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LMK04832-SP: Cold Sparing support

Mark Guastaferro
Genius 11645 points
Part Number: LMK04832-SP


Are the inputs to the LMK04832-SP cold-sparing? Particularly the OSC+/- inputs. In other words, if the device is unpowered but the input is being driven, will this stress the device?

I saw another E2E entry asking about cold-sparing on the SPI inputs and the response indicated that the ESD diodes might start conducting resulting in high current draw.  I assume this will hold for other input control inoput pins but not sure about clock inputs like OSC+/-. 

  • 0
    TI__Mastermind 33970 points

    Mark,

    CLKin and OSCin pins still have an ESD structure on them, but the ESD structure is surprisingly not the biggest issue; there's other circuitry within the input stage that will sink high current, and this becomes an issue for electromigration in the metal layer right above the silicon.

    We're working on exactly what we can specify in the datasheet, but it looks like around 6mA DC is the maximum value up to 105°C PCB/die temperature, and maybe 3mA at 125°C PCB/die temperature. AC-coupling significantly complicates the picture, but in any case the same limits for RMS current while unpowered are unlikely to cause problems. RMS input current is sort of hard to measure, though; conceptually it could be done with a 1Ω resistor in the path after the AC-coupling capacitor and before the pin, along with a differential probe to measure the current. 

    The best option is to disable signal to the OSCin port entirely, if possible. Many devices have either enable/disable support, or a means to disable a buffer providing the signal.

    A less-good but valid option may be to hot spare instead of cold spare. The LMK04832-SP in powerdown condition fully biases the OSCin stage so there will be no stresses on the device, and draws a maximum of 5mA across all operating conditions. It should also be trivial to detect if the powerdown condition has failed even with rudimentary current monitoring on the supply rail, since the device tends to draw hundreds of mA under normal circumstances.

    Regards,

    Derek Payne