Aut o moti ve P ow er ADVANCED SENSE Calibration and Benefits Guide Applic atio n N ote V1.0 2011-04-27 Aut o moti ve P ow er ADVANCED SENSE Calibration and Benefits Guide ADVANCED SENSE Revision History: V1.0, 2011-04-27 Previous Version: none Page Application Note Subjects (major changes since last revision) 2 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Table of Contents Table of Contents 1 Abstract ............................................................................................................................................... 4 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Introduction ........................................................................................................................................ 4 Pin Names and Functions .................................................................................................................... 5 Voltages and Currents ......................................................................................................................... 5 Flowchart Nomenclature ...................................................................................................................... 6 Example Circuit Board Scenario .......................................................................................................... 6 Fundamental Concepts ........................................................................................................................ 7 Problems with Conventional Sense Functions ..................................................................................... 8 Advantages of ADVANCED SENSE Technology .............................................................................. 10 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Calibrating ADVANCED SENSE Enabled Devices ........................................................................ 12 Calibration Nomenclature and Equations .......................................................................................... 12 Types of Calibration ........................................................................................................................... 15 No Calibration (No Cal) ...................................................................................................................... 15 Offset-Only Calibration ....................................................................................................................... 16 Virtual 2-Point Calibration .................................................................................................................. 19 Running Offset Calibration ................................................................................................................. 21 Accuracy of Different Calibration Options .......................................................................................... 23 4 Conclusion ........................................................................................................................................ 24 Application Note 3 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Abstract 1 Abstract Smart, high-side power switches from Infineon® are designed to control all types of resistive, inductive, and capacitive loads. These devices provide protection and diagnostic functions and are specially designed to drive loads in harsh automotive environments. Analog current sense diagnostics signals in high-side switches have inherent inaccuracies associated with them. The two main sources of inaccuracy are the sense offset current, which dominates at lower load currents, and the slope (steepness) inaccuracy, which becomes more significant at higher load currents. Infineon’s ADVANCED SENSE technology allows for multiple calibration techniques offering increased levels of accuracy to be implemented depending on the application requirements. Note: Devices that incorporate ADVANCED SENSE technology will have “Advanced analog load current sense signal” or similar wording in the Features section of their respective datasheets. If a high-side power switch is enabled with Infineon’s ADVANCED SENSE technology, its associated control system can perform a calibration to remove the sense offset current as a source of inaccuracy. Also, ADVANCED SENSE technology supports Virtual 2-Point Calibration, which means that it is possible for manufacturing test to obtain a true 2-point calibration by measuring at only one load current. This allows slope as a source of inaccuracy to be significantly improved. This application note first introduces some fundamental concepts. This is followed by a discussion of the sources of inaccuracy experienced by conventional high-side power switches that do not support ADVANCED SENSE technology. Next, the way in which ADVANCED SENSE technology addresses these issues is discussed. The application note then details the multiple types of ADVANCED SENSE enabled calibration that may be employed during Manufacturing Test and/or by the Application Software. Note: The following information is given as an implementation suggestion only, and shall not be regarded as a description or warranty of a certain functionality, condition, or quality of any device. 