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

When the LTC7810 is powered up (voltage applied to Vin and RUN pin passing 1.2V) DRVcc starts to rise. But at a voltage between 3 and 5 it suddenly gets unstable. Up to the point where it gets above the DRVcc UVLO after which the LTC7810 starts switching and DRVcc is correctly at the set 10V. This behavior causes a lot of variation in time before the 3.3 V becomes available.

DRVcc is only connected to a 4.7uF 25V ceramic capacitor and the NDRV pin. And of course, for both channels, via a bootstrap diode and 10 Ohm bootstrap resistor to the boost pin.

The RUN pin of channel 1 is connected to Vin via a resistor divider and capacitor for a ~10ms delayed start. The RUN pin of channel 2 is kept low to disable this channel until its voltage is required.

Some images of the logged signals:

• Green: Voltage on Vin, 27V
• Blue: DRVcc voltage
• Pink: Run pin voltage (resistor divider and 1uF on run pin for a delayed start)
• Red: 3.3V output voltage on channel 1

Another startup, clearly a different start up time:

We like to inquire about any possible cause and how to solve this.