Predriver circuit for low-noise switching of high currents in a load

Abstract: A predriver circuit (107, 109) for current switching, di/dt, noise control for high current load devices (101) is disclosed. Weak pull up (P3'') and pull down (N3) circuit paths are combined with strong pull up (P1', P2') and pull down (N1, N2) circuit paths, respectively, to provide control over the rate of change of current through the load (101) during predetermined turn-on and turn-off time periods. Predriver control for both NMOS driver, current sink NMOS, and PMOS driver, current source PMOS, are demonstrated. The predriver circuit provides control of the rate of current switching through the load without excessive delay in fully turning on or off the load device.

Description: FIELD OF THE INVENTION The present invention generally relates to driver circuits for loads which draw a large current (such as light emitting diode drivers) and the reduction of electromagnetic inductance noise emissions caused by high current switching through wires to current loads. It also relates to a corresponding method. BACKGROUND OF THE INVENTION In heavy current driver circuits, such as light emitting diode ("LED") drivers and input-output circuits, there is a relatively large current flow through the power pins. This current is switched on and off by a control signal. Rapidly cycling or changing the state of an LED results in the generation of undesirable electromagnetic inductive ("EMI") noise expressed as L*di/dt, where L is circuit bonding wire inductance. A matrix of LED's, as may be found on control panels in a variety of commercial products, magnifies the problem. In addition to affecting the operation of other components of a commercial electronic product, EMI emissions are considered a hazard and are thus controlled under various standards and government regulations. Therefore, there is a need for devices to control emissions. In order to reduce di/dt noise from driving higher current loads, one can control the rate at which the current is being supplied to the load. In general, it is common to slow down the rate at which a driver cycles a heavy current load by providing a predriver circuit sized (that is, chosen to have predetermined operating characteristics) to slow down the rate at which the predriver affects the drive current. However, if the turn on or off rate is high enough to achieve a desired speed of operation, the di/dt noise is also still relatively high. On the other hand, if the predriver turn on or off rate is kept low to reduce noise emissions, it may take too long to drive the load to its new state. A method and apparatus for increasing signal transition speed is shown by Anderson in U.S. Patent No. 5,039,874. For driving an integrated circuit output pad, Anderson switches between two output drivers dependent upon pad noise levels where one driver may be disconnected if the particular application is effected by the increased noise level. In US-A-5 315 173 there is described a data output buffer that includes a data driving circuit having a pull-up transistor responsive to a first signal and a pull-down transistor responsive to a second signal, a first control circuit for regulating the slope of the first signal to be less steep after reaching the threshold of the pull-up transistor than before reaching the threshold of the pull-up- transistor, and a second control circuit for regulating the slope of the second signal to be less steep after reaching the threshold of the pull-down transistor than before reaching the threshold of the pull-down transistor. As a result, noise generated by the transition of the output signal of the data output buffer is reduced without affecting operation speed. US 4 880 997 describes a low noise output buffer with a prodriver having a weat and a strong pull-up transistor in parallel. The strong transistor is switched in response to the output voltage, so that power supply spikes are suppressed. However, there is a need for a general application method and device to control the rate at which the current source and sink driver for a heavy current load is cycled in order to achieve lower di/dt and affect a lower EMI noise output. It is necessary to provide a method and device which will achieve a lower di/dt rate in a high current load without introducing excessive delays in fully switching the load state. The present invention is particularly suited to LED driver circuitry. SUMMARY OF THE INVENTION The present invention is defined by the claims hereinafter. In its basic aspects, the present invention provides a current control circuit device for a high current load, having a driver circuit, in a circuit having a power supply and a signal generator for generating a signal to the load. The current control circuit device includes: an input connected to the signal generator; an output connected to the driver circuit; a pull up circuit connected between the device input and the output