2 Introduction Current sensing is implemented within high-side switches to diagnose systems and to protect them in the event of failures. High-side current sensing is used to protect both the load and the wiring harness, to diagnose the load so as to ensure proper operation, and to measure the output current for the purpose of controlling the output power. Note: Further generic information on high-side switches with diagnostics and protection can be found in the TM Application Note: What the designer should know: Short introduction to PROFET +12V. There are two main problems with conventional high-side current sensing solutions. The first is the inaccuracy that is introduced by the internal amplifier offset voltage. This can deteriorate the current sense accuracy, especially at lower load currents, and can even disable the current sense functionality below certain load current thresholds. The second is the slope (steepness) inaccuracy, which becomes more significant at higher load currents. The solution is high-side power switches that are enabled with Infineon’s ADVANCED SENSE technology, which provides the ability to calibrate the current sense for high accuracy requirements utilizing simple end-ofline measurements and low application software overhead. Application Note 4 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Introduction 2.1 Pin Names and Functions Single-channel, high-side power switches of the general type considered in this paper have five pins (GND, IN, OUT, IS, and VS) as illustrated in Figure 1. Figure 1 Control input from microcontroller IN Current sense output to microcontroller IS VS OUT Main output to drive the load GND Pin names The functions of these pins are detailed in Table 1. Table 1 Pin functions Pin Name Pin Function GND Ground: Ground connection IN Input: Digital 3.3V and 5V compatible logic input; activates the power switch if set to HIGH level (definitions for HIGH and LOW can be found in the parameter tables of the respective device datasheet) Output: Protected high-side power output OUT IS Sense: Analog sense current signal VS 2.2 Supply Voltage: Positive supply voltage for both the logic and power stages Voltages and Currents Figure 2 illustrates the voltages and currents referenced in this application note. The load current IL and the sense current IIS will be the focus of the following discussions. VS VS IS IIN IN IL VS OUT VIN VOUT IIS IS VIS GND IGND GND Figure 2 Definition of currents and voltages These abbreviations are defined in Table 2. Application Note 5 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Introduction Table 2 Voltage and current abbreviations Abbreviation Meaning VS Supply voltage GND VIN Ground VOUT Output voltage driving the load VIS Sense voltage IL Load current IIS Sense current IS Supply current IGND Ground current 2.3 Control input voltage Flowchart Nomenclature With regard to flowcharts used in this application note, the representation of the five main symbols is illustrated in Figure 3. Document / Note Process / Action Decision Pre-defined Process (Subroutine) Internal Storage Figure 3 Flowchart nomenclature 2.4 Example Circuit Board Scenario For the purposes of this application note, it is assumed that a circuit board containing a microcontroller and some number of single-channel, high-side power switches as illustrated in Figure 4. Circuit board High-side power switches Microcontroller Figure 4 Example circuit board scenario Application Note 6 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Introduction The microcontroller is used to turn the high-side power switches ON and OFF, and also to measure the value of the sense current (IIS) outputs from the switches. 2.5 Fundamental Concepts In order to understand the problems associated with conventional current sense functions (and hence the advantages of solutions based on Infineon’s ADVANCED SENSE technology), it is first necessary to be familiar with some fundamental concepts. Let’s start with the general formula for a straight line in "slope-intercept" form, which is presented in Equation (1). Equation (1) In this case, y is the value on the vertical axis (Y), x is the value on the horizontal axis (X), m is the slope of the line, and b – which is known as the y-intercept – is the point at which the line intersects the Y-axis. For the purposes of this application note, only positive slope values are discussed (and are applicable) as illustrated in Figure 5. y y y b x x (a) b = 0 Figure 5 b x (b) Positive b value (c) Negative b value Generic lines with identical positive slopes All three lines in Figure 5 have the same m (slope) value. The difference between the lines is the b (y-intercept) value. The line in Figure 5(a) has a b value of zero; the line in Figure 5(b) has a positive value for b; and the line in Figure 5(c) has a negative value for b. One way in which the characteristics of such a line can be determined is to first identify two points as illustrated in Figure 6(a). y (x2, y2) (x1, y1) y y (x2, y2) (x2, y2) m m (x1, y1) (x1, y1) b x (a) Identify two points Figure 6 x (b) Determine slope x (c) Determine y-intercept Determining the characteristics of a straight line Given a straight line, the slope (m) of the line can be calculated using Equation (2). Application Note 7 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Introduction Equation (2) Once the slope has been determined as illustrated in Figure 6(b), this value can be used to calculate the yintercept (b) as illustrated in Figure 6(c). This can be accomplished by picking the (x,y) values for any point and solving for the y-intercept using Equation (3). Equation (3) After m (slope) and b (y-intercept) have been determined, these values can be substituted into the generic equation for a line as defined in Equation (1), and then this equation can be used to determine the (x, y) values of any other point on the line. 2.6 Problems with Conventional Sense Functions A high-level block diagram for a conventional high-side power switch is illustrated in Figure 7. VS IN IIS IS Input circuit Sense output circuit Power Switch OUT IL Diagnosis and Protection GND Figure 7 High-level block diagram for a conventional high-side power switch The ideal relationship between the sense current IIS and the load current IL is shown in Figure 8(a). In reality, however, there may be slope (steepness) error as illustrated in Figure 8(b). Such slope errors are mainly dependent on part-to-part production variation, and their effects are more pronounced at higher load currents. Meanwhile, the sense offset error – which is introduced by the internal amplifier offset voltage – is strongly dependent on production variation and the operating temperature of the device; the effects of the offset are more pronounced at lower load currents. Also, in the case of a negative offset error, the current sense capability may become disabled below a certain load current threshold, as illustrated by the horizontal portion of the solid green line in Figure 8(c). Application Note 8 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Introduction typ IIS IIS IIS Slope (steepness) error typ IIS typ IIS IIS max IIS Offset error min IIS IL (a) Ideal curve IL (b) With slope error IL (c) With offset error (Offset = 0, Slope = typical) (Offset = 0) (Blue = positive; Green = negative) Figure 8 Relationship between IIS and IL in conventional devices The relationship between the sense current IIS and the load current IL in a high-side power switch can be expressed using Equation (4). Equation (4) The sense current IIS in Equation (4) corresponds to y in Equation (1); the load current IL in Equation (4) corresponds to x in Equation (1); the slope defined by 1/kIS in Equation (4) corresponds to m in Equation (1); and the sense offset current IIS(OFFSET) in Equation (4) corresponds to the y-intercept b in Equation (1). The value of kIS is defined in the corresponding device datasheet. Now consider the information that is available when working with a conventional high-side power switch as illustrated in Figure 9. VIN t IL t tsIS(ON) tsIS(OFF) 90% IIs 10% t IIS can be measured during this time Figure 9 Relationship between IIS and IL in conventional devices The shaded areas of Figure 9 (and any equivalent illustrations later in this application note) indicate the times when the IIS output is transitioning between ON/OFF states. These transition times, which are represented by tsIS(ON) and tsIS(OFF), are the 90% IIS and 10% IIS current sense settling times, respectively. The analog sense current signal is invalid during the current sense settling times. Application Note 9 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Introduction As can be seen in Figure 9, the only value that is available is the sense current IIS. There is no way to calibrate the sense current offset or the slope from a single reading. The only way to calculate these values is to use a 2point calibration technique, which involves switching in two separate loads during manufacturing test as illustrated in Figure 10. VIN t IL Load #1 Load #2 t tsIS(ON) tsIS(OFF) tsIS(ON) tsIS(OFF) IIs t IIS for load #1 can be measured during this time Figure 10 IIS for load #2 can be measured during this time Measuring two loads using conventional devices The value of kIS may now be calculated using Equation (5), where IL1 and IL2 are the two different load currents and IIS(IL1) and IIS(IL2) are the corresponding sense currents. Equation (5) Once the slope has been determined, this value can be used to calculate the sense current offset value by picking the (IL,IIS) values for any point and solving for IIS(OFFSET) using Equation (6). Equation (6) Both of these values may be stored in the microcontroller’s non-volatile memory to be used by the application software. The end result is that, using conventional techniques, the steepness of the slope is difficult to measure by endof-line calibration. This is due to the fact that it requires the use of a full 2-point calibration technique, which involves switching in two separate loads during manufacturing test. Furthermore, the sense current offset cannot be measured dynamically by the application during operation, which means that the temperature dependency associated with the offset will remain in the application. 