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SN74HC4851-Q1: Injection Current Control Details

Peiheng Liu
Prodigy 620 points
Part Number: SN74HC4851-Q1
Other Parts Discussed in Thread: SN74LV4053A-Q1, SN74HC4852-Q1

Hi,

I'm looking into TI analog mux portfolio for multiplexing multiple NTC temperature sensors into 1 or 2 ADC input channel of a MCU. Right now I have SN74LV4053A-Q1, SN74HC4851-Q1 and SN74HC4852-Q1 in mind.

4051 does not specified the 'injection-current effect control' features and the maximum input clamp current rating is in negative. Is the clamp current flow through the internal ESD circuitry when the analog terminal swing below GND? Also, for any circumstances that analog terminal swing above Vcc, a external diode clamp to supply rail is necessary, right?

Besides, from the Fig. 4 of 4851 and 4852 datasheet, I could tell that an intrinsic diode eliminates the need for external clamp outside the analog terminal. Are there any ESD circuitry between the analog terminals and GND?

Great thanks for your help!

  • 0
    TI__Mastermind 45495 points

    Peiheng,

    There are internal esd clamp diodes from the signal path to both Vcc and ground.  This can be seen in the abs max table where the diodes will begin to conduct ~0.5V above the supply or below ground.

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    TI__Prodigy 620 points
    Hi Adam,

    Your info here is very straight forward and helpful. I am wondering why SN74LV4053A-Q1 does not specified a positive input clamp current under VI>0 condition. This means that external schottky clamp is highly recommended when analog input swing higher than Vcc, right?

    Great thanks for your help!
    Peiheng.
  • 0 in reply to Peiheng Liu
    TI__Mastermind 45495 points

    Peiheng,

    The SN74LV4053-Q1 device is not recommended to have a voltage on the I/O signal path pins greater than Vcc.  This would mean it is recommended to prevent voltages on these pins from exceeding Vcc.  You could accomplish this with external clamp diodes.   

    Why is your I/O signal going to be higher than Vcc?

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    TI__Prodigy 620 points
    Hi Adam,
    Thanks for your prompt!
    Customer is looking for analog MUX for NTC temperature sensors in their battery management system design. I discussed these to facilitate the FEMA process. For example, NTC sensors would locate in some points around the battery pack, not just on the PCB. So there are potential broken wire or short to external sources faults, which would post higher voltage on the analog terminals than Vcc. I am proposing 4851 and the injection current control feature is fitting in just right.
    Peiheng.