# Patent

## Title
Efficient reduction of electromagnetic emission in LIN driver

## Abstract

A Local Interconnect Network (LIN) driver circuit employs a charging/discharging current applied to the gate of a driver transistor coupled to an LIN bus. The charging current includes a constant charging current and an additional soft charging current, whereas the discharging current includes a constant discharging current and an additional soft discharging current. As a result of the soft charge/discharge components, there is a significant reduction in electromagnetic emission on the LIN bus.

## Background
PRIORITY CLAIM
This application claims priority from Chinese Application for Patent No. 201110461475.7 filed Dec. 30, 2011, the disclosure of which is incorporated by reference.
BACKGROUND
1. Technical Field
The present invention relates generally to Local Interconnect Network (LIN) circuitry and, more specifically, to reducing electromagnetic emission in an integrated LIN driver.
2. Introduction
Industry specifications for LIN drivers typically mandate that a transmitted signal be shaped with a predefined slope in order to reduce electromagnetic emission at high frequencies. Unfortunately, the requested small slope oftentimes results in an invalid duty cycle, which consequently results in an error in data transmission. In an attempt to solve this issue, many conventional LIN drivers employ a step current as a gate charging/discharging current. However, the sharp edges of the step current contribute to a significant level of radiated emission on the LIN bus. Therefore, there exists a need for an LIN driver operable to ensure signal integrity while achieving reduced electromagnetic emission on the LIN bus.
SUMMARY
The present disclosure provides an integrated LIN driver having reduced electromagnetic emission. One embodiment provides a driver circuit comprising: a driver transistor having a gate terminal; a first circuit operable in response to a first phase of a clock signal to generate a gate charging current for application to the gate terminal; and a second circuit operable in response to a second phase of the clock signal to generate a gate discharging current for application to the gate terminal; wherein said gate charging current comprises a sum of a first exponentially decreasing current and a first constant current; and wherein said gate discharging current comprises a sum of a second exponentially decreasing current and a second constant current.
In another embodiment, the LIN driver circuit comprises: charging circuitry operable to receive a first charging current and generate a second charging current; discharging circuitry operable to receive a first discharging current and generate a second discharging current; and an output transistor operable to receive a gate current, wherein during a charging phase, said gate current comprises said first charging current and said second charging current, and wherein during a discharging phase, said gate current comprises said first discharging current and said second discharging current; wherein during said charging phase, said second charging current dissipates such that said gate current substantially comprises said first charging current; and wherein during said discharging phase, said second discharging current dissipates such that said gate current substantially comprises said first discharging current.
Yet another embodiment of the present disclosure provides a driver circuit comprising: a driver transistor having a gate terminal; a first circuit comprising a first capacitor coupled to a first transistor and a first switch, said first circuit operable to receive a first constant current and generate a first exponentially decreasing current; a second switch operable to couple said first circuit to said gate terminal to apply a gate charging current to said gate terminal; a second circuit coupled to said gate terminal, said second circuit comprising a third switch and a second capacitor, said second circuit operable to generate a second exponentially decreasing current; and a fourth switch coupled to said gate terminal and said second circuitry, wherein said third and fourth switches are operable to apply a gate discharging current to said gate terminal; wherein said gate charging current comprises a sum of said first exponentially decreasing current and said first constant current; and wherein said gate discharging current comprises a sum of said second exponentially decreasing current and a second constant current.
Another embodiment of the present disclosure provides a method for reducing electromagnetic emission in a driver circuit, the method comprising: generating a first exponentially decreasing current in response to a first phase of a clock signal; generating a second exponentially decreasing current in response to a second phase of the clock signal; applying a charging current at a gate terminal of a driver transistor, wherein said charging current comprises a sum of a first constant current and said first exponentially decreasing current; and applying a discharging current at said gate terminal of said driver transistor, wherein said discharging current comprises a sum of a second constant current and said second exponentially decreasing current.
The foregoing and other features and advantages of the present disclosure will become further apparent from the following detailed description of the embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the disclosure, rather than limiting the scope of the invention as defined by the appended claims and equivalents thereof.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments are illustrated by way of example in the accompanying figures not necessarily drawn to scale, in which like reference numbers indicate similar parts, and in which:
FIGS. 1A and 1B illustrate an example embodiment of a conventional LIN driver circuit and the timing diagram and waveforms corresponding to the conventional LIN driver circuit;
FIGS. 2A and 2B illustrate an LIN driver circuit in accordance with an embodiment of the present disclosure and the timing diagram and waveforms corresponding to the disclosed LIN driver circuit; and
FIG. 3 illustrates a comparison of LIN emission simulation results for the LIN driver circuits shown in FIGS. 1A and 2A.



