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Texas Instruments Choosing the Right Voltage Reference for Your Automotive Application Application notes
Choosing the Voltage Reference for Your Automotive
Application
Introduction
5V VDD
In automotive systems such as ADAS, body
electronics, powertrain, etc. there is a need for precise
data converters. For every data converter, a precise
voltage reference (VREF) is often necessary to reach
the lowest possible errors when measuring automotive
signals. While many data converters can incorporate
internal references, it is difficult to find a internal
voltage reference in CMOS technology that can reach
the high accuracy, low temperature drift, and low noise
of a bipolar process. This is even more complicated in
digital processes for MCUs as the internal reference
can be noisy due to all the inherit clocking noise. Due
to this, it is often desirable to use an external voltage
reference to have more precise measurements.
PMIC
Load Switch
Buck/LDO
Processor Power Supply
CMOS Sensor
Imager
ADAS Processor
AIN0
AIN1
AIN2
AIN3
Analog Input Signal
VDD
VREF+
VREF-
3V VREF
OUT
IN
GND
REF3430-Q1
MCU
Copyright © 2017, Texas Instruments Incorporated
Figure 2. REF3430-Q1 with MCU
In order to ensure that a system meets the error
specifications, it is important to characterize the signal
chain to understand the errors of the voltage rail.
Voltage rail errors have become a more stringent as
the total error allowed has been reducing to allow for a
more optimized system. An issue with characterizing
the signal chain error in a MCU is that typically internal
voltage references are not fully characterized as in
depth as external voltage references and often lack
maximum worst case values. Due to this it is difficult to
calculate the worst case error of the system. This
challenge can be resolved by using an external
voltage reference such as REF3430-Q1 as shown in
Figure 2.
DC/DC
Table 1. Typical Core Voltage Rail Monitoring
CAN
(Safety)
MCU
Voltage
Reference
Specification
Ethernet
Digital Processing
Vehicle Interface
Control
Signal
Logic
Supervisor
Sequencer
Voltage
Reference
Diagnostics & Monitoring
Copyright © 2017, Texas Instruments Incorporated
Figure 1. Simplified ADAS Front Camera Diagram
Voltage Reference for Monitoring a 1% 1V Rail
In an automotive advanced driver assistance system
(ADAS) it is important to monitor the voltage rails
being used in the MCU/DSP/FPGAs. Voltage rails are
monitored independently with an ADC and voltage
supervisor combination to ensure that the voltage rail
does not cross a certain voltage which might cause an
undervoltage or overvoltage event which could
damage the MCU/DSP/FPGAs. Typically these ADCs
are internal to a microcontroller (MCU) that are used to
ensure the voltage rails are working correctly. It is not
uncommon to see an external VREF be attached to a
MCU for precision and to ensure that the internal ADC
has a redundant voltage reference for robustness.
With the addition of an external VREF it is possible to
have an accurate ADC that does not need calibration
for monitoring a 1% 1V rail.
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Requirement
Voltage Rail
1V
Max Error (-40C to 125C)
1%
Table 1 shows an example of voltage rail monitoring
requirements for monitoring a 1V rail for a precision
MCU in an ADAS system by a micro. Due to the
stringent requirements of some voltage supply rails to
be within a certain voltage range, we want to make
sure that the total error of the signal chain system is
less than 1% so we can measure the deviation which
in this case is 10mV.
For a 1V DC measurement it possible to use an
external voltage reference to calculate the total error.
There are two ways to calculate error in a system:
worst-case and root sum squared (RSS). The main
difference between the error calculations is how the
individual errors of a system are combined. In worsecase error all the errors are additive of their worst case
which results in a conservative to ensure that every
device will work but main drawback is that 6+ sigma
events are taken into account and this can increase
the cost of a system. A common alternative to the
Choosing the Voltage Reference for Your Automotive Application
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1
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worst-case method is the RSS method that is based
on statistical tolerance analysis. RSS is used because
it provides a more realistic acceptable limit that is
based on distributions. In this example we use RSS
due to its more realistic presentation of error.