2.7 Advantages of ADVANCED SENSE Technology A high-level block diagram for a high-side power switch enabled with Infineon’s ADVANCED SENSE technology is illustrated in Figure 11. Application Note 10 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Introduction VS IN IIS IS Power Switch OUT Input circuit IL Diagnosis and Protection Sense output circuit* V(OFFSET) GND *The sense output circuit provides access to both IIS and IIS(OFFSET) Figure 11 High-level block diagram for an ADVANCE SENSE enabled high-side power switch As can be seen, there are two main differences between this block diagram and that for the conventional highside power switch, which was illustrated in Figure 7. The first difference is the use of special circuitry to introduce a voltage bias called V(OFFSET), which ensures the offset error is always positive as illustrated in Figure 12(b). Ideal curve Ideal curve (Offset = 0, Slope = typical) (Offset = IIS(OFFSET), Slope = 1/kIS) typ IIS IIS Offset error IIS max IIS Offset error min IIS IL (a) Conventional current sense error Figure 12 IL (b) ADVANCED SENSE current sense error Comparison between conventional and ADVANCED SENSE enabled devices The fact that the offset error is always positive significantly simplifies the relationship between the sense current IIS and the load current IL. Knowing that the offset error is always positive means that it can always be subtracted from the measured sense current IIS so as to determine the actual amount of sense current that is associated with the load. It also means that the IIS sense current output always shows a meaningful value, even when the current load IL is small. This “always positive” offset error is one of the major benefits of ADVANCED SENSE because it significantly reduces both manufacturing test software and application software overhead when calibrating for increased accuracy. The second difference and major advantage of an ADVANCED SENSE enabled device is that it provides direct access to the sense current offset as illustrated in Figure 13. Application Note 11 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Introduction tVIN(RISING) tVIN(FALLING) VIN t tRESET IL t tsIS(ON) tsIS(OFF) IIs 1/kIS x IL t IIS can be measured during this time Figure 13 IIS(OFFSET) IIS(OFFSET) can be measured during this time Measuring IIS(OFFSET) with an ADVANCED SENSE enabled device As is illustrated by Figure 13, the difference between reading IIS and IIS(OFFSET) is dependent on the state of the VIN input and the time since the VIN input transitioned from a LOW to a HIGH state, or vice versa (definitions for HIGH and LOW can be found in the parameter tables of the respective device datasheet): The output of the IS pin is IIS when tsIS(ON) < t < tVIN(FALLING) The output of the IS pin is IIS(OFFSET) when tsIS(OFF) < t < tRESET This feature means that a Virtual 2-Point Calibration can be performed by manufacturing test using only a single load as discussed later in this application note. Furthermore, having direct access to the sense current offset in this way means that the application software can compensate for any temperature dependency associated with the offset. 3 Calibrating ADVANCED SENSE Enabled Devices 3.1 Calibration Nomenclature and Equations The nomenclature used in the high-side power switch datasheets and the information presented earlier in this application note references calibration information in terms of current. However, the analog-to-digital converter (ADC) in the microcontroller that is used to monitor the IS (sense current) output from the high-side switch reads voltages, not currents. Thus, the calibration techniques discussed below are presented in terms of voltages because these are what the manufacturing test and application software read. Consider the reference circuit illustrated in Figure 14 (the resistors RINPUT and RSENSE are for protection and have no or minimal effect on the calibration calculations). Application Note 12 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices VS +5V RINPUT VS IN µC (e.g. XC866) IL OUT RSENSE IS GND IIS + VIS – IL RL RIS GND Figure 14 Reference circuit for calibration nomenclature The analog sense current signal IIS flows through resistor RIS. The corresponding voltage potential VIS, which is developed across this resistor, and which is seen by the microcontroller’s ADC input, is determined by Ohm’s law as shown in Equation (7). Equation (7) Also, remembering that the offset current IIS(OFFSET) can also be directly read through the IS pin as discussed in the previous section, the potential VIS(OFFSET) is determined as shown in Equation (8). Equation (8) With the exception of the No Calibration scenario discussed later in this application note, the initial values for kIS and VIS(OFFSET) will be determined by manufacturing test and stored in the microcontroller’s non-volatile memory for use by the application software. Note: This application note assumes that manufacturing test will store VIS(OFFSET) (the voltage value in ADC counts) in the microcontroller’s non-volatile memory; that is, it is assumed that manufacturing test will NOT store