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High Power Dissipation during startup (~1 ms)

Category: Hardware
Product Number: LTC3895

I ran the simulation provided for the LTC3895 (LTC3895 Demo Circuit - High Efficiency High Voltage Buck Converter (14-130V to 12V @ 5.0A) with no changes to the circuit or simulation profile

At 0.15 ms - 0.33 ms the power consumption for the LTC3895 is ~10 W, which would correlate to a junction temperature of 305°C (25°C + 10W * 28°C/W). 

I know this is for a very short amount of time, but would the eat into the life time of the IC?.

For an application we have, we see ~6W of power dissipated by the LTC3895 for a period of ~ 1 ms. Should I be concerned about this?

The 10 W in the simulation correlates to the time where the DRVcc voltage raises from 4 to ~8 V.

The 6 W in out application correlates to the time where the DRVcc voltage raises from 0 to 10 V, right after power on (48 VDC)

best regards

Dennis

  • 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.