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Question

I am using this LDO in my design.The datasheet says the below thing about this LDO Enable pin.

My circuit is given below.

Analog devices AE team which reviewed the schematic says the below thing.

? It is not stated in the datasheet that the EN chip is internally protected, therefore I would recommend including a pull-up resistor.

Do I need to keep it? If yes, can I put 10K between Vin and EN

UPDATE:

BELOW is the response received from the concerned forum of ADI.I received the first response from ADC AE team because this regulator is powering my ADC.Now I received respose from the team which is taking care of this regulator.

Answer 1

The ADM7155 datasheet has this table on page 5:

The line "EN to GND" states that the EN pin will tolerate up to +7V with respect to ground. This makes sense, because that's also the maximum tolerable potential at VIN.

The EN pin doesn't need any more protection than VIN itself. If you tie EN directly to VIN, and you are sure that VIN is never going to exceed +7V, then no damage will occur. Whatever is "protecting" EN inside the IC, if anything, is sufficient already.

I have no idea what the "AE team" is talking about, the datasheet makes it abundantly clear that tying EN to VIN is perfectly fine.

The only hint I could find regarding protection is from this diagram on page 14:

All this does is reinforce my conviction that no damage will occur unless you apply more than +7V to EN, which would cause the zener diode to conduct heavily. However, that zener diode does imply that including a resistor would limit EN current in the event that you apply more than +7V. If you've tied EN directly to VIN, and then set VIN above +7V, then the entire IC is already damaged, and any damage to EN circuitry is moot.

I would suggest that this "resistor protection" might only be relevant if you are controlling the ADM7155 from a separate system, able to apply above +7V to EN. In that case a resistor would permit the internal protection zener diode to "clamp" to the +7V maximum, while the resistor limits the resulting current.

That's still not a great idea, it's best to ensure that the control system never apply more than necessary to "trigger" EN, which is determined by the hysteresis and thresholds described on page 24. I infer from that rather detailed description that it is intended for EN to permit under-voltage lockout, using a resistor divider to apply some fraction of \$V_{IN}\$ to EN; perhaps that's the motivation for a discussion about using resistors with EN.

Answer 2

I look in the the datasheet that there is a 7V zener to GND. But no pull up resistor is needed or required as such. Maximum voltage is specified to 7V.

Answer 3

You have the data sheet that says one thing.

You have official statement from Analog Devices AE who recommend you what you should do.

There is no way for anyone else to know without internal knowledge of the chip what you should do.

Choise is to leave out a resistor and you may end up in problems because not doing what AE suggested.

Another choise is to add a resistor because it was the suggestion, even if it might be possible to leave the resistor out.