DETAILED DESCRIPTION OF THE DRAWINGS
Reference is made to FIGS. 1A and 1B, wherein an example embodiment of a conventional LIN driver circuit 100 is shown in FIG. 1A, and is discussed herein in combination with its corresponding timing diagram and waveforms 150 illustrated in FIG. 1B. The conventional LIN driver circuit 100 comprises a reference current source 102 supplying reference current IR to a first current mirror 104, comprised of transistors M1 and M2. Transistor M1 may be sized to transistor M2 to set mirrored reference current IR′, which is supplied to a second current mirror 206 comprised of transistors M3, M4, and M5. Transistor M4 may be sized relative to transistor M3 to set a constant charge current I1, which is operably connected to the gate of a transistor MD operating as a low side driver by a switch S1 operating on a phase of a first clock signal Φ1. Transistor M5 may be sized relative to transistor M3 to set an additional charge current I1′, which is operably connected to the gate of the driver transistor MD by a second switch S2 operating on a second clock signal Φ1′. In an example embodiment of the circuit 100 illustrated in FIG. 1A, IR=5 uA, IR′=10 uA, I1=20 uA, and I1′=7 uA.
Also included in the first current mirror 104 are transistors M6 and M7. Transistor M6 may be sized relative to transistor M1 to set a constant discharging current I2, which is operably connected to the gate of the driver transistor MD by a third switch S3 operating on an opposite phase of the first clock signal Φ1. Transistor M7 may be sized relative to transistor M2 to set an additional discharge current I2′, which is operably connected to the gate of the driver 108 by a fourth switch S4 operating on a third clock signal Φ1′′. In an example embodiment of the circuit 100 illustrated in FIG. 1A, I2=20 uA and I2′=40 uA.
The LIN driver circuit 100 also includes resistor R1, as well as capacitor C0 coupled in parallel with series connected transistor M8 and resistor R2, which are coupled to the gate of the driver transistor MD. Capacitor C0 is provided to absorb EMC gate coupling of the driver transistor MD, and is also used with resistor R1 to smooth gate charge and discharge of the driver transistor MD. Transistor M8 operates with the driver transistor MD to provide a current mirror, wherein the resistor R2 is provided to adjust the mirror factor. In an example embodiment of the circuit 100 illustrated in FIG. 1A, C0=20 pF.
The source of the driver transistor MD is coupled to the LIN bus 108 via a first blocking diode D1 operable to block current flowing into the LIN bus 108 from ground. The driver circuit 100 also includes a second blocking diode D2 and pull-up resistor R3 coupled between a supply voltage VS and the LIN bus 108, wherein the second blocking diode D2 operates to block current from the LIN bus 108 from reaching the voltage supply VS.
The conventional LIN driver 100 employs step current IGATE as a gate charging/discharging current for the driving transistor MD. In order to achieve the step current IGATE, two constant currents are intermittently switched on and off, thereby switching between the constant charging current I1 and the constant discharging current I2 applied to the gate of MD as step current IGATE. As shown in FIG. 1B, phase clock signal Φ1 is high during the charging phase and low during the discharging phase. Accordingly, charging current I1 is switched on (i.e., applied to the gate of MD) during the charging phase (when Φ1 is high), and is switched off during the discharging phase (when Φ1 is low). Conversely, discharging current I2 is switched off during the charging phase, and is switched on during the discharging phase.
During initial periods of the respective charging and discharging phases, an additional current is switched on to reduce turn-on/off delay time of the driver transistor MD. As illustrated by the timing diagram and waveforms 150 illustrated in FIG. 1B, during the initial period of a charging phase, phase clock signal Φ1′ is high and additional charging current I1′ is switched on. Therefore, when Φ1 and Φ1′ are high, both charging current I1 and additional charging current I1′ are applied to the gate of the driver transistor MD. In other words, when Φ1 and Φ1′ are high, the step current IGATE is the sum of charging current I1 and additional charging current I1′. When Φ1′ goes low and Φ1 remains high, the additional charging current I1′ is switched off and only the charging current I1 is applied to the gate of the driver transistor MD (i.e., IGATE=I1). During the initial period of a discharging phase, phase clock signal Φ1′′ is high and Φ1 is low (Φ1(bar) is high), both discharging current I2 and additional discharging current I2′ are switched on (i.e., IGATE=I2+I2′). When Φ1′′ goes low and Φ1 is low, additional discharging current I2′ is switched off and only discharging current I2 is applied to the gate of MD (i.e., IGATE=I2). Accordingly, the circuit 100 in FIG. 1A produces a step gate current IGATE having sharp edges 160 caused by switching off the additional charging/discharging currents I1′/I2′ responsive to the switching of additional phase clock signals Φ1′ and Φ1′′. The step gate current IGATE is applied to the gate of transistor MD to produce the LIN bus signal 165 shown in FIG. 1B. The embodiment illustrated in FIGS. 1A and 1B produces satisfactory duty cycle with the shortened delay time, however, performance of radiated emission on the LIN bus is strongly degraded due to the sharp edges 160 in the step current IGATE.
