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TPS24712: Power good/ Fault questions

Jack Huang87
Prodigy 20 points
Part Number: TPS24712
Other Parts Discussed in Thread: TPS24740

Hi,

I have questions below. Here's my condition:

TPS24712 will applied at PCIE card hot-plug.

Vin=12V, Cout=3000uf, MOS=NTMFS4H01NT1G

iLOAD=

1. 18.3A (200uS)
2. 14.2A (1mS)
3. 12A (5mS)
4. 5.5A (TDC)

1. The max peak current is 18.3A(200US), can i set Rshunt=1.5mohm to got OCP=16.7A? And set timer longer than 200us to make sure gate won't pull down?

2. PG will be low immediately when load hit current limit point? Or need to wait timer expired? 

3. Only current will trigger timer count?

4. Do i need to use back to back MOSFET? If the load board will be removed and plug back immediately(load board has large cap and still hold 12V), will the reverse current through body diode back to VIN?

Jack CT

  • 0
    TI__Guru 57737 points

    Hi Jack,

    Please see my response below,

    1. Yes, you can set OCP = 16.7 A and timer duration greater than 200 us to allow for the max peak current 18.3A  for a duration of 200us. 

    2.After the hot-swap circuit successfully starts up, the PG pin can return to a low-impedance status status whenever the drain-to-source voltage of MOSFET M1 exceeds its upper threshold of 340 mV, which presents the downstream converters a warning flag. This flag may occur as a result of overload fault, output short fault, input overvoltage, higher die temperature, or the GATE shutdown by UVLO and EN.

    If you want an indication after the timer has expired, you can use FLT pin. . FLT goes high and FLTb pulls low at the end of the fault timer. Otherwise, FLT assumes a low-impedance state and FLTb a high-impedance state.

    3. Current limit or Power limit will trigger timer.

    4. When you plug back the load board and there is no Vin (or Vin < Load board voltage) then there is will be reverse current flowing back into Vin through the body diode. If you connect back to back FET, it help blocking reverse current only in conditions when the GATE is pulled low. This is because if the GATE is high and there is reverse current flow in the FET, the controller has no intelligence to block reverse current by turning OFF the FET.  IF you want true reverse blocking, use TPS24740 family of devices. 

  • 0 in reply to Praveen GD
    TI__Guru 57737 points

    Hi Jack,

    Do you have any other questions ? If not, can we close this thread ? 

  • 0 in reply to Praveen GD
    Prodigy 20 points

    Hi Praveen

    Sorry for delay, I still a little confuse the back-to-back mosfet.

    1. What kind of situation i should using back-to-back MOS to prevent the reverse body diode current?

    2. If in my application, VIN always has 12V and the output PCIE card remove and plug in, do i need the back-to-back mosfet?

    thx.

    by Jack CT Huang

  • 0 in reply to Jack Huang87
    TI__Guru 57737 points

    Hi Jack,

    1. You will need back to back FETs when there is a possibility of reverse current flowing from Vout to Vin. This can happen when Vout is charged and Vin is not present. Using back to back FETs controlled by TPS24740 would help blocking reverse current.

    2. In your case, as Vin is always at 12V and Vout can never be greater than Vin there cannot be reverse current flowing from Vout to Vin. hence, you should be fine using TPS24712 and you do not require back to back FETs. 

    Hope this explanation cleared  your confusion.