Automotive Power - ADVANCED SENSE Calibration and Benefits Guide

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
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Application Note
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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
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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.
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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.
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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
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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).
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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).
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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.
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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.
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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.
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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).
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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.
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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)
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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.)
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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.
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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.
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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.
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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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THE INFORMATION GIVEN IN THIS APPLICATION NOTE IS GIVEN AS A HINT FOR THE
IMPLEMENTATION OF THE INFINEON TECHNOLOGIES COMPONENT ONLY AND SHALL NOT BE
REGARDED AS ANY DESCRIPTION OR WARRANTY OF A CERTAIN FUNCTIONALITY, CONDITION OR
QUALITY OF THE INFINEON TECHNOLOGIES COMPONENT. THE RECIPIENT OF THIS APPLICATION
NOTE MUST VERIFY ANY FUNCTION DESCRIBED HEREIN IN THE REAL APPLICATION. INFINEON
TECHNOLOGIES HEREBY DISCLAIMS ANY AND ALL WARRANTIES AND LIABILITIES OF ANY KIND
(INCLUDING WITHOUT LIMITATION WARRANTIES OF NON-INFRINGEMENT OF INTELLECTUAL
PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION GIVEN IN
THIS APPLICATION NOTE.
Information
For further information on technology, delivery terms and conditions and prices, please contact the nearest
Infineon Technologies Office (www.infineon.com).
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