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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
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.
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