When performing error calculations, the error of the
ADC is independent but the error of the voltage
reference is proportional to the ADC analog input
signal. The VREF total error calculated in is only valid
when the analog input signal is at full scale. In this
example, since the analog input is 1V and not the full
scale voltage, only a fraction of the VREF total error
affects the analog input can be seen in Equation 3.
Table 2. REF3430-Q1 3V Specifications
Specification
Requirement
Reference Voltage + Initial Accuracy
3V ± 0.05%
Temperature Drift (-40°C to 125°C)
6 ppm/°C
Temperature Hysteresis (TempCo)
30 ppm
Long-Term Stability
25 ppm
1/f Noise
15 µVPP
ErrorVREF @ AIN
Total
Accuracy
500ppm
2
2
TempCo
2
TempHyst
6ppm / qC * 165qC
2
2
LongTerm Drift
30ppm
2
25ppm
2
2
ErrorVREF @ AIN
Total (LSB)
* 2 ADC Re solution
370 ppm * 2
1.51 LSB
ErrorVREF
ErrorVREF @ AIN
ADC Total
1.51 LSB
2
(4)
2
ErrorADC
Total
6.92 LSB
2
Total
2
7.08 LSB
2.82 bits
Effective Re solution
(5)
ADC Re solution ErrorVREF
ADC Total
12 bits 2.82 bits
(6)
9.18bits
Table 4. Total Error
2
Specification
Requirement
REF3430-Q1 Total Error for AIN 1.51 LSB
ADC Total Unadjusted Error
Table 3. Example of Internal MCU ADC
Percent Error of Vin
7.08 LSB
0.517%
Requirement
Resolution
12 Bits
Gain Error
4 LSB
Offset Error
4 LSB
INL Error
4 LSB
When choosing an ADC it is important to find an ADC
with the lowest possible errors. Internal MCU ADC with
the specifications from Table 3 are used for this
example. The ADC total error in this situation is also
known as the total unadjusted error and it is calculated
similar to the VREF total error by using the RSS
method.
ErrorADC
4 LSB
Table 4 summarizes the final error of the system as an
external voltage reference can help in characterizing
the error to make sure that the minimum accuracy is
met. In practice the measurements will be more
precise than the total RSS error but this error can
provide a baseline to improve on. The ADC errors of
the system can easily be reduced by choosing a more
accurate ADC since the dominant error is from the
ADC. There are also techniques to improve the
voltage reference error such as using a higher voltage
external voltage reference. In Table 5 there are
alternate voltage reference devices that can help
reduce this error or save power.
Total
Gain Error
2
2
Offset Error
4 LSB
2
2
4 LSB
INL Error
2
2
Table 5. Alternative Device Recommendations
Device
6.92 LSB
(2)
2
Total
12
(1)
Total Unadjusted Error
ErrorVREF @ AIN
2
1/ f Noise
15 PVPP / 3V
(3)
With ADC specifications, the VREF total error is
convert into LSB using Equation 4 which makes it
possible to combine both VREF and ADC errors using
the RSS method in Equation 5.
1110ppm
Specification
Total *AnalogIN
Re ference Voltage
1110ppm * 1V
3V
370ppm
The total error for a VREF reference calculation is a
culmination of all the errors such as initial accuracy,
temperature coefficient, ect. To calculate the total
error, all the errors should be in common units such as
ppm (parts-per million) as in Equation 1. The VREF
total error can be further reduced with calibration, as
calibration can eliminate the static errors such as initial
accuracy and TempCo. For the purposes of this
example, errors such as solder shift, load regulation,
line regulation among others have been omitted but
they can be included to calculate a more accurate
representation of the VREF total error. shows how all
the errors are combined using the RSS method.
ErrorVREF
ErrorVREF @ AIN
Total
Optimized Parameters
REF3130-Q1
Quiescent current, Temperature drift
REF5025-Q1
Initial accuracy, Temperature Drift, Noise
LM4132-Q1
Quiescent current, Temperature drift
LM4128-Q1
Quiescent current, Temperature drift
Choosing the Voltage Reference for Your Automotive Application
Copyright © 2018, Texas Instruments Incorporated
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