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What is a voltage tracker?


1. What is a voltage tracker?

Voltage tracking is an integrated circuit that outputs a voltage to VOUT pin that is equivalent to voltage applied to ADJ (VADJ) pin.

It's called a voltage sensor because it monitors input voltage at ADJ pin. This IC is mainly used as a power supply for discrete sensors and is proportional to supply voltage.

Fig. 1. Example of connecting a voltage monitor

2. When a Voltage Tracking Device is Required

  • Cases requiring high precision detection

  • Motor vehicles developed in recent years contain many sensors because high-precision sensors are needed for advanced automotive control systems.

    If you want to securely power and accurately read signals from a separate sensor mounted away from ECU (Electronic Control Unit), a voltage tracker is best choice due to its excellent voltage tracking accuracy. This point will be explained in more detail in section “3. Characteristics of voltage tracking device.

  • When MCU needs to be protected against power short circuit or ground fault

  • When MCU (Microcontroller Unit) and sensor use same ECU board, they can be easily powered from same power supply.

    Most automotive sensors, however, are discrete sensors mounted on a substrate separate from MCU. If a single sensor and MCU in such a setup were to use same power source, there would be unwanted power shorts and ground faults in wiring harness connecting two boards, which could damage MCU (see Figure 2) .

    Fig. 2. Separate sensor and MCU with common power supply (LDO)

    Voltage tracking separates power supply of split sensor from power supply of motor control unit, and is also equipped with protective functions to protect control unit of microcontroller from unwanted damage caused by short circuits to the power supply and ground faults (see figure 3).

    Fig. 3. Using a voltage tracker to power a separate sensor

    3. Voltage Tracking Device Specifications

    (1) Excellent voltage tracking accuracy

    Voltage tracking is characterized in that difference (bias voltage) between input voltage and output voltage of ADJ terminal can become very small because output voltage follows input voltage of ADJ terminal.

    The following example describes effect on accuracy of a discrete sensor (Ratiometric Linear Hall IC) when using an LDO (Low Drop Out) regulator to power a discrete sensor compared to using a voltage tracker.

    Description of components and pins in block diagram in Table 1

  • Using LDO as a power supply for individual sensors

  • In fig. Figure 4 shows use of an LDO as a power supply for a discrete sensor. Even if "LDO1" and "LDO2" with same output voltage are selected, manufacturing differences as well as temperature and current fluctuations faced by each product will result in different values.

    Assume that only remote sensor supply voltage (LDO2 output voltage) drops from 5.0V to 4.9V due to temperature change. When supply voltage is 5.0 V, separate sensor output is 2.50 V at 0 mT, but drops to 2.45 V because output is proportional to supply voltage. Therefore, an ADC receiving same 5.0 V reference from "LDO1" will have a read error when reading split sensor signal. The same result will take place when output voltage of "LDO1" fluctuates.

    Fig. 4. When using LDO

  • Using a voltage tracker as a power source for discrete sensors

  • However, as shown in fig. 5, using a voltage tracker as a power source for a separate sensor solves these problems.

    Even if output voltage of LDO drops from 5.0V to 4.9V due to temperature changes, output voltage of voltage tracker will match input voltage of ADJ (VADJ) terminal, so reading accuracy of individual sensors will not decrease.

    Fig. 5. Using a Voltage Tracker

    (2) Built-in multiple security features

    Voltage testers have various built-in protection functions.

    Individual sensors used in vehicles usually communicate with ECU via a wiring harness. If wiring harness is shorted to power (=short to power) or to ground (GND) (=short to ground) due to random failure, it may cause ECU to operate abnormally on connection destination.

    The reverse current prevention function built into voltage monitor prevents current from flowing into ECU from output terminal of voltage monitor during a short to power (see Figure 6).

    Fig. 6. Daytime backflow prevention function

    In addition, function of built-in overcurrent protection circuit and thermal shutdown circuit is to protect voltage monitor from overcurrent and self-heating occurring at output of voltage monitor during a ground fault (see Figure 7) . ).

    Voltage tracking can protect ECU and MCU from power short circuit and ground fault with these various protection functions.

    Fig. 7. Overcurrent protection circuit/thermal switch circuit in case of earth fault

    The S-19720 series has a built-in overcurrent protection circuit, an overtemperature shutdown circuit, and a reverse current prevention function that prevents damage to equipment due to power short circuits and ground faults. In addition, difference between input and output voltage (bias voltage) of voltage tracking device is ±5 mV, which provides industry's lowest bias voltage. This improves accuracy of split sensor and helps keep vehicle safe.