4xxx input protection clamping diodes

[Max]

Hi, Im evaluating a design with an HEF40106 with a +12V supply
with inputs that can exceed the VDD limit!
My question is: can I rely only on an external resistor (say 100k)
in series with the input and the HEF-internal clamping diode?
Obviously I can put an external protection diode, too.
Has anyone already experienced this issue?

Thanks in advance for your suggestions, Max

 

[Ken Smith]

In article <Xns94B3C50C7974EMax@195.31.190.132>,
Max <betaware@hotmail.com> wrote:

Hi, Im evaluating a design with an HEF40106 with a +12V supply
with inputs that can exceed the VDD limit!
My question is: can I rely only on an external resistor (say 100k)
in series with the input and the HEF-internal clamping diode?
Obviously I can put an external protection diode, too.
Has anyone already experienced this issue?

If you keep the current in the input diode low enough and you don't
require the circuit to react fast, the 100K idea works.

On the CD4000 series chips, the input protection diode looks like the
emitter of a low gain PNP transistor. The collect(s) of this transistor
is every other bit of P doped semiconductor in the chip. As a result,
when the protection diode has current in it, the Icc of the device
increases.


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