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TPS54202H: Device Enable

Noriyuki Takahashi
Genius 11050 points
Part Number: TPS54202H

Hi team,

Customer is trying to tie DC24Vinput to EN-pin via 510kOhm as the attached. Could you confirm this is OK? The abs max of EN pin is 7V, it seems it is banned to apply 24V to EN-pin. But 7.3.5 Device Enable mentions that it is OK. How should we understand this?

example.pptx

Regards,

Noriyuki Takahashi

  • 0
    TI__Genius 9540 points

    Hi,

    Yes, it's okay to use as attached .

    BRs,

    Young

  • 0 in reply to Young Zhao
    TI__Genius 11050 points

    Young,

    Thanks.

    Could you tell the reason why it's OK to apply 24V via 510kOhm to EN-pin?

    It seems that it exceeds abs max voltage, 7V of EN-pin.

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Genius 9540 points

    Hi ,

    As there is a internal pull down resistor , ~1Mohm to gng. 

    Also a Zener diode (typical break down voltage 6.9 V) is used to clamp the EN input voltage.

    BRs,

    Young

  • 0 in reply to Young Zhao
    TI__Genius 11050 points

    Hi Young,

    How much current can be passed through the diode?

    Customer wants to understand it, because they assume that 47uA at max can be passed through the diode like the attached.

    In order to confirm the 47uA can be no problem for the diode, they want to understand it. Though not sure yet how to calculate the 47uA is true or not.

    example (3).pptx

    Regards,

    Noriyuki Takahashi

  • +1 in reply to Noriyuki Takahashi
    TI__Genius 9540 points

    Hi ,

    Calculation in the attachment is not correct, it should be 24V/(510K+1M)=16uA.

    And this 16uA won't pass through the Zener diode totally . And for the Zener diode , when the stress voltage is less than break down voltage 6.9 V(typ), the current is very small , When the voltage is larger than the break down voltage , the current will go up quickly . You can refer the typical Zenner diode VI curve .

    BRs,

    Young

  • 0 in reply to Young Zhao
    TI__Genius 11050 points

    Young,

    Please let me ask again. I misunderstood their question.

    How much current can be passed through EN-pin?

    They want to reduce the resistor value of R4 in the attached, less than 50kOhm, they hope. This is because they want to reduce the voltage across the R4.

    For example, if the input current at the EN terminal is max1μA, if R4=510k Ω, the EN sense voltage will have an error of max1 uA x 510k Ω=510mV. This is not an ignored for them. If R4 = 51k Ω, the voltage is max1 μA * 51k Ω = 51mV. If R4 = 5.1k Ω, the voltage is max1 μ a x 5.1k Ω = 5.1 mV. The smaller the resistance, the smaller the error due to the input current.  

    That's why they want to understand the value of current that can be passed thought EN-pin. They will use the value in order to decide the value of R4 in their design.

    example (4).pptx

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Genius 9540 points

    Hi,

    Pls note 2 items for this Vin-Ven voltage:

    1. Rpd is around 1M ohm and will diver the Vin voltage on EN pin

    2.  There is one Zener diode on EN pin.

    So Vin-Ven can be easily to get when Ven<6.9V , ~ R4/(R4+Rpd) *Vin. So smaller R4 will bring smaller divider voltage

    If Ven >6.9V, the current would be larger than above because of Zener is broke down and R4 will take more voltage.

    BRs,

    Young

  • 0 in reply to Young Zhao
    TI__Genius 11050 points

    Young,

    Please let me ask again about their question.

    They want to understand 3 things as below in order to minimize the value of R4. Could you confirm these things again? Please also see the following explanation to understand why they need these.

    - Rpd's (min and max value)

    - the permitted current that I_Zen can pass

    - I_EN, that is input current a min of EN pin,

    7271.example (4).pptx

    1. They want to minimize the value of R4. R4 can be calculated using the following equation, and the max I_EN value must be known in order to determine the minimum value of R4.
    24V – I_EN * R4 = 6.9

    2. I_EN is determined by the following equation: I_EN = I_Rpd + I_Zen

    3. I_Rpd can be calculated using the following equation: Rpd is 1Mohm(typ), but Rpd's (min and max value) should also be checked.
    I_Rpd * Rpd = 6.9

    4. I_Zen regulates the current to maintain 6.9 V. I_Zen needs to know the permitted current that I_Zen can pass (the minimum current that the zener can pass) so that the IC will not break even if the voltage at the EN pin is less than 7V. If the minimum I_Zen value is known, the maximum I_EN value is also recognized and the minimum R4 value can be calculated.

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Genius 9540 points

    Hi ,

    Sorry for misleading you. Actually Ven is almost be constant when Zener diode voltage is over than its breakdown voltage , typ 6.9V. As after 6.9V, the current would be increasing quickly while voltage doesn't have much variation, this means that the EN current would be large and much power loss .

    And there is not a true minimum R4 if not considering the eff and power loss. There is a trade off for performance and eff .

    Another concern is that the heat of R4 when it's current is very large . So suggest to follow TI datasheet recommendation .

    BRs,

    Young

  • 0 in reply to Young Zhao
    TI__Genius 11050 points

    Hi Young,

    They want to understand following things in order to calculate the minimize the value of R4 even if the voltage at the EN pin is less than 7V. Could you tell these?

    - Rpd's (min and max value)

    - the permitted current that I_Zen can pass

    - I_EN, that is input current a min of EN pin,

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Genius 9540 points

    Hi,

    As we talked above, a smaller R4 is not helpful to reduce the VIN-VEN voltage , as VEN would clamped at around 6.9V .

    And the Rpd is M ohn level , the current would be uA level , much less than the I_Zen(~mA level). You can just ignore this Rpd variance.

    Regarding the I_Zen, it would be ~mA level , Will check this value internal . And feedback to you soon.

    Regarding the I_EN, what's the meaning of this item ? the minimum EN current ? 

    You can refer below EC table.

    BRs,

    Young

  • 0 in reply to Young Zhao
    TI__Genius 11050 points

    Young,

    I_EN is the input current of EN-pin as the attached. I just used this to explain what I want to confirm to you. Customer wants to confirm how much current can be passed in EN-pin, so that the device can't be destroyed as I explained.

     8688.example (4).pptx

    Regards,

    Noriyuki Takahashi

  • 0 in reply to Noriyuki Takahashi
    TI__Genius 9540 points

    Hi ,

    I will check this item with design team and feedback to you later.

    BRs,

    Young

  • +1 in reply to Noriyuki Takahashi
    TI__Genius 9540 points

    Hi ,

    Per checked with design, this EN current is not suggested larger than100uA.

    BRs,

    Young