FIG. 2A illustrates an LIN driver circuit 200 in accordance with an example embodiment of the present disclosure. The disclosed LIN driver circuit 200 comprises a reference current source 202 supplying reference current IR to a first current mirror 204, comprised of transistors M11, M12, and M13. Transistor M11 may be sized to transistor M12 to set mirrored reference current IR′, which is supplied to a second current mirror 206 comprised of transistors M14 and M15. Transistor M15 may be sized relative to transistor M14 to set a constant charge current I1, and transistor M13 may be sized relative to transistor M11 to set a constant discharge current I2. In an example embodiment of the circuit 200 illustrated in FIG. 2A, IR=5 uA, IR′=10 uA, I1=20 uA, and I2=20 uA.
Coupled to the second current mirror 206 is charging circuitry 210. The charging circuitry 210 comprises a switch 212 operating on an opposite phase of a clock signal Φ1, and a diode connected transistor 214 coupled in parallel with series connected capacitor C1 and resistor RC. The charging circuitry 210 and second current mirror 206 are coupled via switch 216 to the gate of a transistor MD operating as a low side driver, wherein switch 216 operates on a phase of the clock signal Φ1.
Discharging circuitry 220 is coupled to the first current mirror 204 by switch 218 operating on the opposite phase of the clock signal Φ1. The discharging circuitry 220 comprises a switch 222 coupled in series with a second capacitor C2 and resistor RD, wherein switch 222 operates on the opposite phase of the clock signal Φ1.
The disclosed LIN driver circuit 200 also includes resistor R1, as well as capacitor C0 coupled in parallel with series connected transistor M16 and resistor R2, which are coupled to the gate of the driver transistor MD. As mentioned above, capacitor C0 is provided to absorb EMC gate coupling of the driver transistor MD, and is also used with resistor R1 to smooth gate charge and discharge of the driver transistor MD. Transistor M16 operates with the driver transistor MD to provide a current mirror, wherein the resistor R2 is provided to adjust the mirror factor. In an example embodiment of the circuit 200 illustrated in FIG. 2A, C0=20 pF.
The source of the driver transistor MD is coupled to the LIN bus 230 via a first blocking diode D1 operable to block current flowing into the LIN bus 230 from ground. The driver circuit 200 also includes a second blocking diode D2 and pull-up resistor R3 coupled between a supply voltage VS and the LIN bus 230, wherein the second blocking diode D2 operates to block current from the LIN bus 230 from reaching the voltage supply VS.
The LIN driver circuit 200 illustrated in FIG. 2A is similar to that shown in FIG. 1A in that the LIN driver circuit 200 employs a charging/discharging current IGATE2 applied to the gate of the driver transistor MD coupled to the LIN bus 230. Like the LIN driver circuit 100 in FIG. 1A, the LIN driver circuit 200 achieves satisfactory duty cycle by shortening delay time. However, when compared to the LIN driver circuit 100 in FIG. 1A, the LIN driver circuit 200 illustrated in FIG. 2A provides a significant reduction in electromagnetic emission by replacing the sharp edges 160 shown in FIG. 1B, with a smooth, step-less current change as shown by the timing diagram and waveforms 250 illustrated in FIG. 2B.
In order to achieve the current IGATE2, charging and discharging currents are intermittently switched on and off, thereby switching between a charging current and a discharging current applied to the gate of MD as gate current IGATE2. During a charging phase, the gate current IGATE2 initially comprises the sum of the constant charge current I1 and an additional soft charge current I1′. During a discharging phase, the gate current IGATE2 initially comprises the sum of the constant discharge current I2 and an additional soft discharge current I2′. The charging and discharging phases are described in greater detail below with reference to the timing diagram and waveforms 250 illustrated in FIG. 2B.