for controlling the rate of current change through the load, having a weak pull up device and a strong pull up device connected in parallel; and a pull down device connected between the device input and output for controlling the rate of current change through the load, having a weak pull down device and a strong pull down device connected in parallel. It is an advantage of the present invention to provide a heavy current load predriver circuitry that controls the rate of current switching through a source or sink that suppresses the noise generated by L*di/dt. It is another advantage of the present invention that it provides a method and device that controls the rate at which the current source and sink are turned on and off without excessive delay in fully changing the load state condition. Other objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings, in which like reference designations represent like features throughout the FIGURES. DESCRIPTION OF THE DRAWINGS FIGURE 1 is a circuit block diagram of the present invention in conjunction with an exemplary load. FIGURE 2 is a circuit block diagram of a predriver circuit of the present invention for a NMOS load driver. FIGURE 3 is a circuit block diagram of a predriver circuit of the present invention for a PMOS load driver. FIGURES 4A through 4D are waveform diagrams for the present invention as shown in FIGURE 1 in which: FIGURE 4A is a voltage waveform at the output of the predriver circuit of the present invention as shown in FIGURE 3, FIGURE 4B is a voltage waveform at the output of the predriver circuit of the present invention as shown in FIGURE 2, and FIGURE 4C is a current waveform through the load in conjunction with the present invention as related to FIGURES 4A and 4B. The drawings should be understood as not being to scale except where specifically noted. DETAILED DESCRIPTION OF THE INVENTION Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventors for practicing the invention. Alternative embodiments are also briefly described as applicable. In an exemplary embodiment, an application for an LED driver adaptation is described. However, it will be recognized by a person skilled in the art that the present invention is adaptable to a variety of high current load applications. No limitation is intended by the inventors nor should any be implied from the use of an exemplary embodiment used to facilitate the description of the invention. The present invention as shown in FIGURE 1 is adapted to a high current load 101 to control the rate of current switching through a source (PMOS) or sink (NMOS) in order to suppress the noise generated by a current change, di/dt, as the load is cycled, e.g., a matrix of LED indicators in a control panel apparatus in which LED's are selectively turned on and off. Each LED has a source driver 103 and a sink driver 105 as would be known in the art. In general, the matrix would be adapted to have plurality of addressable column drivers 103 and field drivers 105 to control the selection of LED's to be switched in accordance with predetermined operational user indications, namely by a particular column and field driver set being switched by the control signal. A field predriver circuit 107 for the field driver(s) 105 and a column predriver circuit 109 are provided. The field predriver circuit 107 of the present invention as shown in FIGURE 1 is shown and detailed in FIGURE 2. The implementation is for a pull down NMOS driver 105, or a current sink. The control signal for the field select is shown a Vin107 and will generally be, for example, a HIGH (or digital 1) or a LOW (or a digital 0), depending on whether the field driver 105 associated therewith is to be ENABLED or DISABLED, respectively. There are two parallel paths from Vout107 to circuit ground in the field predriver circuit 107. The components of these paths will be used to control the turn off rate of the LED load 101. The first path is through the series combination of transistors N and N2. NMOS transistor N1 is connected between Vout107 with its gate to Vin107, with its drain to Vout, and its source to the drain of N2. NMOS transistor N2 is diode connected with its drain to the source of N1 and its source to ground. These transistors are sized to provide a strong pull down. The second of the two parallel paths from Vout107 to circuit ground is through transistor N3. NMOS transistor N3 is sized to provide a weak pull down. Field predriver circuit 107 further includes a half latch connected between Vout107 and the circuit bias voltage supply, Vdd, and a PMOS transistor P1 connected between Vdd and Vout107. PMOS transistor P1 has its gate connected to Vin107, its source to Vdd and its drain to Vout107. The half latch is formed by inverter 201, having its input coupled to Vout107 and its output coupled to a PMOS transistor P2 at its gate. PMOS transistor P2 has its source connected to Vdd and its drain to a source terminal of another PMOS transistor