Short to Battery Protection Circutiry for Automotive Video Decoder

Dear Sir/Madam,


One of my customers is using an ADI video decoder (ADV7280) in an automotive environment. They are feeding analog video over a CVBS cable into the ADV7280.

Our recommended input circuit to the ADV7280 uses a resistor divider (24 Ohm and 51 Ohm) and an 100nF AC coupling capacitor. Please see the attached input network circuit.

The customer also requires that the analog video input network needs to be robust to 24V short to battery events (STB) i.e. car battery accidentally getting shorted onto the analog video input line.

The AC coupling capacitor will protect the ADV7280, however the 24 Ohm and 51 Ohm resistors could be damaged during the STB event.

We cannot move the location of the AC coupling capacitor or insert additional DC blocking capacitors without upsetting the operation of the video decoder.


I was hoping that some of the voltage protection devices designed for power supply applications could be used to protect the 24 Ohm and 51 Ohm input network resistors during STB events. I was particularly wondering if I could just place an ADG465 in series to protect the resistors. 

http://www.analog.com/static/imported-files/data_sheets/ADG465.pdf

I would appreciate any feedback anyone could give on whether this is a valid solution. Once I have a solution I will add this as a recommended solution to all our video decoder customers.

FYI analog CVBS video signals are generally 1V peak to peak. The DC level can vary from (bottom of CVBS signal at) 0V up to (top of CVBS signal at) 1.8V .

Regards,

Robert Hinchy,

Senior Applications Engineer,

Digital Video Products Group,

Analog Devices Inc,

  • 0
    •  Analog Employees 
    on Oct 31, 2014 6:07 PM over 6 years ago

    Hi Stephen,

    Theresa is out until next Wednesday.  Maybe I can help here.

    Overall I think your approach looks fine.  But can you clarify how your want to drive IN2 to toggle the switch?  Do you want to toggle the switch up to such high speeds as 10MHz?  Of are you asking about this 10MHz signal from a noise point of view on IN2?  From a noise point of view have a look at Figure 23 ACPSRR, this is noise on the power supply but it would give you an idea of what could leak through. 

    There is no automotive qualified version of this part.  But it is qualified to operate up to +125C if that helps.

    Regards,

    Eric.


  • Hi Luca,

    Thanks for your quick reply.

    Looking forward to some help from the switch group.

    regards,

    Robert Hinchy,

    Senior Applications Engineer,

    Digital Video Products Group,

    Analog Devices Inc,

  • HI Rob,

    We have a new family of low Ron (10ohm) Over-voltage protected switches - ADG5412F (4xSPST).  This will protect against ±55V over-voltage events in both the powered and unpowered state.  See attached datasheet.  The ADG5412F needs an 8V min supply rail to operate but to benefit from the low and flat Ron for a 1Vpp signal I would recommend the part be powered off a bipolar rail (±5Vmin).See fig 4 and 5 for reference, d/s page 14.

    http://www.analog.com/static/imported-files/data_sheets/ADG5412F_5413F.pdf

    We have a quad channel protector, ADG5462F due to release in January.

    Rgds,

    Theresa.

  • 0
    •  Analog Employees 
    on Oct 31, 2014 9:08 PM over 6 years ago

    Bob,

    I think it is a good idea, but the switch already has 80ohm resistance itself.

    Not sure if there is a lower RDSon version.

    I am moving the discussion  to the switch forum to see if someone can help you here.

    LucaV

  • Hi Theresa,

    Thanks so much for getting back to me.

    The solution you have proposed looks really good. Do you mind if I describe the setup and how I think the circuit will work. Is the below correct?

    Setup:

    Use an ADG5413F.

    Set Vdd to +5V. Set VSS to -5V. Set GND to 0V of video decoder.

    Feed the CVBS analog video into S2 and IN2.

    Normal Operation:

    In normal operation the CVBS signal is fed from S2 to D2. The switch in the ADG5413F from S2 to D2 is normally closed.

    The FF fault flag pin will drive high.


    Short to Battery Event Occurs:

    When the CVBS analog video input exceeds VDD +Vt then the switch in the ADG5413F opens. The ADG5413F will output a constant 5V Vdd voltage.

    The FF fault flag pin will drive low.

    Short to Battery Event Ends:

    When the CVBS analog video input drops back to VDD +Vt then the switch in the ADG5413F closes. The CVBS signal is sent from S2 to D2 again.

    The FF fault flag pin will drive high again.


    Additional Questions:

    1) The CVBS signal can have frequency content up to10MHz.  The CVBS signal is connected directly to IN2 pin. Will this affect the operation of the switch control on IN2 pin?

        

    2) Is there an automotive version of the ADG5413F available?


    Thanks and Regards,

    Robert Hinchy