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SN74HC151-Q1: 5V Input Capability under VCC=3.3V

hideyuki sakai
Mastermind 6250 points
Part Number: SN74HC151-Q1
Other Parts Discussed in Thread: SN74CBTLV3861-Q1, SN74HC151

Dear, Sir.

My customer's P.C.B was completed, after then they are facing a neceesity of modification.

The voltage on input line is changed to 5V, output is 3.3V which is connected to MCU.

The power supply is providing 3.3V for SN74HC151-Q1 VCC, too.

The best way would be put LDO, Voltage translator, etc, but there is no space for such modification.

They are considering more simple way as ;

SN74HC151-Q1 has the internal power clamp diode on input, so the series resistors will be put on input

pins to limit the input current.

I believe the real input pins voltage would be 3.3V(VCC) + Vclamp(diode VF), even if almost of voltage

will be dropped at the series resistors.

I would like to get your advice on followings ;

1. Is it no problem to realize 5V input capability taking above simple way?

2. On the datasheet, VI MAX(Input volatge MAX) is limitted on VCC.

   Can the nromal operation be expected under 3.3V(VCC) + Vclamp(diode VF)?

Best Regards,

H. Sakai

  • 0
    TI__Guru* 97046 points
    Hello Sakai-san,
    Yes, you can use series resistors on the inputs to fix this system. As long as the input current remains below 20mA, the clamp diodes will maintain a safe voltage on the inputs. The recommended values for input voltage are listed that way to prevent people from activating the clamp diodes by mistake.
  • 0
    TI__Intellectual 2900 points

    Hi Sakai and Emrys,

    There is an idea just came across. Please allow me to go through a question list.

    Your requirements are

    1. automotive qualified

    2. 8:1 multiplexer

    3. input signal is 5V and output is 3.3V. voltage translation is required

    Here are my solutions:

    Solution 1. using SN74CBTLV3861-Q1 (10 channel 1:1 switch) to do voltage translation and then SN74HC151-Q1 (8:1 MUX)

    Here is the way how to use SN74CBTLV3861-Q1 to do 5V to 3.3V translation.

    Solution 2: If the customer thinks the above self-biasing circuitry is too complex, they may use SN74CB3T16210-Q1 + SN74HC151-Q1. This is TI's integrated solution. It's 20 channel. Unfortunately, we don't have 8 channel version for automotive. Here is how to use it. Very simple. 

    Solution 3: I am not sure if the customer can use SN74CBTLV3861-Q1 or SN74CB3T16210-Q1 without SN74HC151-Q1. It depends if the customer really needs signal driving capability and inverting output. 

    The voltage translating switches I am talking about do not have signal buffering/repeating capability, while SN74HC151 does. In addition, SN74HC151 has an inverting output, W pin, that voltage translating switches do not have. 

  • 0 in reply to Fan.Wang
    Mastermind 6250 points

    Dear, Wang-san and Emrys-san. 

    I am feeling a mount of gratitude for your valuable advice. 

    1. The reason why they selected SN74HC151-Q1 is a kind of solution against shortage of MCU I/O's. 

       So, data selector(multiplexer) must be needed. 

    2. Thank you for advice about self-biasing. 5V is come from sensor(switches) lines, 

       So, they don't want make the influence to external sensor lines due to IDD current flow. 

    I will memorize your advice and will propose if the situation would be changed. 

    BTW, I would like to double-confirm just one thing. 

    I & Emrys-san was talking about the series resistor on inputs. 

    I wonder the normal operation on SN74HC151-Q1 could be assured under this condition?

    Best Regards, 

    H. Sakai

  • 0 in reply to hideyuki sakai
    TI__Guru* 97046 points
    Hi Sakai-san,
    Since this condition is listed as acceptable in the datasheet (subnote 2 in the absolute maximum ratings table), then TI will guarantee normal operation.