P3. PMOS transistor P3 has its gate connected to Vin107 and its drain connected to Vout 107 . The column predriver circuit 109 is shown in FIGURE 3. The circuit is equivalent to the field predriver circuit but configured for a PMOS current source, or pull up PMOS column driver 103. As would be known in the art, therefore, it is biased and connected as a pull up circuit. There are two parallel paths from Vout109 to Vdd. One path is through the series combination of P1' and P2'. P2' is diode connected and P1' is connected between Vout109 and the drain terminal of P2'. As in the field predriver circuit 107, a weak pull up transistor P3' is connected between Vout109 and Vdd, only in this configuration a PMOS type. A half latch is connected between Vout109 and ground. This half latch is formed by inverter 301 and transistors N2' and N3'. Similarly, a weak pull down is provided in the form of NMOS transistor N1' connected between Vout109 and ground. With the field predriver 107 connected to the load 101 via the field driver 105 (pull down NMOS current sink) and the column predriver 109 connected to the load 101 via the column driver 103 (pull up PMOS current source) as shown in FIGURE 1, the rate of current through the load when switching between states is controlled. In other words, the rate at which the current source and sink are turned on and off is controlled to achieve a lower di/dt and, therefore, less EMI noise is generated during current switching. Operation of the circuit is demonstrated in conjunction with the waveforms as shown in FIGURE 4. The output voltage of the column predriver 109, Vout109, is plotted in FIGURE 4A. The output voltage of the field predriver 107, Vout107, is plotted in FIGURE 4B. The load 101 current is plotted in FIGURE 4C. Note, however, that the di/dt shown in FIGURE 4D is concentrated at time periods t 2 and t 4 of FIGURES 4A and 4B. As such, the goal of the present invention is met. NMOS (FIELD) PREDRIVER Vin107 is generally a HIGH or LOW indicating that a field select line for a matrix of LED's is enabled or disabled. As Vin107 goes HIGH the gate voltage on transistors N1 and N3 rises, transistors N1 and N3 turn on. and transistor N3 starts conducting since it is sized as a weak pull down. Recall that transistors N1 and N2 are selected to provide a strong pull down to the load, V L . As the voltage across transistor N2 approaches V tn , where "V tn " is the threshold voltage of NMOS transistor N2 (namely, the turn-on/turn-off point depending on the direction of the gate swing), the current through transistor N2 falls to a very low value. That is, this is the point where the design in accordance with the present invention is meant to traverse relatively slowly. The series combination of transistors N1 and N2 have a high resistance to ground and the parallel combination with transistor N3 thus determines the rate at which Vout 107 falls to ground. In other words, by preselecting appropriately sized transistors, the circuit can be tailored such that the driver 105 gate voltage will have two different rates of fall. Initially, Vout 107 will fall rapidly (see FIGURE 4B, t 1 ) as the path through transistors N1 and N2 control; negligible current is switched (FIGURE 4C). As Vout 107 approaches V tn , transistor N3 controls and the rate of fall will become slower (FIGURE 4B, t 2 ). Current is switched (FIGURE 4C). Note that this initial fast rate of fall Vout 107 , and hence the gate voltage of the driver 105 gate voltage, reduces the time to turn off the driver 105 fully whereas the subsequent slower rate of fall helps keep di/dt noise under control (FIGURE 4D) as the bulk of the current switching takes place in this region, where the gate voltage passes through V tn . In region t 3 , the current reaches a steady state; the LED would be off. Similarly, there are two regions in the turn on process; an initial slow rise of driver 105 gate voltage and a subsequent quick rise time to Vdd. For the turn on process, the weak pull up transistor P1 causes Vout 107 to rise at a slow rate during an initial turn on period, t 4 . As Vout 107 rises, the inverter 201 starts to turn on while the weak pull up transistor PI is controlling. During period t 5 , the strong pull up formed by transistor P2 and P3 causes Vout 107 to rise at a much faster rate, but not much current is switched. Transistors P2 and P3 are sized to get the desired time constant for the latter time period, whereas the threshold voltage of the inverter 201 determines the voltage at which the strong pull-up path starts turning on. Again note that the bulk of the load current is switched in the region where the gate voltage is varying slowly. This keeps the di/dt noise within the predetermined limits, yet the fast Vout 107 rise gets the driver 105 to a full turn on with a minimal delay. It is when the gate-source voltage of the NMOS driver is close to V tn that the bulk of the load current is switched. Hence, to keep di/dt