Automotive ICs

What is a Voltage Tracker?

 

1. What is a voltage tracker?

A voltage tracker is an IC that outputs a voltage to the VOUT pin that is equal to the voltage (VADJ) input to the ADJ pin.
It is called a voltage tracker because it tracks the voltage input to the ADJ pin. Such an IC is mainly used as a power supply for an automotive off-board ratiometric* sensor. *Proportional to power supply voltage

Figure.1 Example showing a voltage tracker connection
Figure.1 Example showing a voltage tracker connection

 

2. When a voltage tracker is needed

・When high-accuracy sensing is required

Automobiles developed in recent years contain a wide variety of sensors as high-accuracy sensing is indispensable to advanced automotive control systems.
These voltage tracker ICs are ideal for ensuring that electricity is safely delivered to off-board sensors situated away from ECUs (Electronic Control Units) and that signals are correctly read. The reason for this lies in the superior voltage tracking accuracy of these ICs, which is described in “3. Voltage tracker characteristics”.

・When an MCU needs protection from short to power/ground faults

Figure.2 Off-board sensor and MCU sharing a power supply (LDO)
Figure.2 Off-board sensor and MCU sharing a power supply (LDO)

When an MCU (Microcontroller Unit) and a sensor share the same ECU board, their power voltage can easily be provided by one power supply.
However, most automotive sensors are off-board sensors mounted on boards separate from the MCU. If an off-board sensor and MCU in such an arrangement share the same power supply, unwanted short to power and short to ground faults in the wire harness linking the two boards could damage the MCU (see Figure.2).

 

Figure.3 Using a voltage tracker to power an off-board sensor
Figure.3 Using a voltage tracker to power an off-board sensor

A voltage tracker separates the power supply to the off-board sensor from that to the ECU and is also equipped with protection functions to protect an MCU from unwanted short to power and short to ground faults (see Figure.3).

 

Figure.4 Contributes to the heat dissipation of the LDO
Figure.4 Contributes to the heat dissipation of the LDO

 

 

Additionally, the use of a voltage tracker contributes to the heat dissipation of the LDO (see Figure.4).

 

 

3. Voltage tracker characteristics

(1) Superior voltage tracking accuracy

A characteristic of a voltage tracker is that the output voltage tracks the voltage input to the ADJ pin making it possible to minimize the difference (offset voltage) between the input voltage to the ADJ pin and the output voltage.
The following example describes the impact on the reading accuracy of an off-board sensor (Ratiometric Linear Hall IC) when using an LDO (Low Drop Out) regulator to power an off-board sensor versus using a voltage tracker.

Table.1 Explanation of elements and pins in the block diagram
Elements/pins Explanation
ECU An abbreviation for Electronic Control Unit, a device used to control the engine, air conditioning, the transmission and other systems.
An average automobile contains tens of ECUs.
MCU An abbreviation for Microcontroller Unit, a semiconductor chip controlling functions and characteristics of electronic devices.
ADC An abbreviation for Analog-to-Digital Converter, an electronic circuit that converts analog signals to digital signals.
Vref Indicates the full-scale ADC reference voltage.
LDO
≫What is a LDO?
The abbreviation for Low Drop Out, a linear regulator capable of controlling the output voltage even when the difference between input and output voltage is small.
Ratiometric sensor A sensor that uses voltage changes to output signals.
Since sensor sensitivity is proportional to power supply voltage, the output voltage changes with changes in the power supply voltage.
An example is the Ratiometric Linear Hall IC, which is used as a current sensor throughout an automobile.
Using an LDO as a power supply for the off-board sensor

Figure.5 shows how an LDO can be used as a power supply for the off-board sensor. Even if products with the same output voltage are selected for LDO1 and LDO2, manufacturing variations as well as the temperature and current changes that each is exposed to will result in different values.
Let’s assume that only the power supply voltage (LDO2 output voltage) of the off-board sensor drops from 5.0V to 4.9V due to a temperature change. As the off-board sensor output signal is 2.50V at 0mT when the power supply voltage is 5.0V, off-board sensor output drops in proportion to the supply voltage to 2.45V. As a result, an ADC receiving the same reference voltage of 5.0V from LDO1 will record an error when reading the off-board sensor signal. The same result occurs also when only the output voltage of LDO1 changes.

Figure.5 Using an LDO
Figure.5 Using an LDO

Using a voltage tracker as a power supply for the off-board sensor

However, as shown in Figure.6, use of a voltage tracker as a power supply for the off-board sensor solves these problems.
Even if the output voltage of the LDO drops from 5.0V to 4.9V due to a temperature change, the reading accuracy of the off-board sensor does not decrease because the voltage tracker output voltage tracks the voltage (VADJ) input to the ADJ pin.

Figure.6 Using a voltage tracker
Figure.6 Using a voltage tracker

 

(2) A variety of protection functions are integrated

Figure.7 Operation of the reverse current protection function in a short to power fault
Figure.7 Operation of the reverse current protection function in a short to power fault

A voltage tracker integrates a variety of protection functions.
Automotive off-board sensors normally exchange signals with an ECU via a wire harness. If the wire harness is short-circuited to the power line (= shorted to power) or shorted to GND (= shorted to ground) due to unwanted failure, ECU operation may also be adversely affected.
The reverse current protection function in the voltage tracker prevents current from entering the core of the ECU from the output pin of the voltage tracker in a short to power fault (see Figure.7).

Figure.8 Overcurrent protection circuit and thermal shutdown circuit protect against short to ground faults
Figure.8 Overcurrent protection circuit and thermal shutdown circuit protect against short to ground faults

The overcurrent protection circuit and the thermal shutdown circuit integrated in the voltage tracker are functions that protect the voltage tracker from high current or self-generated heat due to a short to ground fault in the output pin of the voltage tracker (see Figure.8).
A voltage tracker uses these protection functions to protect ECUs and MCUs from short to power and short to ground faults.

 

 

ABLIC’s S-19720 Series of voltage trackers also come equipped with an overcurrent protection circuit a thermal shutdown circuit and reverse current protection function to protect devices against destruction from short to power or short to ground faults. In addition, the input and output voltage difference (offset voltage) of the voltage tracker is ±5mV, the industry’s smallest offset voltage. These features improve off-board sensor reading accuracy and helps ensure automotive safety.


4. ABLIC’s Voltage Tracker

S-19720 Offset voltage: ±5mV
Output current: 50mA

Datasheet

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S-19721 Offset voltage: ±4.5mV
Output current: 250mA

Datasheet

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