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Original question:

LM5170: Initial fault check without using circuit breakers

LM5170: Disabling fault detection

Marc Johnson
Prodigy 60 points
Part Number: LM5170

The datasheet describes that the failure detection mechanism is disabled by placing a 10k pulldown on the SYNCOUT pin (section 8.3.16.1). The datasheet further states that the BRKG and BRKS pins must be terminated with 20k resistors, which is illustrated in Figure 35, but provides no explanation regarding how these values are chosen. The example in Figure 55 of the datasheet shows 10k resistors instead. Is the resistance here critical, and can you provide details regarding what an acceptable range is?

Thanks!

  • 0
    TI__Guru 73270 points

    Hi Marc,

    Thank you for posting.  The 10k at SYNCOUT is a signature resistor telling the IC to skip the fault detection.  

    The 20k resistors at the BRKG and BRKS are recommendations.  The BRKG is a current source pin, and it will force the ~300uA.  If you leave the pin floating, it will raise the BRKG voltage to VIN.  Since BRKG pin max rating is just 65V, it may be subject to damage if VIN is >65V.  

    However, we do not recommend to ground the BRKG, otherwise the internal losses would be VIN x 300uA.   Placing terminal resistor would help remove some of the losses to the external resistor, and 20k is just a reasonable value. 

    Similar considerations for BRKS.

    Thanks,

    Youhao Xi, Application Engineering

  • 0 in reply to Youhao Xi
    Prodigy 60 points

    Based upon this description, it sounds like the resistance values for the terminations on the BRKS and BRKG, when the circuit breaker is not being used, are not critical. More particularly, the 10k resistors shown in some examples in the datasheet should also serve the required purpose. Please let me know if this is correct.

  • 0 in reply to Marc Johnson
    TI__Guru 73270 points

    Yes you are right. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 60 points

    Thank you for the information and for the fast responses.

    M. Johnson