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[FAQ] What are some commonly asked questions about voltage tracking LDOs?

Ishaan Shetye
Genius 14030 points


  1. What is the difference between a tracking LDO and a regular LDO?

A regular LDO reacts to changes in the input voltage, VIN and the load current, IOUT by dynamically minimizing error between the feedback voltage, VFB and an internal reference voltage, VREF, thereby regulating the output voltage, VOUT.

Unlike standard LDOs, tracking LDOs utilize an externally provided reference voltage which allows greater flexibility on choosing what this reference voltage can be. Tracking LDOs also have enhanced protection features compared to regular LDOs

 

  1. What can serve as the reference for a tracking LDO?

In most applications, a clean and stable voltage is chosen as the reference (input to the ADJ pin). Using feedback resistors, the output voltage may be a scaled up value of the reference voltage. The same circuitry that gains up the precision (external) reference will also gain up the inherent noise from the reference, thus low noise references are commonly used. Common external references include: any precision voltage reference (eg. REF5050-Q1), a high accuracy LDO output, or a DC source used to power an MCU (which is getting inputs from sensors to be powered by the tracking LDO) may be used as the reference.

  1. What are some common applications of tracking LDOs?

Tracking LDOs are used in two types of applications most commonly: where high accuracy of output voltage is prioritized (eg. In powering ratiometric sensors) and where enhanced protective features are required to protect the main power distribution board while powering off-board loads.

In the former application, a tracking LDO helps maintain a tight tolerance between VCC (input voltage of the sensor i.e. Vout of the LDO) and VREF (reference voltage of the LDO which also powers the ADC in the MCU receiving inputs from the sensor) while also protecting the control PCB from faults on the cables. This way, the fidelity of ratiometric sensor measurements is maintained.

In the second application, long cables are used to supply power to off-board loads from a control PCB. The long cables are susceptible to failure. Wear and tear and degradation of insulation can cause the LDO output voltage to short to the battery voltage or to ground. These long cables also have significantly large inductance and this can cause inductive kickback during a power off sequence which can cause a negative voltage to appear momentarily on the output of the LDO. Integrated protective features protect the LDO and thus, the main control PCB from any faults on the cables. The LDO thus acts as a buffer.

 

  1. What kind of load currents can tracking LDOs support?

TI offers tracking LDOs for output current levels of 70 mA, 150 mA and 300 mA. Tracking LDOs can also be paralleled to deliver higher load currents.

 

  1. Does the output voltage always have to be higher than the reference voltage?

No. The output voltage can be set greater than, equal to or lesser than the reference voltage using the FB pin on the tracking LDO.

  • For VOUT < VREF: Use a resistor divider between VREF , ADJ, and GND to scale down the voltage at the ADJ pin. Connect OUT to FB
  • For VOUT = VREF: Connect VREF to ADJ, Connect OUT to FB directly

  • For VOUT > VREF: Connect VREF to ADJ, use a resistor divider between OUT, FB and GND to scale up VOUT 

 

  1. Can tracking LDOs be directly connected to automotive battery?

Yes. TI’s tracking LDOs have integrated protective features which protect against load dump, cold crank, and short to battery. This allows direct connection of IN to a 12V automotive battery.

 

  1. Is the accuracy of tracking LDOs impacted by resistors used in the feedback network?

Yes. There are three sources which can introduce error in the output voltage of a tracking LDO. First, the offset voltage of the internal error amplifier which is inherent to the LDO. This is around 6mV. Second, the accuracy of the reference voltage. The tracking error is scaled by the same factor used to scale up VOUT from VADJ/EN. The third source is the feedback resistor tolerance. High accuracy resistors must therefore be chosen to minimize overall error in VOUT

 

  1. What protective features are present in TI’s family of tracking LDOs?

TI’s tracking LDOs conform to the standards established by the Automotive Electronics Council (AEC) by implementing integrated protection features such as protection against reverse polarity, reverse current, overtemperature (thermal shutdown), overcurrent (current limit), short to ground, short to battery and an extended absolute minimum Vout value.

 

  1. What kind of technical resources/ training material are available on Tracking LDOs?

Please see this app note on various applications of voltage tracking LDOs , this product overview of TI’s tracking LDO portfolio, and this video on applications of tracking LDOs and their implementation in systems. We also have some more comprehensive documents on Seismic re: our new tracking portfolio, including fighting guides against competition and transition guide.