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I'm trying to design a circuit with MAX8934AETI+. I'm confused about how to connect a pin to logic level high. For example, pin 6 in the picture below is meant to be connected to logic level high to assign the input current limit to 500mA, but does this mean I need to put a resistor in series with it? I do not see anything else in the datasheet about this.

Typical Application

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2 Answers 2

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On your datasheet, the table on page 7 gives the logic thresholds as low max of 0.4 V, high min of 1.3 V. The absolute max ratings allow 0 to 6 V on PEN2. This means it can be connected directly to a VCC of 5 V.

However, resistors are cheap, expose a test point, and allow you ground that point during debug or test if that's a useful thing to do, or easily hand modify a PCB to change the function. If you've got space and the budget for a resistor, then it doesn't hurt to use one. Use one on every pullup and pull down if you can, you never know when the ability to easily change a 'fixed' pin is going to save you some trouble.

You might wonder why I looked at the absolute max ratings. The idea of pulling inputs up with a resistor being a very good idea dates back to the days of TTL, when the VCC absolute max was 7 V, but the inputs could only tolerate 5.5 V. If the rail spiked to between 5.5 V and 7 V during some fault condition, then ICs with a resistor pullup would survive, those with direct pullups might not.

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For ATE testability, resistors are used, but not necessary for CMOS, otherwise.

However for TTL due to failure modes with reverse Vbe inputs >5.5 V a current limiting 10k resistor can be shared by 10 inputs.

These design rules were set about 50 yrs ago.

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