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LMH1218 - Enable control using multiple LMH1218 devices

Kawai
Guru 19785 points
Other Parts Discussed in Thread: LMH1218, LMH1219

Hi Team,

Our customer is considering to control Enable pin to power and power down the LMH1218. They will control 8 LMH1218 Enable pins simultaneously.

LMH1218 Enable pin is a 4-level input pin and requires 1k pull up(Enable, disable at LOS) or 1k pull down(Disable) to enable/disable the device.

Q1). Do we need 1k pull up or 1k pull down resistor for each device, or should we use one common pull up or one pull down resistor for several devices ?

Q2). Could you please tell us the internal circuit of this 4-level input, such like internal pull up and pull down resistor value ?

Best Regards,

Kawai

  • 0
    TI__Mastermind 29655 points
    Hi Kawai-san,

    1. You can either use a 1k pull-up/pull-down for each device, or you can use a common pull-up/pull-down for all 8 devices. Note that the effective resistance will be divided by 8, since the pull-up for 8 LMH1218 devices is like having 8 x 1kOhm resistors in parallel. I expect, therefore, that the pull-up/pull-down value for a common resistor controlling 8 LMH1218 devices is 1k / 8 = 125 Ohms.

    2. There is an internal 30k pull-up and 60k pull-down on the 4-level inputs.

    Thanks,

    Michael
  • 0 in reply to Michael Lu (Santa Clara)
    Guru 19785 points
    Hi Michael-san,

    Thank you for the detail explanation. Internal resistor information helps us very much.

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    Guru 19785 points
    Hi Michael-san,

    We have additional question to this 4-level input function.

    When user wants to enable/disable the device, you would require a switch to be used to 1k pull up or 1k pull down. Question is that cheap switch may have a few hundreds of ON resistance. How much additional resistance would be acceptable to the pull up, pull down resistors in series ? i.e. what is the MIN/MAX voltage of each input level ?

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    TI__Mastermind 29655 points
    Hi Kawai-san,

    The threshold for these pin logic levels can be found in the LMH1219 datasheet as well as other datacenter logic:

    Typical 4-Level Input Thresholds:
    • Internal Threshold between L and R = 0.2 × VDD
    • Internal Threshold between R and F = 0.5 × VDD
    • Internal Threshold between F and H = 0.8 × VDD

    As long as the resulting voltage on the pin has margin from the 4-level threshold of the logic you need, then you can add the resistance. For example, for a logic Level H, the voltage at the 4-level pin must be at least 0.8 x 2.5 V = 2.0 V. This means that the resulting voltage at the pin for each device must be at least 2.0 V in order for the device to recognize the logic as Level H and not Level F.

    To determine if your switch is accessible, please calculate whether the added resistance of the switch will still allow you to be above 2.0 V on the pin per device for Level H and below 0.2 x 2.5 V = 0.5 V for Level L.

    Thanks,

    Michael
  • 0 in reply to Michael Lu (Santa Clara)
    Guru 19785 points
    Hello Michael-san,

    Thank you very much for the detail information. It may be better to have this information described in the LMH1218 datasheet, too.

    Best Regards,
    Kawai