In automotive ECUs, microcontrollers and other parts of the system have to be supplied by a stable and reliable voltage that is lower than the battery voltage (e.g. 3.3V or 5V) and works over the entire temperature range (from -40°C to 150°C). Use of discrete solutions does not manage to fulfill those conditions because of voltage dependency on load-current (e.g. resistor divider) or on temperature (e.g. Zener diode).
A linear voltage regulator converts a DC input voltage (e.g. battery line) into a pre-defined lower DC output voltage (e.g. 5V). In spite of input voltage variations, the output voltage remains steady and stable, as long as the input voltage is greater than the output voltage. Linear voltage regulators are the most frequently used electronic power supplies in automotive applications.
Linear voltage regulator block diagram
Every linear voltage regulator consists of an internal reference voltage, an error amplifier, a feedback voltage divider and a pass transistor. The output current is delivered via the pass element controlled by the error amplifier. The error amplifier compares the reference and output feedback voltages.
If the output feedback voltage is lower than the reference, the error amplifier allows more current to flow through the pass transistor, hence increasing the output voltage.
On the contrary, if the feedback voltage is higher than the reference voltage, the error amplifier allows less current to flow through the pass transistor, hence decreasing the output voltage.
NPN linear regulatorsConventional linear regulators use NPN bipolar transistors as the pass element. Usually the pass element is composed of a PNP base current driver transistor and a single NPN power transistor, therefore the drop voltage, i.e. the minimum voltage difference between input and output, is equal to
V SAT(PNP) + V BE(NPN), which is about 1.2V. Functionalities and integrated protection are limited and additional protection circuitries are required.
PNP linear regulatorsWith only a single PNP bipolar transistor as the pass element, the drop voltage of PNP regulators is about 0.5V. For this reason, this type of regulator is called Low Drop Out (LDO). This enables it to operate during a drop in battery voltage (e.g. cranking). PNP regulators are protected against reverse polarity faults.
NMOS linear regulatorsNMOS pass transistors provide very low drop-out voltage and minimal quiescent current. A charge pump is necessary to achieve low drop-out voltage, because the gate of the NMOS needs to be ~2V higher than the voltage at source to drive the pass element open. However, the change pump also introduces additional line noise.
PMOS linear regulatorsPMOS linear regulators provide very low drop-out voltage and minimal quiescent current. An internal charge pump is not necessary for the PMOS pass element. The new control loop concept in the new Infineon PMOS linear regulators allows a faster regulation loop and better stability, requiring only a single 1µF output capacitor for stable operation.
The output voltage of some linear voltage regulators can be adjusted by an external resistor divider, connected to the voltage adjust pin named as ADJ or VA.
For a certain output voltage, the value of the external resistors can be easily calculated with the formula:
- R 2 < 50kΩ to neglect the current flowing into the ADJ/VA pin.
- Internal reference voltage V ref is device-dependent. The V ref value of a specific device can be found in its datasheet.
If an output voltage equal to the reference voltage is needed, the output pin Q has to be directly connected to the voltage adjust pin ADJ/VA.
Selection of the external resistors for TLE42764GV/ DV
According to the datasheet
- Internal reference voltage V ref : typically 2.5V,
- Output voltage V OUT adjustable between 2.5V and 20V,
Required output voltage: V OUT= 3.3V.
The following resistors could be selected:
- R 1 = 12kΩ, R 2 = 39kΩ
It must be taken into consideration that the accuracy of the resistors R 1 and R 2 adds an additional error to the output voltage tolerance.
Application diagram TLE42764GV/ DV