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TPS51124: pull up resistor selection on PG pins

FRANK1
Expert 8675 points
Part Number: TPS51124


Hi Team,

A Japanese customer has a inquiry on how to select pull up resistor on PG pins.

Can I get your valuable suggestion?

He want to know the relationship between the pull-up resistance value of PGOOD1 and 2 of TPS51124 and the IPGMAX current value.

Q1: Since the current value of IPGMAX is MIN2.5mA, should I set the PGOODx pull-up resistance value to 1.2kΩ for 3V voltage?

Q2: When PGOODx pull-up resistance value is set to 10kΩ Is the current value of IPGMAX about 0.3mA at 3V voltage?

IPGmax is Min 2.5mA, typ. 5.0mA in the datasheet.

Customer is anxiously waiting for this reply, I appreciate for your quick response. 

Thanks.

Regards,

Frank

  • 0
    TI__Expert 7320 points

    Hi Frank,

    The principle of PGOOD is shown in the figure below.PG pin output consists of a triode and a pullup resistor.

    The pullup resistor cannot be selected too small because of the limited capability of the triode output current, otherwise the output voltage of PG remains high when the triode is on.

    Q1: 1.2 kΩ is a little small, a 100k pull-up resistor is optional per EVM user guide. Also, based on the recommendation of DATASHEET, we recommend to pull up to V5IN.

    Q2: Yes, you can estimate IPG like this. Just remember to design with IPG less than IPGMAX.

  • 0 in reply to Athos Zhao
    TI__Expert 8675 points

    Hi Athos,

    Unfortunately, my design used the PGoodx as 3V.
    is there any issue if I use PGoodx to pull up 3V.

    BTW, there is description of 'Pull up with a resistor to 5 V, or appropriate signal voltage' in the PGood Pin fuctions.

    Q3: What is specific to the appropriate signal voltage?
    Can you say more details?

    Q4: Current capability is 5 mA, so the pull up resistance value is 5V/5mA.
    It's going to be 1 kohm, is this calculated method correct?

    Q5: why the EVM use 100 kohm pull up resistors on the PGood pins? I still don't quite understand.
    The IPG current is limited to 5uA at 5V and it is difficult to drive the signal to get the PGOOD signal.
    I think power on to Hi output immediately after Pgoodx low output.
    I hope you will answer these issues.

    Thanks.

    Regards,

    Frank

  • +1 in reply to FRANK1
    TI__Expert 7320 points

    Hi Frank,

    It is also OK to pull PG up to 3V.

    It is important to note that the PGOOD pin chip is internally used to feedback the output state, which is essentially a collector of a triode as follow.

    When the output of the power supply is correct, the triode operates in the cutoff region, PG voltage is connected to 3 V through a pull-up resistor, and VPG = 3 V.

    When the output is incorrect, the triode is turned on. VPG = Vcc-IPG*Rup, where IPG is less than IPGMAX.

    if Rup> Vcc/IPGMAX then VPG = 0V(or VPG=~0.2V) where PG pin is functional. If Rup is too small, VGP= Vcc-IPGMAX*Rup>0, where PG pin is not functional. So Rup can be larger but can't be too small.

    Q3:PG is used to detect the output state, so the appropriate signal voltage (Vcc) is selected for external circuit to judge if PG pin is high or low. Thus the appropriate signal voltage depends on how you use PG pin in the external circuit.

    For example, if Vcc is too low like 0.5V, it is difficult to distinguish the state of the PG. 

    Q4:You can calculate the min value of Rup using 5V/5mA(Vcc/IPGMAX). Rup can't be less than 1kΩ.

    Q5:100kΩ is ok according to above explanation.

    Please let me know if you have other questions.

    Athos