Abstract:
An optimized power output clamping structure, includes a power output transistor having a first breakdown voltage and a breakdown structure having a second breakdown voltage coupled to the power output transistor. The second breakdown voltage is less than the first breakdown voltage and follows the first breakdown voltage across all temperature and semiconductor process variations. This feature allows a reduction in breakdown voltage guardbanding and increases output structure reliability. A method of protecting a circuit from inductive flyback is also disclosed. The method includes the steps of driving an inductive load with drive circuitry, turning off the inductive load, and clamping an inductive voltage at a voltage magnitude that protects the drive circuitry from breakdown across all temperature and processing variations.

Description:
FIELD OF THE INVENTION 
     This invention is in the area of integrated circuits and is more particularly related to output clamping mechanisms in power integrated circuits. 
     BACKGROUND OF THE INVENTION 
     Many power output structures are required to dissipated energy stored in inductive loads that are being driven by the power output structure. A prior art power output circuit 10 is illustrated in prior art FIG. 1. A power MOS transistor 14 is connected in series with an inductive load 12 and a power supply Vcc. Power MOS transistor 14 also is connected to circuit ground. Power MOS transistor 14 has a diode 16 connected in parallel. Diode 16 typically is implicit in the power MOS transistor layout itself. The mode of the inductive transient is as follows: when a power output which is driving an inductive load is turned off, the voltage at the load begins to rise for a low side driver output configuration and fall for a high side driver output configuration. In either case, the voltage across the power output device increases until the output structure breaks down. When this occurs the energy stored in inductive load 12 is snubbed to ground through the output device. In such a configuration, power MOS transistor 14 is forced to dissipate the inductive energy in a reverse biased mode of operation. In this manner, the breakdown is localized to areas within power MOS transistor 14 structure that have the lowest voltage junction breakdown, thus overall energy handling capability of the output structure is minimized. 
     Prior art FIG. 2 illustrates another common solution that attempts to increase the energy handling capability of power MOS transistor 14. Prior art circuit 20 of FIG. 2 adds a zener diode 18 between the drain and gate of power MOS transistor 14. Zener diode 18 is designed to breakdown before power MOS transistor 14. In so doing, the zener diode breaks down, thereby charging the gate of power MOS transistor 14 and causing power MOS transistor 14 to turn on in its forward biased mode. In this operation, the inductive energy is dissipated more efficiently and the reliability of power MOS transistor 14 is not compromised. Prior art circuit 20 has a disadvantage in that it has a substantially lower breakdown voltage which is undesirable and well known and understood by those skilled in the art. All present schemes for designing the zener diode, due to processing and temperature variations, require a significant amount of voltage headroom to ensure that, in every case, zener diode 18 breaks down before power MOS transistor 14. Therefore the breakdown voltage of circuit 20 is greatly reduced. 
     It is an object of this invention to provide a means and methodology for improving the breakdown voltage of the output circuit while simultaneously maintaining a rugged output structure that can withstand substantial inductive energy transients. 
     It is another object of the invention to provide an output circuit and method that improves the breakdown voltage without substantially increasing circuit area which in prior art solutions has been directly proportional to the output breakdown voltage. 
     Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification and drawings. 
     SUMMARY OF THE INVENTION 
     An optimized power output clamping structure, includes a power output transistor having a first breakdown voltage and a breakdown structure having a second breakdown voltage coupled to the power output transistor. The second breakdown voltage is less than the first breakdown voltage and follows the first breakdown voltage across all temperature and semiconductor process variations. This feature allows a reduction in breakdown voltage guardbanding and increases output structure reliability. A method of protecting a circuit from inductive flyback is also disclosed. The method includes the steps of driving an inductive load with drive circuitry, turning off the inductive load, and clamping an inductive voltage at a voltage magnitude that protects the drive circuitry from breakdown across all temperature and processing variations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art circuit diagram illustrating an output circuit 10. 
     FIG. 2 is another prior art circuit diagram illustrating another output circuit 20. 
