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LM3881: Max Fan Out from Each Flag and Pull-Up Value for each Flag

SANTANU KUMAR SINHA
Prodigy 175 points
Part Number: LM3881

Tool/software:

Hi TI Team,

Can you confirm the driving capability of LM3881 sequencer for the below scenario  

Flag # 1 - # 2 (two) inhibit signal of power supply (LMZ Series),

Flag # 2 - # 4 (four) signals consisting of two power supply (LMZ Series), one load switch and one LDO

Flag # 3 - # 2 (two) signals consisting of one LDO and one load switch

What should be the Pull-Up values for each Flags under above scenario ? 

Thanks & Regards,

Santanu Kumar Sinha

  • 0
    TI__Guru 65840 points

    HI

    You have to ensure the pull up resistor does not violate the following spec。

    The max current to the pin is 1.2mA.

    Regards

    Yihe 

  • 0 in reply to yihe
    Prodigy 175 points

    Hi Yihe, 

    Considering Max Current of 1.2 mA, The max Pull-up resistor would be 2.75 Kohm. 

    As per the recommendation in data sheet, I have used 100 Kohm, Pull-Up resistor on all Flags to 3.3V. 

    The Flag # 1, Output Voltage was read as 3.3 V.

    However, a voltage drop is observed in Flag # 2 & Flag # 3. and voltage measured is close to 1.8 V for both. 

    Can you give some insight why i am seeing this behaviour ? 

    I would also like to increase the voltages at Flag # 2 and Flag # 3 to 3.3V as 1.8V (actual measured) is just 0.5 V above the threshold limit of power supply IC connected to respective flags. 

    Thanks and Regards,

    Santanu Kumar Sinha

  • 0 in reply to SANTANU KUMAR SINHA
    TI__Guru 65840 points

    Hi

    Do you use 100KOhms for FLAG1/2/3?

    You may try to disconnect the connection between FLAG2 and FLAG3 to the outside circuit and check the where the drop from.

    Regards

    Yihe 

  • 0 in reply to yihe
    Prodigy 175 points

    Hi Yihe,

    Yes, 100KOhms is used for FLAG1/2/3.

    Will there be any change in voltage if I change Pull-up resistor to 2.75K ?

  • 0 in reply to SANTANU KUMAR SINHA
    TI__Guru 65840 points

    Hi

    Have you followed my previous suggestion to narrow down the source?

    I would try 10K first.

    Regards

    Yihe 

  • 0 in reply to yihe
    Prodigy 175 points

    Hi Yihe,

    I have isolated the devices connected to Flag 2 and Flag 3. 

    I am able to measure 3.3V on these rails too now with 100K pull up. 

    Thanks and Regards,

    Santanu Kumar Sinha.