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LM3881: the problem of usage

run Ren
Prodigy 220 points
Part Number: LM3881

Hello :

I use TI LM3881 and have several questions to consult:

1.Whether the delay can be adjusted between each flag, I need about 3--7ms, whether the capacitance of TADJ can be increased, and whether there is an upper capacity limit?

2.Can a flag pin drive two MOS tubes at the same time (control 2-way DCDC)?

Schematic diagram is as follows:

  • 0
    TI__Guru 66240 points

    Hello

    Please see my comments below.

    #1. the delay between each flag output is not adjustable. the delay between FLAG1->FLAG2->FLAG3 are fixed, the fix value is decided by the Cadj. You can choose a proper Cadj based on your own timing need.

    #2. Sure, you can use single FLAG output to control multiple power supply.

    Regards

    Yihe

  • 0 in reply to yihe
    TI__Guru 66240 points

    Hello

    We haven't heard from you for a while and assumed that the issue was solved. Please reply if you need further help.

    Regards

    Yihe

  • 0 in reply to yihe
    Intellectual 450 points

    hello

         wether there is an upper capacity limit of the TADJ? i need delay 7ms for every stage

    the EN pin of the DCDC conventor need  high level to work,but the flag(connected to DCDC's en pin)is pull up to high,so wether the DCDC conventor is work by power ON,then the LM3881 is work , the flag output low level,and the DCDC conventor is OFF,then delay 1ms or other time set by TADJ capacitor, the flag output high the DCDC conventor is work again,so mybe it suitable that  the pull up resistor on the flag change to pull down,but the output is Open drain.

    or i need MOSFET on the EN pin,can you give me your advice?

    thanks

  • 0 in reply to user1359349
    TI__Guru 66240 points

    Hello

    There is no issue to support 7ms delay.

    have you tried the LM3881 EVM? Before the EN signal off the LM3881 is asserted, the FLAG is output low, if you have a 100K pull up, the flag shall stay at low which will not turn on the DC-DC convert. Before the delay expires, the flag output would be low.

    Regards

    Yihe.