ADM13307 - Can the Sense Pins Be Driven When Unpowered?

Hi there,

by breaking down and rewrite that equation it can be changed to:

SENSEmax=(VDD + 0.3V) * [VIT1/VREF1]

where Vit1/Vref1 is the divider ratio.

VDD + 0.3V is a indication of ESD diode clamping to VDD.

What this means is the node after the divider is clamped to VDD pin via a ESD diode.

If it is a 2.5V threshold input (SENSE1), the divider is internal and adjustable ones has external divider.

Based on the equation you can not have SENSE2 at 0.6V while VDD is at 0V.

In reality, what happens is the when VDD goes down first, current will leak from the sense pins through the internal ESD diode to VDD pin. and because the current are limited by the dividers, it is safe enough for a short (or even long) period of time.

In such event the current into the pin will be limited by the top resistor in the divider, so user needs to make sure it is reasonably big for the external ones. For Sense1 with internal divider, the spec table says the max input current is 8uA at 5.5V, that's mostly due to leakage through the internal divider, with 2.25 threshold, the divider ratio is about 1:1. So leakage with only top resistor should be around 16uA. so you get 4.8uW across the ESD diode during the 5-10ms power down, which I'm sure the diode can handle.

Ren

Hi Richard,

i think the value you've chosen are reasonably safe.

the leakage on these pins are low enough (50nA and 25nA) so even double the resistance wouldn't cause much accuracy shift due to leakage.

Ren

    Good morning Ren.  My customer is asking for verification of his current circuit as shown below for each SENSE input.  I have included my comments in parentheses based on my understanding of your previous response.  Please confirm.

     Thank you.

     1)  SENSE1 is used for 2.5V.  We do not have an external resistor on SENSE1 (Rich - since the divider is internal, there should be no need for an external current limiting resistor).  Do we need one to limit current further or is the internal resistor R1 good enough?

     2)  SENSE2 is used for 1.8V and we have 16.9K as top resistor so current is 1.8V/16.9K = 106.5uA (Rich - therefore the power would be 185.9uW which seems high to me given your example for SENSE1). Is this good enough or do we need a higher resistor?

     3)  SENSE3 is used for 1.1V and we have a 6.49K as top resistor so current is 1.1V/6.4K = 169.5uA (Rich - therefore the power would be 181.7uW which seems high to me given your example for SENSE1).  Is this good enough or do we need a higher resistor?