LTC7810 - DRVcc not stable until the DRVcc UVLO is reached. Leading to various startup times that can be as long as a few seconds.

Since it's been over a month now with no response, I do not expect too much from this forum. But to add to the issue for future reference:

The LTC7810 was replaced but the behavior remains about the same.

DRVcc capacitor, bootstrap capacitors, bootstrap diodes and bootstrap resistors were replaced, but the behavior remains about the same.

Other PCBA's with the same circuit are functioning fine.

It looked like the cause and solution were found. Because of clearance rules, there was only limited connection between PGND and GND on the PCBA. DRVcc was referenced and connected to PGND via the recommended 1uF capacitor. What should have been a good low-impedance connection between PGND and GND, wasn't. Shorting PGND to GND seemed to solve the issue for a moment.

However, it later returned and the previous observation seemed a lucky shot. Replacing the LTC7810 again, just to be sure, did not solve the issue and DRVcc (blue line) does never reaches its required value:

• Green: Voltage on Vin, 20V
• Blue: DRVcc voltage
• Yellow: Run pin voltage (resistor divider and 1uF on run pin for a delayed start)
• Red: 3.3V output voltage on channel 1

Zooming in on DRVcc, a saw tooth profile is observed:

Is this erroneous behavior of board components that are related to the internal LDO?

Finally the cause and solution were found: A small 2.2 Ohm resistor was connected in series with Vin of the LTC7810.

Though visually fine and the correct value, it measured a high resistance instead, causing the voltage on Vin pin to drop as soon as DRVcc was started. Replacing the faulty resistor solved the issue and resulted in a stable DRVcc and operation of the LTC7810 as intended.