This design note describes how to use the external power input pin (EXTVCC) to provide bias power to a high input voltage step-down controller, increasing the efficiency of the converter. This method can be used with controllers such as the LTC3890 (dual output) and LTC3891 (single output).
High voltage buck DC/DC controllers such as the LTC3890 and LTC3891 are popular in automotive and telecom applications due to their wide 4V to 60V input range. While these chips typically use an on-chip LDO to generate a 5V supply (INTVcc) from the high input voltage, this method can be inefficient when the input is consistently high.
To improve efficiency, the note suggests using the external power input (EXTVcc) instead. This allows the chip to be biased by stepping down the converter's output voltage rather than generating 5V from the high input. The document provides circuit solutions for outputs above 4.7V and below 2.5V.
The output can be directly connected to the EXTVcc pin as long as the output voltage is above 4.7V.
A simple voltage doubler scheme based on small P-channel and N-channel MOSFETs is used.
A multivibrator-based circuit consisting of transistors and an inductor is used to step up the voltage.