The power supply of an ECU

How does the power supply of an ECU work?

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The power supply supplies power to all other units on the ECU. Different functional blocks need energy in different forms.

In a car, the available voltage source is the battery, which in operation has terminals 13.5 - 14V. This voltage value can vary depending on several factors. Every functional block needs constant voltage to function properly.

KL30, KL31

KL is the abbreviation for "klemme", which is the German term for connector / connection. These two connections conventionally represent the battery terminals. KL30 represents the "+" terminal and KL31 represents the "-" terminal. These terminals are the power supply of the ECU.

Power sub-blocks

Each independent circuit on the ECU can have its own power supply. Dedicated power supplies they can be permanently active or they can be activated when necessary. These sub-power supplies are individually filtered. Several active power supply sub-blocks constitute several parallel current paths.

A component often used for the power supply sub-blocks of the ECU is NCV4274 / NCV4274A of the ON Semiconductor.


VDD / VCC is a conventional notation for the supply voltage of one or more dedicated functional blocks. In automotive, VDD / VCC is used for the supply voltage of logic blocks, uC, AO, MUX. The value of the VDD / VCC voltage is usually 5V, and is obtained by a sub-block of the power supply block.

The voltage of 5V is obtained using a DC "VTC" descending voltage. Descending VTC circuits are also called voltage stabilizers.

The 5V VDD / VCC stabilized voltage is filtered with both high value and low value capacities to minimize voltage variations regardless of their frequency.

The capacities are positioned both before and after the stabilizer to filter both the voltage variations induced from the outside of the block and those induced from inside. The voltage stabilizer can be in a discrete capsule or incorporated in a chip that performs several functions (ex: SBC- System Basis Chips - TLE6266G).


Some analog circuits need reference voltages in their operation. The reference voltage is a fixed value voltage, established, which is used as a benchmark.

The most common example is the value of the "GND" voltage which is considered 0V and is used as a benchmark both in analog voltage measurements and in the logic levels of digital circuits.

The reference voltage can be any fixed value of the voltage used for comparison. A digital comparator will output 1 logic if the signal at the input is greater than Vref, or 0 logic if the input is less than Vref.

In electrical tests, GND is usually used as the reference voltage, but VDD or other voltages on the ECU can also be used as reference for measurements. Usually if GND is not the benchmark, this is mentioned.


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  1. Car electronics is nasty in that sense that battery voltage drops a lot, where starter motor is engaged. That would dim out the lights, for example, but looks cheap - so you’ll need a buck-boost regulator to keep the voltage even during the dip.

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