High Power Dissipation during startup (~1 ms)

You are OK. Although a transient thermal impedance graph is not included, there are 140V applications with 4.7uF on DRVCC shown in the Data Sheet. As you mentioned, 1ms is a short time and the theta j-c is only ~7 deg C/W (https://www.analog.com/media/en/package-pcb-resources/package/thermal-table.pdf).

A quick look at a 2N7002 (60V MOSFET) in SOT23 on Digikey indicates it can take 3W+ for 1ms and has a theta ja 10 times higher than the LTC3895.

As a side note, I would consider adding the optional external boost diode (DRVCC to BOOST - see diode recommendations in the LTC7810/7862 Data Sheets). The external diode provides a lower impedance/faster boost refresh especially at high duty cycles and I recommend when using the LTC3895 or LTC7801. These are all similar 100% duty parts.

You are OK. Although a transient thermal impedance graph is not included, there are 140V applications with 4.7uF on DRVCC shown in the Data Sheet. As you mentioned, 1ms is a short time and the theta j-c is only ~7 deg C/W (https://www.analog.com/media/en/package-pcb-resources/package/thermal-table.pdf).

A quick look at a 2N7002 (60V MOSFET) in SOT23 on Digikey indicates it can take 3W+ for 1ms and has a theta ja 10 times higher than the LTC3895.

As a side note, I would consider adding the optional external boost diode (DRVCC to BOOST - see diode recommendations in the LTC7810/7862 Data Sheets). The external diode provides a lower impedance/faster boost refresh especially at high duty cycles and I recommend when using the LTC3895 or LTC7801. These are all similar 100% duty parts.

Hi Kurk

Thanks, this was the information I needed.

I will look into the external boost diode, but my main priority was to clarify the thermal aspects at power-up.