage” determined by the VEC (that is equal to VEB+VGS(TH) for the MOSFET or VEB+VF for the LED), and an initial small current flows (order of microamperes) from the emitter to the collector, since there is now sufficient current flowing through the transistor base. As the voltage between emitter (or the “cathode”) and the collector (or the “anode”) goes higher, the current also flowing through the emitter-collector is higher because more base current is also flowing through it. The transistor gain acts to “square” the diode forward characteristic, or the gate-to source voltage characteristic in case of the MOS transistor configurations shown in FIGS. 1(a) and 1(b).
Referring to FIGS. 4(a) to (c), there is shown the emitter-collector voltage versus current characteristic traces at three levels of current: namely 2 microamps; 200 microamps 10 milliamps, showing graphically the operation of the devices for a Red LED and a PNP transistor combination of the present invention.
In terms of off-state capacitance, it has been observed that the MOSFET-bipolar or LED-bipolar device combinations of the present invention depend merely on the output capacitance of the transistor, which can be tailored to be smaller than any low voltage TVS/Zener.
The transistor may be selected to be small enough to have an off-state capacitance of around 6-7 pF, with a maximum of 10 pF. This is significantly lower than that of a conventional 2.5V ½ W Zener, which has 290 pF, or the punch-through structure used on the SLVU2.8 (without the diode in series) of 30 pF, at 1 MHz 0.1 Vrms signal and zero dc bias.
The lower capacitance value represents less insertion loss at upper band frequencies required on a USB (6 MHz and 240 MHz) transmission line or FireWire (200 MHz).
The flexibility of the devices of the present invention allows also for the increase in the surge voltage by changing the bipolar transistor size, thereby reducing the switching resistance.
An advantage of the present invention is that it can be easily constructed using standard components that allow the number of devices to be easily scaled for a particular application. For example, FIGS. 5(a), (b) and (c) show respectively a die layout, a corresponding schematic diagram and equivalent circuit for a two device configuration according to the present invention.
In general the devices are unidirectional, but could offer a bi-directional protection (for negative spikes) if connected as a pair in anti-parallel. The maximum blocking voltage in reverse mode is about 7-8V but is not intended to operate in reverse mode. In terms of insertion loss the device is fully dependable on the transmission line used. Under USB 1.1 or 2.0 or IEEE 1394 (Fire Wire) the device could add no more than 3 dB attenuation at 240 MHz.
Although preferred embodiments of the invention have been described herein, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.
Claims
1. A semiconductor device for coupling a transient voltage from an input node to a reference node, the device comprising:
- a) a bipolar transistor adapted to couple its collector to said input node and its emitter to the reference node; and
- b) a driver device adapted to be coupled between the input node and the base terminal of the transistor such that the driver device is responsive to a transient voltage at the input node to turn on the transistor, thereby shunting the transient voltage to the reference node.
2. A device as defined in claim 1, said transistor being an NPN device.
3. A device as defined in claim 1, said transistor being an PNP device.
4. A device as defined in claim 1, said driver device being a gate-drain coupled n-channel MOSFET device.
5. A device as defined in claim 1, said driver device being an LED.
6. A device as defined in claim 1, said input node being coupled to a high speed data transmission line.
7. A device as defined in claim 6 said transmission line operating below 5v and the reference node being coupled to ground terminal.
8. A method for coupling a transient voltage from an input node to a reference node, said method comprising the steps of:
- a) coupling a collector terminal of a bipolar transistor to the input node and its emitter terminal to the reference node; and
- b) coupling a driver device between the input node and the base terminal of the transistor such that the driver device is responsive to a transient voltage at the input node to turn on the transistor, thereby shunting the transient voltage to the reference node.
9. A device as defined in claim 1, said device having a plurality of input nodes and a corresponding number of transistor and driver devices.
Type: Application
Filed: Mar 9, 2005
Publication Date: Apr 20, 2006
Inventors: Alfredo Ochoa (Cave Creek, AZ), George Templeton (Tempe, AZ), James Washburn (Scottsdale, AZ)
Application Number: 11/075,416
International Classification: H02H 9/00 (20060101);
