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ESDS302: Continuous current rating for ESD devices

John Griffith
Mastermind 21646 points
Part Number: ESDS302
Other Parts Discussed in Thread: TPD2E2U06, TS5USBC402, , ESD752

Hello, 

My customer is trying to figure out protection for a USB port that has cabling that runs connector as RS232. They want to protect against the possibility of a DC short to RS232 lines that can go down to -13.2V. I looked at a number of ESD devices and was trying to figure out how to calculate the series resistance needed to protect the ESD device from a DC short.

Is the correct method to just figure out the clamping voltage and the current that would occur. Multiply those together for a power rating and check if the part will heat above its rated temperature using the theta JA numbers? I opened a number of ESD device datasheets and none of them have a continuous current rating or anything similar to help figure this out. 

I am trying to protect the TS5USBC402 device which has a negative max voltage of 0.5V. So I was thinking about a unidirectional ESD device like the ESDS302 or TPD2E2U06.

Thanks in advance! 

John

  • 0
    TI__Genius 16440 points

    Hi John,

    Our ESD diodes are designed to protect from transient events (nS - µS pulse durations).They are not built to withstand being broken down over a long period of time from a static DC voltage. Once the diode is broken down, it generates a significant amount of heat as current goes through it. This will eventually cause thermal failures.

    This is why we recommend that you only apply static DC voltages across the diode that are at or below the rated Vrwm.

    Regards,

    Sebastian 

  • 0 in reply to Sebastian Muriel
    TI__Mastermind 21646 points

    Hi Sebastian,

    Understood, and thanks for the quick response. This is a very unlikely fault that they want to protect the system against. If we were to exaggerate one option, which is putting a series resistor before the diode, we would have:

    • Let's say we have a 1MΩ series resistance
    • the current sinking in the TPD2E2U06 which breaks down at 6.5V would be 6.7µA of current (13.2-6.5) / 1MΩ
    • Taking this one step further, 6.7µA of current at 6.5V is ~44µW of power
    • Even a 300°C/W junction to ambient temperature rise would result in 0.013°C temperature rise 

    Is there no guidance you can help provide for calculating what could be an acceptable amount of current the diode could sustain?

    John

  • 0 in reply to John Griffith
    TI__Genius 16440 points

    Hi John,

    The max DC current through the TPD2E2U06 is 1mA at Vbr. 

    I would recommend using one of our newest devices, the ESD752 for protecting RS232. The ESD752 is a bi-directional diode with a Vrwm = +/-24 V, which is sufficient for RS232 voltage levels that go down to -13.2 V in your application. 

    For the ESD752 the max DC current is 10mA at Vbr. 

    Regards,

    Sebastian 

  • 0 in reply to Sebastian Muriel
    TI__Mastermind 21646 points

    Thanks Sebastian! This helps a lot.

    The ESD752 will not work since the device after the diode is a USB MUX which cannot handle more than 5.5V. I was looking at unidirectional devices since the MUX device after the ESD diode can be damaged at -0.5V. So I was trying to clamp the voltage negatively at less than -0.5V and positively at less than 5.5V with the ESD device at an acceptable current for whatever device is chosen. Are there any unidirectional low voltage devices I should be considering for this that can handle more than 1mA of DC current at Vbr?

    Thanks,

    John

  • 0 in reply to John Griffith
    TI__Genius 16440 points

    Hi John,

    Note that the DC current at Vbr is a test condition. Vbr is specified at 1mA for most of our devices, this includes all our low voltage devices. It's possible for the diode to withstand a higher current at Vbr, however we can't guarantee any value. I would conduct some bench testing if you want to verify if they withstand a higher current.  

    It would be easier to use a Zener diode that is designed to regulate DC voltage or an overvoltage protection IC. 

    Regards,

    Sebastian