IIS(OFFSET) (the current value). From Figure 10 and Equation (5), kIS is traditionally calculated using a 2-point calibration technique, which involves switching in two separate loads during manufacturing test. From Equation (7), IIS = VIS / RIS, so substituting for IIS in Equation (5) allows kIS to be calculated as shown in Equation (9). Equation (9) Figure 13 illustrated that one of the major benefits of an ADVANCED SENSE enabled device is that it provides direct access to the sense current offset. This means that Virtual 2-Point Calibration can be performed by manufacturing test using only a single load as illustrated in Figure 15. Application Note 13 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices tVIN(RISING) tVIN(FALLING) VIN t tRESET IL t tsIS(ON) tsIS(OFF) IIs 1/kIS x IL t IIS can be measured during this time Load #1 (Real) Figure 15 IIS(OFFSET) IIS(OFFSET) can be measured during this time Load #2 (Virtual) Performing Virtual 2-Point Calibration using only a single load The single real load, shown as Load #1 in Figure 15, equates to IL1 in Equation (9). The virtual load, shown as Load #2 in Figure 15, equates to IL2 in Equation (9); this is equivalent to a no load (open load) condition, which means that IL2 = 0 in Equation (9). From Figure 15, the sense current associated with Load #2 is IIS(OFFSET). From Equation (8), IIS(OFFSET) = VIS(OFFSET) / RIS, so substituting this in Equation (9) gives the formula used by manufacturing test to calculate kIS as shown in Equation (10). Equation (10) From Equation (4), IIS = (1/kIS × IL) + IIS(OFFSET), which means that the load current IL can be calculated as shown in Equation (11). Equation (11) From Equation (7) and Equation (8), IIS = VIS / RIS and IIS(OFFSET) = VIS(OFFSET) / RIS, so substituting these values into Equation (11) yields Equation (12). Equation (12) Factoring Equation (12) allows the application software to calculate the load current IL as shown in Equation (13). Equation (13) Application Note 14 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices 3.2 Types of Calibration The various types of calibration that may be performed using ADVANCED SENSE enabled devices are summarized in Table 3. Table 3 Types of calibration supported by ADVANCED SENSE enabled devices Calibration Performed By Used By No Calibration N/A Application Software Offset-Only Manufacturing Test Application Software Virtual 2-Point Manufacturing Test Application Software Running Offset Application Software* Application Software Note: * The application software is dynamically updating the 25C value written to NVM by manufacturing test. 3.3 No Calibration (No Cal) With this calibration option, no calibration is performed by manufacturing test; thus, no values for VIS(OFFSET) and kIS are stored in the microcontroller’s non-volatile memory. Instead, the application developer simply uses typical values for VIS(OFFSET) [calculated using Equation (8)] and kIS as specified in the datasheet. This scheme is the least expensive in terms of time and manufacturing cost, but it also yields the least accuracy. The term DUT (Device Under Test) refers to the high-side power switch that is being calibrated by manufacturing test or measured by the application software. The flowchart in Figure 16 summarizes the process used by the application software when the No Calibration option is being used. 1 The datasheet will actually reference IIS(OFFSET). The application developer will have to convert this into an equivalent VIS(OFFSET) value using Equation (8) Use kIS from datasheet During normal output turn ON Use VIS(OFFSET) from datasheet1 Delay for t > tsIS(ON) Convert/Read DUT (VIS) with µC ADC Calculate IL using Equation (13) Figure 16 Use result IL for diagnostics and protection Application software procedure for No Calibration option During normal device/load turn-on cycles, the software reads the IS pin from the ADC after delaying for the current sense settling time. It then uses the datasheet values for kIS and VIS(OFFSET) to calculate the load current IL using Equation (13). The application software would then compare the calculated load current value to diagnostic threshold limits stored in the microcontroller’s non-volatile memory to determine the load condition (normal, short-to-battery, short-to-ground, etc.) Application Note 15 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices 3.4 Offset-Only Calibration With this calibration option, manufacturing test measures the value of VIS(OFFSET) at an ambient temperature of 25°C. This measured value will be stored in the microcontroller’s non-volatile memory along with the typical datasheet value of kIS, and these are the values that will be used by the application software. Single-point calibration involves switching a known load at a known temperature (typically 25°C) and then measuring the analog sense current. With conventional high-side switches, the polarity of the offset must be determined and tracked such that the software can add or subtract the offset value from the measured values. To determine the value and polarity of the offset, additional points above and beyond the single-point calibration must be