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Texas Instruments Automotive Off-Board Sensor Power Considerations Application notes
Automotive Off-Board Sensor Power Considerations
One doesn’t need to look long at modern cars to see
that sensors are ubiquitous and plentiful throughout
the body of a vehicle. And their purposes are diverse,
from keeping the cabin climate at a cool, stable
temperature to detecting when a door is ajar to
determining throttle position. However, depending on
what is being sensed, their location may often times be
in remote areas that are not near a control module. In
such cases, their excitation voltage and signal,
whether analog or digital, is transmitted via wire
harnessing.
This presents its own set of challenges since wire
harnessing introduces another potential point of failure.
Wire harnessing, as seen in Figure 1, typically
bunches together many wires that route along the
vehicle frame. Such proximity to other wires and the
chassis can lead to unwanted shorting that must be
anticipated and protected against.
IN
IN
DC/DC converter
or LDO
5V
Tracking LDO
ADJ
OUT
OUT
(Wire
Harness
Inductance)
5V
MCU
ADC
Sensor
Control Unit Board
Figure 2. A typical configuration for providing offboard power using a tracking LDO
Consider a short-to-GND failure, as shown in Figure 3.
In such an event, the regulator or load switch
supplying the sensor must have a current limit that
activates quickly. In the case of an LDO, this puts the
linear regulator into a constant current mode that can
subsequently trigger thermal shutdown due to the
increased dissipation across the device. The MCU and
ADC, unaffected by the failure event, will be able to
detect a failure given the absence of a sensor signal.
A short-to-battery failure is also possible. This could
potentially bias the output of the regulator higher than
the voltage at the input terminal, as shown in Figure 3.
Unimpeded, current will try to flow backward through
the regulator. Fortunately, there are robust LDOs that
incorporate back-to-back MOSFETs that prevent
reverse current flow.
Figure 1. Typical wiring harness found in a vehicle
Failure Modes and Protection
There are four main failure modes as regards wire
harnessing: a short to the chassis (or GND), a short to
the battery potential, a short to another potential, or an
open circuit. Before considering the impacts of each
failure, it’s helpful to look at a simplified control unit
sourcing power to the remote sensor like the
configuration shown in Figure 2.
One DC/DC converter or LDO, is used to supply 5V to
the MCU and ADC. However, an additional regulator
(a tracking LDO in this case) or load switch is required
to provide 5V to the off-board sensor. This is because,
in the case of a failure event with the wire harnessing,
the control unit needs to be protected.
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An alternative, common option is to add a diode in
series with the input to prevent reverse current and
subsequent damage to the regulator. This option,
however, adds cost and contributes to the voltage drop
across the device in the case of an LDO. Again, with
either implementation, the MCU, ADC and rest of the
control unit is unaffected by the wire harness failure.
It should be noted that the shorting of the output of the
regulator to any voltage higher than the regulated
voltage has the potential to damage an IC. This is
why, whether the output is shorted to 9V, a 12V
battery potential or any other voltage, the chosen
regulator must be able to withstand such exposure
without blowing the internal ESD cells. This rating is
typically included in the Absolute Maximum Ratings
section of the device datasheet.
Automotive Off-Board Sensor Power Considerations
Copyright © 2018, Texas Instruments Incorporated
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Short To Ground Protection
IN
OUT
7V
IOUT = ILIM
IN
DC/DC
Converter
OUT
Tracking
LDO
GND
To understand the value of a tracker LDO, it helps to
look at an example. Consider an HVAC system where
the temperature in the cabin is being gauged by a
remote NTC and an 8-bit ADC.
Short To Battery Protection
IN
OUT
DC/DC
Converter
7V
Ireverse = 0mA
IN
OUT
Tracking
LDO
IN
IN
Figure 3. Two failure scenarios for off-board power
supplies
Since any of these failures is possible with wire
harnessing, the regulator or load switch supplying offboard power must be robust and incorporate various
kinds of protection to shield the control unit from
damage. This, however, is only the first consideration
of a regulator powering an off-board sensor.
DC/DC converter
or LDO
Traditional LDO
4.9V
OUT
OUT
5V
8-bit
ADC
MCU
Control Unit Board
NTC
Tracking LDOs and Measurement Accuracy
In addition to protecting the control unit from wire
harness failures, a special type of regulator is
sometimes required to supply off-board sensors. This
type of regulator, a tracking LDO, is specially designed
to supply power to sensors with ratiometric outputs.
These include many temperature sensors (like NTCs)
and rotational sensors (like hall-effect sensors).
Since a sensor’s ratiometric output is proportional to
the excitation voltage, an accurate reading from the
ADC requires its reference voltage to be the same as
that of the sensor supply voltage. This is problematic
with a traditional LDO given that the output voltage of
one LDO may not be exactly the same as another due
to differences in process, junction temperature, and
line and load variations. Such incongruence can lead
to inaccurate readings.
Figure 4. Providing power to a ratiometric sensor
with a non-tracking LDO
If the voltage being supplied to the NTC is only 4.9V,
then the voltage being read at the sensor will shift
versus when it was supplied by 5V. 25°C may have
once corresponded with a 2.5V reading. However,
since the excitation voltage is now 4.9V, 25°C now
corresponds with a 2.45V output resulting in a 50mV
delta from the true value. Since the ADC codes in
~19.5mV steps, the temperature will be read out as
slightly hotter (by two bits) than it actually is. If each bit
corresponds to a 0.5°C difference, then the control
module will cool longer, or heat shorter, by 1°C to
reach the target temperature in the cabin. This
problem is exacerbated when the reference voltage
and sensor excitation voltage are even further apart.
To overcome this, a tracking LDO does not have an
internal reference voltage like a traditional LDO.
Instead, the inverting input of the internal error
amplifier is bonded out to an ADJ pin. This pin can
then be connected to the voltage rail being supplied to
the ADC reference, as shown in Figure 2. The LDO is
then put into a unity gain configuration.
This example goes to show that having a tracker LDO
will drastically improve sensor readings and lead to
more accurate control systems when a sensor is being
powered remotely. This is in addition to the value the
tracker LDO brings in protecting the control unit from
any wire harnessing failures.
This topology allows the tracker LDO output to track
the supply voltage of the ADC reference. As such, the
excitation voltage of the sensor will be very close to
that of the ADC reference voltage. With the
TPS7B4253-Q1, the output will track the ADJ pin
voltage with a maximum deviation of ±4mV.
Table 1. Device Recommendations
2
Device
Tracking
Accuracy
Output
Current
Package
TPS7B4250-Q1
±5 mV
50 mA
SOT23-5
TPS7B4254-Q1
±4 mV
150mA
SO PowerPAD-8
TPS7B4253-Q1
±4 mV
300mA
SO PowerPAD-8
HTSSOP-20
Automotive Off-Board Sensor Power Considerations
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