Re: Interfacing 5V -> 3.3V with just a serial resistor



jrwalliker@xxxxxxxxx wrote:

JJ wrote:
I would like to interface an IC with CMOS 5V output to an CMOS 3.3V
input which can have max 3.3V+0.5V. The simplest method I can think of
is using a serial resistor with value 5V-3.3V/Imax. But I don't see
this method used a lot, specialy not with CMOS I/O. Can somebody please
tell me why?

The method works fine, but there are a few things to watch out for.

With most devices, any attempts to drive more than Vdd +0.6 V will turn
on a protection diode between the input pin and Vdd. This is fine, so
long as the current is low (no more than a few mA) AND the power supply
voltage of the low-voltage device will not be made to rise unacceptably
by this extra current.

If too high a current (usually >100mA) is driven into the protection
diode, the chip may latch up and self destruct.

All these problems are solved by using two resistors as a resistive
divider.

The final thing to look out for is the effect on the speed of the
circuit. It will be slowed down in proportion to the series resistance
and the stray capacitance plus the device input capacitance.

All very true which is why an LVC part makes more sense.

Graham

.

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