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TPS709: EN connected to VIN via high ohmic resistor

Lutz Naumann
Expert 6450 points
Part Number: TPS709

Dear team,

we understand the EN pin must not be tied to VIN as it is limited to 6.5V. We also understand I can be left floating to turn on because the is internal 300nA current source pulling up the signal. This is then clamped by an internal 6.5V Zener. This lead to the assumption that the Zener can digest a certain continuous current during normal operation.

Because of second source qualification the layout and population of the board needs to be done such, that another LDO can be installed which needs to have the EN connect to VIN. We can not ensure the resistor is NOT populated in case of TPS709.

So here is the questing. We hope that a high ohmic resistor still can be connected from EN to Vin. The question would be how much current can we allow into the internal clamping 6,5V Zener in addition to the 300nA. Is this feasible at all and what resistor value can we use for Vin = 15V?

many thanks

Lutz   

  • 0
    TI__Guru 55940 points
    Hi Lutz,

    While there is theoretically a large resistor that will limit the current into the EN pin without damaging the Zener, TPS709 was designed, tested, and characterized for the EN pin to be left floating. We do not recommend having EN tied to IN even using a large resistor.

    I am not sure I understand why a BOM change can be made to replace the linear regulator but one cannot be accepted to remove a resistor. Do you have any details on why the resistor cannot be removed altogether for the variant of the board with TPS709?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    TI__Expert 6450 points

    Thanks Ryan,

    understood. The manufacturing problem the customer is facing is that they want to qualify second source components that can be used either or. But they are not able to define if component A is used the passives around also for option A and if component B is used the passives are for option B. In their case component A has to work with passives option B and vice versa. This means the can have different resistor manufacturers but no change in value.

    thanks for your help. 

    Lutz