performed. With ADVANCED SENSE enabled devices, the offset is provided by the device under the conditions described in Figure 13; switching a known load is not required. Furthermore the offset value with ADVANCED SENSE enabled devices is always positive, which means that it is always subtracted and always easily measured. There are two ways in which the measurement of IIS(OFFSET) (in the form of VIS(OFFSET)) may be performed in the manufacturing test environment using ADVANCED SENSE enabled devices. The typical technique is illustrated in Figure 17. tVIN(RISING) tVIN(FALLING) VIN t tRESET IL t tsIS(ON) tsIS(OFF) IIs 1/kIS x IL t IIS(OFFSET) IIS(OFFSET) (in the form of VIS(OFFSET) ) can be measured during this time Figure 17 Typical technique for measuring IIS(OFFSET) In this case, the measurement is performed at time t, where t is defined by Equation (14) and the values for tsIS(OFF) and tRESET may be determined from the appropriate device datasheet: Equation (14) A more efficient technique may be performed in the case where no load (open load) is applied to the manufacturing test setup as illustrated in Figure 18. Application Note 16 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices tVIN(RISING) tVIN(FALLING) VIN t tRESET IL t tsIS(ON) tsIS(OFF) IIs 1/kIS x IL t IIS(OFFSET) IIS(OFFSET) (in the form of VIS(OFFSET) ) can be measured during this time Figure 18 More efficient technique for measuring IIS(OFFSET) In this case, the measurement is performed at time t, where t is defined by Equation (15), the value for tsIS(ON) may be determined from the appropriate device datasheet, and the value for tVIN(FALLING) is defined by the manufacturing test program: Equation (15) This more efficient approach is preferred for Offset-Only calibration because it minimizes the amount of time spent on calibration during manufacturing test (this is the approach that will be assumed in the following flowcharts and examples associated with this mode of calibration). Figure 19 summarizes the process used by manufacturing test when the Offset-Only calibration option is being used. Application Note 17 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices With no load (open load) on DUT to calibrate Read kIS from datasheet Turn DUT output ON to calibrate Delay for t > tsIS(ON) Convert/Read DUT (VIS(OFFSET) with µC ADC Store offset VIS(OFFSET) in NVM Store kIS in NVM Turn DUT output OFF Figure 19 Manufacturing test procedure for the Offset-Only calibration option The manufacturing test turns the device input ON with no load connected to the device (open load), delays for the current sense settling time, and then reads and stores the VIS(OFFSET) value (along with the typical datasheet kIS value) in the microcontroller’s non-volatile memory (NVM). Figure 20 summarizes the process used by the application software when the Offset-Only calibration option is being used. Application Note 18 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices During normal output turn ON Use kIS from NVM Use VIS(OFFSET) from NVM Delay for t > tsIS(ON) Convert/Read DUT (VIS) with µC ADC Calculate IL using Equation (13) Figure 20 Use result IL for diagnostics and protection Application software procedure for the Offset-Only calibration option During normal device/load turn-on cycles, the software reads the IS pin from the ADC after delaying for the current sense settling time. It then uses the values for kIS and VIS(OFFSET) stored in the microcontroller’s nonvolatile memory to calculate the load current IL using Equation (13). This load current is then compared to normal or faulted threshold limits to determine the condition of the load. Consider the following example: Test conditions: Application load resistance RL = 1.5; manufacturing test supply voltage 13.5V (real load current IL = 9A under these conditions); manufacturing test temperature 25°C; R IS = 2k; kIS(ACTUAL) = 14,000 (but assume that a typical value of 13,000 from the datasheet is stored in the microcontrollers nonvolatile memory by manufacturing test). Manufacturing test measurement stored in non-volatile memory: VIS(OFFSET) = 0.4V Application software measurement from ADC: VIS = 1.69V Application software calculation from Equation (13): IL = (13k/2k) × (1.69V – 0.4V) = 8.39A It can be seen above that the calculated load current is 8.39A while the actual load was 9A. The application software would then compare the calculated load current value to diagnostic threshold limits stored in the microcontroller’s non-volatile memory to determine the load condition (normal, short-to-battery, short-to-ground, etc.) 3.5 Virtual 2-Point Calibration With this calibration option, manufacturing test measures the values of VIS(OFFSET) and kIS at an ambient temperature of 25°C. Both of these measured values will be stored in the microcontroller’s non-volatile memory to be used by the application software. The traditional method for determining the value of kIS is called 2-point calibration. This involves switching two known loads at known temperatures, measuring the analog sense current for each load, and then using these measurements to calculate the kIS and VIS(OFFSET) values as discussed