MAX31915 input logic levels; supply recommendations

Hi,

DB0 and DB1 are 5V CMOS logic inputs, referenced to GND and roughly centered at 2.5V.

150ohm resistor is a current limiting resistor protecting the VCC24V from overcurrent condition, for example lightning surge transients. The VCC24V supply current is small, 2.3mA max when 5VOUT is not loaded, so the 150ohm won't create too much voltage drop on the supply line.

if large 5VOUT load current is required in the application, we can replace D0 with a TVS diode and remove the 150ohm from the circuit. During high voltage transients, the TVS diode will clamp the voltage to a safe level while conducting to absorb the excessive current.

Thanks,

Wei

Thank you. Final question - is it possible to drive DB0 and DB1 from 3.3V logic (STM ARM processor)?

I took a look at the logic input specs of another digital input chip, MAX31913, that share the same DB0/DB1 and 5VDOUT design.

Here is the VIH/VIL spec. BTW this is not a conventional way to spec VIH and VIL...

The input high spec is 0.7*V5VOUT = 0.7*5.25 = 3.675V max. We couldn't guarantee the 3.3V logic would work over the process variation and operating temperature ranges.

Thanks,

Wei

I took a look at the logic input specs of another digital input chip, MAX31913, that share the same DB0/DB1 and 5VDOUT design.

Here is the VIH/VIL spec. BTW this is not a conventional way to spec VIH and VIL...

The input high spec is 0.7*V5VOUT = 0.7*5.25 = 3.675V max. We couldn't guarantee the 3.3V logic would work over the process variation and operating temperature ranges.

Thanks,

Wei

Thx

One possible solution is to add a pullup resistor to 5VOUT when using the 3.3V logic to drive DB0 and DB1.

I choose more robust solution with logic-level MOSFET