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Part Number: SN74HC4851
My automotive customer has a requirement of Mux having following features:
1. No of channels- 8:1/16:1 2. Vcc/Logic supply available for MUX : 5 (+/-0.5) V 3. MCU logic available: CMOS 4. All channels should have internal clamping diode protection, as all the channels have to handle voltage greater than the Vcc (5V) from 0 to 22V during normal operation. Customer can provide a 47K series resistor to limit the current through the internal clamping diodes. 5. This clamping current injected into the disabled channels should not affect the ICs functionality/performance. 6. Also the enabled channel also will have input voltage greater than 5V Vcc (0 to 22 V).
Could you please confirm if SN74HC4851 (which has injection current effect control) is right fit here or suggest appropriate Mux ?
Per SN74HC4851 datasheet, note 1 on page 3 for abs max input voltage range VI says "The input and output voltage ratings may be exceeded if the input and output current ratings are observed." - does this mean it can take 22V.
Do you see any challenge implementing this spec?
We do not have a multiplexer that meets your requirements.
We do have a new product TMUX1072 that has overvoltage protection but it is a 2-channel 2:1.
What is causing the need to survive 22V? What signals are passing through 8:1 mux normally?
No, this does not mean it can take 22V. Please read note 2 that states this voltage cannot exceed 5.5 V
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In reply to Adam Torma:
Thanks for the feedback.
We understood from the datasheet like this, that note 2 regarding the input voltage limitation of 5.5V to MUX inputs is applicable only if we do not limiting the current through the internal diode, that is if we directly connect 22V source to MUX inputs without any resistance. However if we limit maximum current to/from an input by series resistor, the ICLAMP rating supersedes the VI rating. We are planning to keep 47K resistor between the 22V source and MUX inputs. This produces a clamping current of 0.3 mA (22-5.5 /47000) which is well below the internal diode current limit of 20 mA.
We need confirmation on the below points
* All channels should have to handle voltage greater than the Vcc (5V) from 0 to 22V during normal operation with the above clamping protection. * This clamping current injected into the disabled channels should not affect the ICs functionality/performance as shown below
* Also the enabled channel also will have input voltage greater than 5V Vcc (0 to 22 V) under normal operation.
In reply to Rajesh Kumar2:
There are many different failure mechanisms for an IC and the abs max table is used to describe the conditions to avoid those gross or reliability over time failures that can include both current and voltage. Based on the abs max table there is risk of gross failure or reliability concerns placing more than 5.5 V on the signal path as mentioned in the datasheet.
The closest device I can find from our current switch portfolio is the new TMUX1072 device.
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