     FIG. 3 is a circuit diagram illustrating an embodiment of the invention 30 in a low side driver configuration. 
     FIG. 4 is a circuit diagram illustrating another embodiment of the invention 40 in a low side driver configuration. 
     FIG. 5 is a cross-sectional diagram illustrating a RESURF DMOS transistor 50. 
     FIG. 5a is a cross-sectional diagram illustrating a standard DMOS transistor 70 with an added drift region enhancer 55. 
     FIG. 6 is a circuit diagram illustrating an embodiment of the invention in a high side driver configuration 60. 
     FIG. 7 is a circuit device layout diagram illustrating circuit 30 in a layout embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3 is a schematic diagram illustrating a circuit 30 as an embodiment of the invention. Circuit 30 is a low side driver configuration. Circuit 30 has an NMOS transistor 22 coupled in parallel with a diode 26. NMOS transistor, in this particular embodiment is a power DMOS type transistor and diode 26 is implicit in transistor 22&#39;s semiconductor layout. However, it should be understood that all types of transistors such as PMOS or bipolar transistors would fall within the scope of this invention and may or may not have diode implicit in the device layouts. Transistor 22 is connected in series between a circuit ground potential and an inductive load 12. Inductive load 12 is also connected to a voltage supply potential Vcc. Although this particular implements circuit 30 with a load connected to a voltage supply Vcc, it should be understood that any appropriate voltage potential falls within the scope of this invention. A second NMOS transistor 24 is connected in series with a blocking diode 25. Another diode 28 is connected in parallel with transistor 24. Again, diode 28 is implicit in the device layout of transistor 24, however a separate, diode structure falls within the scope of this invention. Transistor 24 and diode 25 are connected between the drain terminal and the gate terminal of transistor 22. The gate of transistor 22 is also coupled to input type circuitry that is not shown. The input type circuitry provides appropriate output circuitry control for switching inductive load 12 and is well known by those skilled in the art of circuit design. 
     FIG. 4 is a schematic diagram illustrating a circuit 40 as another embodiment of the invention. Circuit 40 is a low side driver configuration. Circuit 40 has NMOS transistor 22 coupled in parallel with diode 26. Transistor 22 is connected in series between circuit ground potential and inductive load 12. Inductive load 12 is also connected to voltage supply Vcc. Second NMOS transistor 24 is connected in series with diode 25. Transistor 24 and diode 25 are connected between the drain terminal and the gate terminal of transistor 22. Diode 28 is connected in parallel with transistor 24. A zener diode 32 is connected in parallel with transistor 24 and diode 25. The gate of transistor 22 is coupled to input type circuitry that is not shown. 
     FIG. 5 is a cross-sectional diagram illustrating a RESURF type power lateral DMOS transistor 50. Transistor 50 has a P-type DWELL region 54 and an N-type RESURF drift region 56 formed in a semiconductor substrate 52. A source region is formed in DWELL region 54 with N+ region 62. A P+ region 60 acts as a backgate connection A drain region is formed within drift region 56 with N+ region 58. An oxide layer 64 is formed over the face of circuit 50 and contact holes are made for the source backgate and drain to source backgate region 60 and 62 and drain region 58. A polysilicon gate 66 is selectively formed over oxide layer 64 to form the gate of transistor 50. A lateral distance labelled &#34;X&#34; is illustrated in drift region 56. 
     FIG. 5a is a cross-sectional diagram illustrating, a standard LDMOS type power transistor 70 having an enhanced drift region diffusion 55. Transistor 70 has an N type region 53 formed over semiconductor substrate 52. N type region 53 has a P-type DWELL region 54 and a low voltage N-tank region 55 (LV N-TANK) formed within it Source region is formed in DWELL region 54 with P+ region 60 and N+ region 62. Drain region is formed within LV N-TANK region 55 with N+ region 58. Oxide layer 64 is formed over the face of circuit 70 and contact holes are made for the source backgate and drain to source backgate region 60 and 62 and drain region 58. Polysilicon gate 66 is selectively formed over oxide layer 64 to form the gate of transistor 70. A lateral distance labelled &#34;Y&#34; is illustrated in LV N-TANK region 55. 