low, this region, where the gate-source voltage is close to V tn , is traversed very slowly. When the pull-down transistors are being turned on by Vin 107 , the pull-up transistors are being turned off. The opposite is also true. This allows Vout 107 to swing from rail to rail as in a steady state only if one of them is fully on and the other is fully off. PMOS (COLUMN) PREDRIVER Operation of the column predriver circuit 109 for a PMOS driver 103, e.g., a current source for an LED or a column is shown in FIGURE 4A. As voltage Vin 109 falls, transistors P1' and P3' will turn on and transistor P2' starts conducting, providing a strong pull up toward Vdd. Transistor Nl' and N3' will turn off, allowing Vin 109 to rise. The series combination of transistors P1' and P2' will have resistance to Vdd and the parallel combination with P3' will determine the rate at which Vout 109 rises to Vdd, the rate slowing as the driver 103 gate voltage reaches Vdd when current switching is most prominent and hence most di/dt noise is generated. In other words, by selecting appropriate design characteristics for transistors P1' and P2' to provide a strong pull up and transistor P3' to provide a weak pull up, the driver 103 gate voltage is tailored to have two different transition rates. As shown in FIGURE 4A, initially, during time t 1 the output voltage of the column predriver 109, Vout 109 , and hence the gate voltage of the drive 103, rises relatively rapidly. As Vout 109 approaches Vdd and transistor P3' takes control, the rise time will lengthen (FIGURE 4A, t 2 ) until Vdd - |V tp |, where V tp is the threshold voltage of the PMOS transistor, is achieved. The initial fast rate of rise of the driver 103 gate voltage thus reduces the time to turn off its PMOS driver transistor fully, whereas the low time constant keeps di/dt noise under control as the bulk of the current switching takes place during this latter time period, t 2 . For the turn on process, transistor N1' acts as a weak pull down connection between ground and Vout 109 . During turn on operation, the weak pull down transistor N1' causes Vout 109 to fall at an initial slow rate (FIGURE 4A, t 4 ). As the output voltage falls, the inverter 301 starts turning on and the strong pull down formed by transistors N2 and N3 causes Vout 109 to fall faster than the initial rate (FIGURE 4A., t 5 ). The predetermined threshold voltage of the inverter 301 determines the voltage at which the strong pull down pair starts turning on. Once again, note that the bulk of the current switching is in the region where the gate voltage of the drive 103 is varying slowly. This keeps di/dt noise under control, yet the fast Vout 109 initially keeps the PMOS driver full turn on time to a minimum. In other words, there are two regions in the turn off process; first, when the gate voltage rises rapidly towards Vdd- |V tp |; and, second, when Vout 109 gradually rises to Vdd. Similarly, there are two regions in the turn on of the PMOS driver process; first, when Vout 109 falls slowly and subsequently when it falls rapidly to ground. It is when the gate-source voltage of the PMOS driver is close to V tp that the bulk of the load current is switched. Hence, by varying the gate-source voltage of the PMOS driver very slowly in this region, di/dt is kept low. Once again, note that when the pull-up transistors are being turned on, the pull-down transistors are being turned off. In steady state, only one of the two is fully on and the other is fully off. This allows Vout 109 to swing from rail to rail. In summary, the use of the disclosed combination of weak elements and strong elements provides a scheme where the bulk of current switching occurs when the weak elements are active. This helps keep noise low. The strong elements help reduce transition time, but hardly participate in current switching whereas the weak elements participate strongly in the current switching but are slower. As shown in FIGURES 4A to 4D, at times t 2 and t 4 , all the current transition is localized, all di/dt is localized, the gate voltages of the driver have low slopes, and the gate-source voltage is close to the threshold voltage. Thus, the present invention provides predriver devices for PMOS and NMOS type driver circuits that will reduce the di/dt noise due to high current load switching, yet which will permit driver turn on and turn off times to be kept to an acceptable minimum. The foregoing description of the preferred embodiment of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result. The embodiment was chosen and described in order to best explain the principles of the invention and its best mode practical application to thereby enable others skilled in the art to understand the invention for various applications with various modifications are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims: A circuit for providing controlled switching on or off of an electronic load (101), which circuit comprises: a