in Figure 10 and Equation (5) and Equation (6). Application Note 19 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices By comparison, in the case of ADVANCED SENSE enabled devices, the offset is given by the device, thereby removing the requirement for switching in two known loads during manufacturing test. As discussed in Figure 15, ADVANCED SENSE technology supports the concept of Virtual 2-Point calibration, which means that it is possible for manufacturing test to obtain a true 2-point calibration by measuring at only one load current. Figure 21 summarizes the process used by manufacturing test when the Virtual 2-Point calibration option is being used. Switch in Load #1 (IL1) to DUT output to calibrate Figure 21 Turn DUT output ON to calibrate Turn DUT output OFF Delay for t > tsIS(ON) Delay for tsIS(OFF) < t < tRESET Convert/Read DUT ( VIS(IL1) ) With µC ADC Convert/Read DUT (VIS(OFFSET)) With µC ADC Calculate kIS using Equation (10) Store VIS(IL1) for future calculations Store offset VIS(OFFSET) in NVM Store kIS in NVM Manufacturing test procedure for the Virtual 2-Point calibration option The manufacturing test turns the device input ON with a known load connected to the device, delays for the current sense settling time, and then reads and stores the corresponding VIS value. The device input is then turned OFF, the test software delays for the current sense settling time, and then reads and stores the corresponding VIS(OFFSET) value. Finally, the manufacturing test software calculates the slope using Equation (10) and stores both the kIS and VIS(OFFSET) values in the microcontroller’s non-volatile memory (NVM). Figure 22 summarizes the process used by the application software when the Virtual 2-Point calibration option is being used. Application Note 20 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices During normal output turn ON Use kIS from NVM Use VIS(OFFSET) from NVM Delay for t > tsIS(ON) Convert/Read DUT (VIS) with µC ADC Calculate IL using Equation (13) Figure 22 Use result IL for diagnostics and protection Application software procedure for the Virtual 2-Point calibration option During normal device/load turn-on cycles, the software reads the IS pin from the ADC after delaying for the current sense settling time. It then uses the values for kIS and VIS(OFFSET) stored in the microcontroller’s nonvolatile memory to calculate the load current IL using Equation (13). This load current is then compared to normal or faulted threshold limits to determine the condition of the load. Consider the following example: Test conditions: Application load resistance RL = 1.5; manufacturing test supply voltage 13.5V (real load current IL = 9A under these conditions); manufacturing test temperature 25°C; R IS = 2k; kIS(ACTUAL) = 14,000. Manufacturing test switches in known load: RL = 1.35 Manufacturing test measurement for known load: VIS = 1.83V Manufacturing test measurement stored in non-volatile memory: VIS(OFFSET) = 0.4V Manufacturing test calculation from Equation (10) stored in non-volatile memory: kIS = 13,986 Application software measurement from ADC: VIS = 1.69V Application software calculation from Equation (13): IL = (13,986/2k) × (1.69V – 0.4V) = 9.02A Note: The actual current value is 9A. The result from the application calculation based on Virtual 2-Point calibration (9.02A) is significantly more accurate than the result from the application calculation based on Offset-Only calibration (8.39A). The application software would then compare the calculated load current value to diagnostic threshold limits stored in the microcontroller’s non-volatile memory to determine the load condition (normal, short-to-battery, short-to-ground, etc.) 3.6 Running Offset Calibration This calibration option provides the highest accuracy (see also the Accuracy of Different Calibration Options later in this application note). Application Note 21 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices In fact, Running Offset is not really a calibration mode in its own right. There are only two main calibration modes: Offset Only, which is traditionally called “single-point calibration,” and Virtual 2-Point, which is traditionally called “2-point calibration.” Running Offset can be performed by the application software to enhance the accuracy of the Offset Only and Virtual 2-Point calibration measurements performed during manufacturing test. The application software achieves this by updating the sense offset value so as to compensate for temperature changes in the high-side switch’s operating environment. This technique is particularly effective at lower load currents where the effects of the offset are more pronounced. When launched, the application initially uses the VIS(OFFSET) and kIS values captured during manufacturing test and stored in the microcontroller’s non-volatile memory (NVM). Whenever the application turns the high-side switch OFF, it reads the current offset value, stores this value in the system’s volatile memory (VM), and subsequently uses this new value in its calculations. In some cases, the application can periodically turn the switch OFF for short periods (measured in