The phase clock signal Φ1 represents the charging/discharging phases, wherein Φ1 is high during the charging phase and low during the discharging phase. Accordingly, charging currents I1 and I1′ are switched on (i.e., applied to the gate of MD) during the charging phase (when Φ1 is high), and are switched off during the discharging phase (when Φ1 is low). Conversely, discharging currents I2 and I2′ are switched off during the charging phase, and are switched on during the discharging phase.
Referring again to the circuit 200 illustrated in FIG. 2A, additional soft charging current I1′ is provided by charging circuitry 210. During the gate discharging phase (when Φ1 is low), switch 212 closes and capacitor C1 is charged by the constant charge current I1 to voltage VGS. During the charging phase (when Φ1 is high), switch 216 closes (and switch 212 opens) and the charge stored on capacitor C1 is transferred as the additional soft charge current I1′ to the gate of MD (along with charge current I1). Accordingly, during the charging phase, gate current IGATE2 is comprised of the sum of constant charge current I1 and additional soft charge current I1′. As the charge on the capacitor C1 dissipates, additional soft charge current I1′ decreases exponentially until the gate current IGATE2 is essentially comprised of only constant charge current I1, thus smoothly transitioning from a larger initial charge current (I1+I1′) to the constant charge current I1. The larger initial charge current reduces the turn-on delay time of the driver transistor MD, whereas the smooth exponential transition, shown in FIG. 2B as curve 260, reduces the sharp edges of the gate current IGATE2, thereby resulting in decreased electromagnetic emission on the LIN bus 230. In an example embodiment of the circuit 200 illustrated in FIG. 2A, C1=10 pF, I1=20 uA, and I1′=220 uA (peak).
Additional soft discharging current I2′ is provided by discharging circuitry 220. During the discharging phase, switches 218 and 222 close and second capacitor C2 charges to produce the additional soft discharging current I2′ sunk from the gate of MD with constant discharging current I2. Accordingly, during the discharging phase, gate current IGATE2 is initially comprised of the sum of constant discharge current I2 and additional soft discharge current I2′. As the charge on the second capacitor C2 increases exponentially, additional soft discharge current I2′ exponentially decreases until the gate current IGATE2 is essentially comprised of only constant discharge current I2, thus smoothly transitioning from a larger initial discharge current (I2+I2′) to the constant discharge current I2. The larger initial discharge current reduces the turn-off delay time of the driver transistor MD, whereas the smooth exponential transition, shown in FIG. 2B as curve 265, reduces the sharp edges of the gate current IGATE2, thereby resulting in decreased electromagnetic emission on the LIN bus. In an example embodiment of the circuit 200 illustrated in FIG. 2A, C2=15 pF, I2=20 uA, and I2′=350 uA (peak).
When compared to the conventional LIN driver circuit 100, the disclosed LIN driver 200 provides several advantages including, but not limited to, a) reduced emission degradation of the LIN output, and b) reduced electromagnetic emission on the LIN bus. The disclosed LIN driver circuit 200 eliminates the need for the two additional phase clock signals Φ1′ and Φ1′′ of FIG. 1A, thereby eliminating emission degradation of the LIN output caused by the spread of clock timing. Additionally, the smooth charge/discharge current comprising the gate current IGATE2 provides for reduced electromagnetic emission on the LIN bus 230 as illustrated by the simulation results 300 provided in FIG. 3.
The simulation results 300 provided in FIG. 3 compare electromagnetic emission on the LIN bus for the circuit 100 illustrated in FIG. 1A (this electromagnetic emission is identified in FIG. 3 as element 305) with electromagnetic emission on the LIN bus for the circuit 200 illustrated in FIG. 2A (this electromagnetic emission is identified in FIG. 3 as element 310). The simulation results 300 illustrate a reduction of up to 17 dBuV in electromagnetic emission on the LIN bus 230 of the circuit 200 illustrated in FIG. 2A, particularly for frequencies ranging from 1 MHz to 10 MHz. As such, the disclosed LIN driver circuit 200 achieves satisfactory duty cycle by shortening delay time while providing a significant reduction in electromagnetic emission by employing a smooth, step-less gate current.
It should be appreciated by one of ordinary skill in the art that the embodiment disclosed herein is provided to illustrate one example for implementing an LIN driver circuit in accordance with the present disclosure. As such, variations to the circuit illustrated in FIG. 2A may be made without departing from the spirit or scope of the present disclosure as set forth in the claims provided below. For example, although a LIN driver is shown, the techniques described herein may be applied to any low side driver circuit, and further, may be used in both sides of a push-pull driver circuit.