     FIG. 6 is a schematic diagram illustrating a circuit 60 as another embodiment of the invention. Circuit 60 is a high side driver configuration. Transistor 22 is connected in parallel with diode 26. Transistor 22 is connected in series with inductive load 12 and voltage supply potential Vcc. Transistor 24 is connected in parallel with diode 28. Transistor 24 is connected in series with a blocking diode 68. Transistor 24 and diode 68 are connected between the drain terminal and gate terminal of transistor 60. A control switch circuit 72 is connected between the gate terminal and source terminal of transistor 22. Control switch circuit 72 is also connected to input control circuitry (not shown) for controlling the switching of circuit 60. 
     Circuit 60 would also operate using vertical DMOS structures as well as for NPN and PNP bipolar structures. This invention contemplates all types of transistor output structures that utilize an alteration in design spacing to ensure breakdown protection across all temperature and semiconductor processing variations. 
     FIG. 7 is a top view diagram illustrating a semiconductor circuit layout of circuit 30 of FIG. 3. FIG. 7 illustrates a multi-fingered output DMOS type transistor 22 with transistor 24 embedded within transistor 22 layout. Having transistor 24 physically embedded within transistor 22 layout provides improved thermal following by ensuring that both transistor 22 and 24 are experiencing identical thermal gradients that exist across semiconductor dies. 
     Circuit 30 of FIG. 3 operates in the following manner. When transistor 22 is conducting input circuitry (not shown) has the gate terminal at a sufficient voltage to keep transistor 22 on. The drain terminal, during conduction of transistor 22, is approximately at V DRAIN  =I DRAIN  X Rds(on) and is therefore dependent upon the product of the drain current and the Rds(on) of transistor 22. Blocking diode 25 acts to prevent current from flowing through diode 28 to transistor 22 when transistor 22 is on. When input circuitry turns transistor 22 off the gate voltage drops to approximately circuit ground potential When transistor 22 stops conducting inductive flyback occurs at its drain terminal because current cannot change instantaneously through an inductor. Transistor 24 is designed with the same semiconductor device layout as transistor 22 except it has different device spacings that ensure it has a breakdown voltage that is less than the breakdown voltage of transistor 22. Such variations in device spacings may be accomplished in various ways. One manner of modifying layout spacings for transistor 24 is illustrated in FIGS. 5 and 5a. In FIG. 5 a RESURF DMOS transistor 50 may modify its RESURF drift region spacing labelled &#34;X&#34; to vary its device breakdown. Similarly, in FIG. 5a a standard DMOS transistor 70 may vary the lateral spacing labelled &#34;Y&#34; (the spacing between tanks 54 and 55). Other spacing modifications that vary breakdown voltage also fall within the scope of this invention. Therefore, by modifying transistor 24&#39;s layout spacing one can ensure that its breakdown voltage (BV dss1 ) is lower in magnitude than the breakdown voltage of transistor 22 (BV dss2 ). When transistor 24 breaks down current flows through transistor 24 and diode 25 to the gate of transistor 22, which charges the gate capacitance and thereby increases the voltage at the gate of transistor 22. When the gate voltage reaches a threshold voltage above ground potential, transistor 22 turns on again and discharges the energy stored in inductive load 12. In this manner, the energy stored in inductive load 12 is discharged through transistor 22 while transistor 22 is in its forward biased mode, rather than in its reverse biased breakdown mode. Therefore the energy stored in inductive load is discharged in an efficient manner. 