load driver transistor (103;105); an input line (Vin 107 ; Vin 109 ); an output line (Vout 107 ; Vout 109 ) connected to the control electrode of the load driver transistor; pull up means (P3', P2', P1'; P1, P2, P3) connected between the input and the output line, and thereby to the load driver transistor (103, 105), for controlling the rate of current change through the load, having a weak pull up means (P3'; P1) and a strong pull up means (P2', P1'; P2, P3); pull down means (N1', N2', N3'; N1, N2, N3), including a first and a second transistor, connected between the input and the output line, and thereby to the load driver transistor (103, 105), for controlling the rate of current change through the load, having a weak pull down means (N1'; N3) and a strong pull down means (N3', N2'; N1, N2) wherein the pull up and pull down means are configured to provide current at a first and a second rate on the output line, the second rate being less than the first rate, such that the second rate is obtained by disabling the strong pull up means and the strong pull down means in response to the output voltage (Vout 107 ; Vout 109 ) being in the vicinity of a preselected threshold voltage (V tp ; V tn ), the first rate being utilised after the bulk of the load current has been switched. A circuit according to Claim 1, wherein the pull down means comprises current limiting means (N2; N2') connected between the second transistor and a ground line for conducting current between the second transistor and the ground line. A circuit according to Claim 2, wherein the current limiting means (N2) comprises a diode-connected transistor connected between the second transistor and the ground line. A circuit according to Claim 2, wherein the current limiting means comprises a third transistor (N2') connected between the second transistor and the ground line, and an inverter (301) connected between the output line and the gate of the third transistor. A method of operating a circuit having an input (Vin 107 ; Vin 109 ) and having an output (Vout 107 ; Vout 109 ) connected via a load driver transistor (103; 105) to an electronic load (101) for providing controlled switching on or off of the load, which method comprises: providing a predriver circuit (107; 109) having strong and weak pull up means and strong and weak pull down means having outputs connected to the control electrode of the load driver transistor forming said output; providing a first input voltage to the input; while providing the first input voltage, maintaining the output at a first output voltage; switching the voltage at the input to a second, different input voltage; in response to switching to the second input voltage, transitioning the output voltage from the first output voltage to a second, different output voltage; transitioning the output voltage in two phases: (i) a first transitioning phase (T1; T5) comprising changing from the first output voltage at a first rate to an intermediate output voltage (V tp ; V tn ) having a value intermediate the first and second output voltages, the first transition being controlled by one of the strong pull up means or strong pull down means; (ii) a second transitioning phase (T2; T4) for switching the bulk of the load current, comprising changing from the intermediate output voltage to the second output voltage at a second rate slower than the first rate, the second transition being controlled by one of the weak pull up means or weak pull down means; transitioning the output voltage during the first transitioning phase by providing current flow between the output and an electrical element maintained at the second output voltage via a first path and a second path; and transitioning the output voltage during the second transitioning phase by providing current flow between the output and an electrical element maintained at the second output voltage only via the second path, the first path being disabled in response to the output voltage being in the vicinity of said intermediate output voltage. A method according to Claim 5 wherein the first path includes a diode-connected transistor (N2) selected to prevent the flow of current when the voltage across the diode-connected transistor is below a preselected value. A method according to claim 5 or 6 wherein transitioning the output voltage during the first transitioning phase includes providing current flow between the output and the electrical element via a first device having a first current flow capacity, and transitioning the output voltage during the second transitioning phase includes providing current flow between the output and the electrical element via a second device having a second current flow capacity which is less than the first current flow capacity.

Inventors: THAYER BILLY E, WALIA RAJAN, THAYER, BILLY E., WALIA, RAJAN
Applicants: AGILENT TECHNOLOGIES INC, AGILENT TECHNOLOGIES, INC.
Application year: 1995
Main classification: H03K3
IPC codes: H03K19, H03K17