## Claims

1. A driver circuit, comprising:
a driver transistor having a gate terminal;
a first circuit operable in response to a first state of a signal to generate a gate charging current for application to the gate terminal; and
a second circuit operable in response to a second state of the signal to generate a gate discharging current for application to the gate terminal;
wherein said gate charging current comprises a sum of a first exponentially decreasing current and a first constant current; and
wherein said gate discharging current comprises a sum of a second exponentially decreasing current and a second constant current;
a first capacitor wherein said first constant current is applied to said first capacitor; and
a clamping transistor selectively coupled in parallel to said first capacitor to generate a charge across said first capacitor for generating said first exponentially decreasing current.

2. The driver circuit as set forth in claim 1, wherein said first circuit comprises a first switch actuated by the first state of the signal to pass the first constant current toward the gate terminal.

3. The driver circuit as set forth in claim 2, wherein said first capacitor configure to be charge during the second state of the signal and discharged during the first state of the signal to generate the first exponentially decreasing current.

4. The driver circuit as set forth in claim 2, wherein said first switch is further operable to pass the first exponentially decreasing current toward the gate terminal.

5. The driver circuit as set forth in claim 1, wherein said second circuit comprises a second switch actuated by the second state of the signal to generate the second constant current toward a ground node.

6. The driver circuit as set forth in claim 1, wherein said second circuit comprises a third switch actuated by the second state of the signal to generate the second exponentially decreasing current.

7. The driver circuit as set forth in claim 6, wherein said second circuit further comprises a second capacitor that is charged during the second state of the signal to generate the second exponentially decreasing current.

8. The driver circuit as set forth in claim 1, wherein the first exponentially decreasing current commences coincident with said first state of the signal.

9. The driver circuit as set forth in claim 1, wherein the second exponentially decreasing current commences coincident with said second state of the signal.

10. An integrated driver circuit, comprising:
charging circuitry operable to receive a first charging current and generate a second charging current;
discharging circuitry operable to receive a first discharging current and generate a second discharging current; and
an output transistor operable to receive a gate current, wherein during a charging phase, said gate current comprises said first charging current and said second charging current, and wherein during a discharging phase, said gate current comprises said first discharging current and said second discharging current;
wherein during said charging phase, said second charging current dissipates such that said gate current substantially comprises said first charging current; and
wherein during said discharging phase, said second discharging current dissipates such that said gate current substantially comprises said first discharging current.

11. The integrated driver circuit as set forth in claim 10, wherein said second charging current commences coincident with said discharging phase.