milliseconds) without inconveniencing the end-user or disturbing the application load as illustrated in Figure 23. If the application is using PWM operation, the offset current can easily be measured during the off time of the PWM cycle so long as the tsIS(OFF) settling time is met. tVIN(RISING) tVIN(FALLING) tVIN(RISING) tVIN(FALLING) VIN t tRESET tRESET IL t tsIS(ON) tsIS(OFF) tsIS(ON) tsIS(OFF) IIs t IIS(OFFSET) (in the form of VIS(OFFSET) ) can be measured during these times Figure 23 Running Offset calibration timing Following the first turn-on-off cycle, the application software may simply use the most recently measured value of VIS(OFFSET), or it may keep a running average, or it may use some other technique. Figure 24 summarizes the process used by the application software when Running Offset is being used. Application Note 22 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices During normal operation Yes Use kIS and VIS(OFFSET) from NVM First ON/OFF cycle? Turn high-side switch ON Turn high-side switch OFF Delay for t > tsIS(ON) Delay for tsIS(OFF) < t < tRESET Convert/Read DUT (VIS) with µC ADC Convert/Read DUT (VIS(OFFSET)) With µC ADC Calculate IL using Equation (13) Store offset VIS(OFFSET) in VM No Use kIS from NVM and VIS(OFFSET) from VM Use result IL for diagnostics and protection Figure 24 Application software procedure for Running Offset During normal device/load turn-on cycles, the software reads the IS pin from the ADC after delaying for the current sense settling time. If this is the first turn-on cycle, the application software uses the values for kIS and VIS(OFFSET) from the microcontroller’s non-volatile memory to calculate the load current IL using Equation (13). This load current is then compared to normal or faulted threshold limits to determine the condition of the load. During each turn-off cycle, the application software will read the value of VIS(OFFSET) and store it in the system’s volatile memory. For subsequent turn-on cycles, the application software will use the value of VIS(OFFSET) that is stored in volatile memory. 3.7 Accuracy of Different Calibration Options Figure 25 illustrates the accuracy provided by the various calibration options discussed above. In the case of the sense current graphs, the red lines represent the typical slopes, the blue lines represent the maximum deviation from typical, and the green lines represent the minimum deviation from typical. Application Note 23 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Calibrating ADVANCED SENSE Enabled Devices Sense current Sense current percentage error +150% +100% +50% 0% -50% -100% -150% IIS (a) No calibration IL IL +150% +100% +50% 0% -50% -100% -150% IIS (b) Offset-Only IL ~+25% ~-15% IL +150% +100% +50% 0% -50% -100% -150% IIS (c) Offset + Virtual 2-Point IL ~+5% ~-5% IL +150% +100% +50% 0% -50% -100% -150% IIS (d) Running Offset IL Figure 25 ~+40% ~-20% ~+2% ~-2% IL Accuracy of different calibration options The sense current percentage error graphs clearly show that ADVANCED SENSE technology allows the flexibility to meet the accuracy constraints for many given load requirements. In addition, the calibration effort for both manufacturing test and application software is greatly reduced as compared to traditional and competitive devices. Note: These graphs are based upon Equation (4) and datasheet parameters for slope (steepness) and offset current including temperature variations. Example datasheet symbols are kIS, kIS(Temp), IIS(OFFSET), and IIS(OFFSET). It was assumed for the running offset case that there is no (zero) offset error. 4 Conclusion Current sensing is a well-accepted feature in high-side power switches. Traditional devices have an offset current that deteriorates the current sense accuracy, especially at lower load currents, and that may disable the current sense functionality below certain load current thresholds. Furthermore, the offset current may only be Application Note 24 V1.0, 2011-04-27 ADVANCED SENSE Calibration and Benefits Guide Conclusion calculated at some nominal temperature during manufacturing test; the application software has no way to access or calculate a new offset current value to compensate for changes (such as temperature) in the operating environment. Infineon’s high-side power switches enhanced with ADVANCED SENSE technology addresses both of these issues by moving the offset to an always positive value and by allowing the offset value to be measured by the application software. ADVANCED SENSE enabled devices also reduce time and cost in manufacturing test by supporting Virtual 2Point calibration, which provides the accuracy of a traditional 2-point calibration for slope while measuring only a single load. Infineon’s ADVANCED SENSE technology enables the compensation of sense current offset and offers high accuracy for load current measurements, all with minimal end-of-line calibration and low application software effort. Application Note 25 V1.0, 2011-04-27 Edition 2011-04-27 Published by Infineon Technologies AG 81726 Munich, Germany © 2011 Infineon Technologies AG All Rights Reserved. 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