     Circuit 30 presents a substantial improvement over prior art solutions such as prior art zener stack circuit 20 of FIG. 2. Prior art circuit 20 has a zener diode 18 that has a breakdown voltage V Z  that varies over temperature and process variations in a manner substantially different from the breakdown voltage variation of transistor 14 (BV dss ). In other words, the temperature coefficients of zener diode 18 and transistor 14 vary from one another and the component&#39;s respective breakdown voltage characteristics vary differently over semiconductor process variations. Therefore, to ensure zener diode breaks down prior to BV dss  in FIG. 2 across all temperature and process variations, substantial guardband is required for zener diode 18, thereby decreasing circuit performance as is well known by those skilled in the art. Circuit 30, however, has transistor 24 that has a breakdown voltage BV dss1  that directly follows the breakdown voltage of transistor 22 (BV dss2 ) across all temperature and processing variations because the breakdown voltage is directly proportional to the device layout spacing. The thermal following is achieved since the physics impacting the breakdown voltage are the same for the like devices. Therefore, by design (varying the semiconductor device layout spacing), transistor 24 may provide effective, efficient breakdown protection of transistor 22 without unnecessary guardband protection, thereby substantially improving circuit performance and reliability. 
     In circuit 40 of FIG. 4 a zener diode 32 is coupled in parallel with transistor 24. This circuit implementation may be desirable when a maximum range is specified for the output breakdown voltage. This type of breakdown circuitry may be beneficial if, for example, the breakdown voltage of transistor 22 (BV dss2 ) was in the range of 60-75V over all process and temperature variations and an application specification required an output breakdown voltage from 60-65V. Zener diode may be centered at 63V and with process and temperature variations range from 61-65V. In this example, if the breakdown voltage of transistor 22 is 60V then transistor 24 will by design due to variations in device layout spacings breakdown before it. Yet if, due to semiconductor processing variation, the breakdown voltage of transistor 22 is 75V, zener diode 32 will ensure that it breaks down at a voltage no higher than 65V. 
     FIG. 6 is a high side driver solution that utilizes the same novel temperature and process following methodology to ensure low guardbanded, efficient breakdown protection for circuit 60. When input circuitry (not shown) is driving output circuit 60, control switch 72 has an effective closed circuit between the input circuitry and the gate of transistor 22 and an effective open circuit between the gate and source of transistor 22. When input circuitry turns off transistor 22, control switch 72 creates an effective resistive short circuit between the gate and source of transistor 22, thus discontinuing its conduction. When transistor 22 turns off, the source of transistor 22 goes low due to the inductive flyback of inductive load 12. Because an approximate short circuit (yet still somewhat resistive) exists between the gate and source of transistor 22, transistor 24 and blocking diode 68 are effectively in parallel with transistor 22 and will breakdown first because transistor 24 is designed to have a lower breakdown. Therefore &lt;BV dss1  &lt;BV dss2 . As stated above, transistor 24 may be altered as illustrated in FIG. 5 or 5a to have a lower breakdown voltage by design. Further, the breakdown voltage of transistor 24 will follow the breakdown voltage of transistor 22 across all temperature and processing characteristics. When transistor 24 breaks down it charges the gate of transistor 22 to a higher potential than the source and transistor 22 turns back on, effectively dissipating the energy stored in inductive load efficiently in a forward biased mode. 
     In FIG. 7, circuit 30 of FIG. 3 is illustrated in a semiconductor circuit layout. More specifically, FIG. 7 illustrates a device layout of transistor 22. Transistor 22 has multiple drain diffusions 82 and source diffusions 84. Source diffusions appear surrounded by polysilicon gates 86. However, as is well known by those skilled in the art, top view layouts do not provide a fully accurate representation of a transistor&#39;s cross section. In FIG. 7, breakdown transistor 24 is embedded within transistor 22. Therefore, the thermal gradients experienced by transistor 22 will also be experienced by transistor 24. This feature allows guardbanding between the breakdown voltages of transistors 22 and 24 to be reduced even further. 
     Alternatively, thermal effects on breakdown voltage can be further reduced by laying out multiple, small LDMOS transistors within the large LDMOS structure, thereby making the breakdown performance even less sensitive to localized thermal gradients. 
     Although the invention has been described with reference to the embodiments herein, this description is not to be construed in a limiting sense. Various modifications of the disclosed embodiments will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.