12. The integrated driver circuit as set forth in claim 10, wherein said second discharging current commences coincident with said discharging phase.

13. The integrated driver circuit as set forth in claim 10, wherein said charging circuitry comprises a first capacitor selectively coupled in parallel with a clamping transistor.

14. The integrated driver circuit as set forth in claim 10, wherein said discharging circuitry comprises a second capacitor operable to generate said second discharging current.

15. The integrated driver circuit as set forth in claim 10, wherein said second charging current dissipates such that said gate current achieves a smooth reduction in charge current.

16. The integrated driver circuit as set forth in claim 10, wherein said second discharging current dissipates such that said gate current achieves a smooth reduction in discharge current.

17. The integrated driver circuit as set forth in claim 10, wherein said gate current alternates between said charging phase and said discharging phase.

18. The integrated driver circuit as set forth in claim 10, wherein said first charging current is a constant current.

19. The integrated driver circuit as set forth in claim 10, wherein said first discharging current is a constant current.

20. A driver circuit, comprising:
a driver transistor having a gate terminal;
a first circuit comprising a first capacitor coupled to a first transistor and a first switch, said first circuit operable to receive a first constant current and generate a first exponentially decreasing current;
a second switch operable to couple said first circuit to said gate terminal to apply a gate charging current to said gate terminal;
a second circuit coupled to said gate terminal, said second circuit comprising a third switch and a second capacitor, said second circuit operable to generate a second exponentially decreasing current; and
a fourth switch coupled to said gate terminal and said second circuitry, wherein said third and fourth switches are operable to apply a gate discharging current to said gate terminal;
wherein said gate charging current comprises a sum of said first exponentially decreasing current and said first constant current; and
wherein said gate discharging current comprises a sum of said second exponentially decreasing current and a second constant current.

21. The driver circuit as set forth in claim 20, further comprising a reference current generator.

22. The driver circuit as set forth in claim 20, further comprising a first current mirror coupled to said second circuit and said fourth switch.

23. The driver circuit as set forth in claim 22, wherein said first current mirror is operable to generate said second constant current.

24. The driver circuit as set forth in claim 20, further comprising a second current mirror coupled to said first circuit and said second switch.

25. The driver circuit as set forth in claim 24, wherein said second current mirror is operable to generate said first constant current.

26. The driver circuit as set forth in claim 20, further comprising a gate resistor coupled to said gate terminal.

27. The driver circuit as set forth in claim 20, further comprising a third capacitor coupled to said gate terminal, said third capacitor coupled in parallel with said second circuit.

28. The driver circuit as set forth in claim 20, further comprising a second transistor coupled to said gate terminal.

29. The driver circuit as set forth in claim 28, further comprising a second resistor coupled in series with said second transistor, said second resistor and second transistor coupled in parallel with said second circuit.

30. The driver circuit as set forth in claim 20, further comprising an output bus coupled to a source terminal of said driver transistor.

31. The driver circuit as set forth in claim 30, further comprising a first diode coupled between said source terminal and said output bus.

32. The driver circuit as set forth in claim 30, further comprising a second diode and pull-up resistor coupled in series between said output bus and a voltage source.

33. The driver circuit as set forth in claim 20, wherein said first circuit further comprises a fourth resistor coupled to said first capacitor.

34. The driver circuit as set forth in claim 20, wherein said second circuit further comprises a fifth resistor coupled to said second capacitor.

35. A method, comprising:
generating a first exponentially decreasing current in response to a first state of a signal;
generating a second exponentially decreasing current in response to a second state of the signal;
applying a charging current at a gate terminal of a driver transistor, wherein said charging current comprises a sum of a first constant current and said first exponentially decreasing current; and
applying a discharging current at said gate terminal of said driver transistor, wherein said discharging current comprises a sum of a second constant current and said second exponentially decreasing current;
wherein generating said first exponentially decreasing current comprises applying said first constant current to a first capacitor selectively coupled in parallel with a clamping transistor to generate a charge across said first capacitor.

36. The method as set forth in claim 35, wherein generating said second exponentially decreasing current comprises applying current drawn from said gate terminal across a second capacitor.