LT8362: Protect Vin from Reverse Polarity?

I am designing a SEPIC converter using the LT8362 to output 5V in a 12V automotive application.  The LT8362 was chosen to keep the 5V circuit alive even during cold cranking.  Here is the datasheet:

http://cds.linear.com/docs/en/datasheet/8362fa.pdf 

Other LT chip datasheets often mention the need for a Schottky diode (or active protection) on Vin to protect against reverse polarity when needed, but the LT8362 datasheet does not specifically mention that at all.  Does that lack of mention in the datasheet mean no reverse polarity protection is needed in LT8362 circuits?  

Asking the same question another way, is reverse polarity protection needed for the LT8362 when both Vin & EN/UVLO connect to +12 (car battery) voltage?  (In other words, if GND is accidentally connected to Vin & EN/UVLO, and if +12V is connected accidentally to GND, without any Schottky protection between Vin and Battery Voltage, will reverse polarity fry the LT8362?)

The reason I ask is because if no reverse polarity protection is needed, I need not worry about a voltage drop across a Schottky (and I can avoid the expense of an active reverse polarity protection circuit).

Last question...

The LT8362 datasheet mentions that adding a 100k resistor between the SYNC pin and GND maintains BURST mode operation at light loads but adds SSFM at heavy loads for "reduced EMI."  No caveats are mentioned at all.  Datasheet pages 27 & 28 show 2 circuits that use the 100k resistor, but both are BOOST, not SEPIC circuits.  My question therefore is twofold: (1) What is the disadvantage of using the 100k resistor instead of just tying the SYNC pin to GND? and (2) Will use of the 100k resistor result in any benefits in a SEPIC circuit?

Thank you.

  • Thank you for your reply. But as I asked in my opening post, what are the “caveats“ of using the 100K resistor between the SYNC pin and GND to enable SSFM?  You only explained the benefit of SSFM. I want to know the CAVEAT. And surely there must be a caveat because if there was not, why would SSFM not be enabled by default? 

  • 0
    •  Analog Employees 
    on Mar 29, 2018 5:51 PM

    The LT8362 does not include reverse polarity protection. If this protection is needed, it must be added externally.

    On the use of the SYNC pin:

    Connecting a 100K resistor between the SYNC pin and ground enables SSFM at heavy loads, and Burst operation at light loads, regardless of the topology used.

    If the SYNC pin is grounded, the converter will only have Burst operation at light load.

    Use of SSFM is at the discretion of the user. If the application must abide by a stringent EMI standard, SSFM will make it easier to comply. If there is no EMI requirement, the SYNC pin can be grounded and the benefits of Burst mode operation will still be available at light load.

    SSFM helps with EMI problems, regardless of the topology used, but it's not a silver bullet. Good layout technique and component selection are still necessary for a sound design.

  • 0
    •  Analog Employees 
    on Apr 2, 2018 5:09 PM

    You are correct, I did not address your question on "caveats".

    The LT8362 is a fixed frequency converter. When SSFM is enabled, the switching frequency is modulated between "F" (frequency selected), and "F"+20%, as explained on the datasheet, page 11. This wider switching frequency variation must be taken into account when choosing external components, particularly the inductor value. 

    Are there CAVEATS particular to specific applications? I do not know.

  • 0
    •  Analog Employees 
    on Aug 2, 2018 4:03 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin