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# ADM13307 - Can the Sense Pins Be Driven When Unpowered?

Good afternoon.  I have a customer using the ADM13307-4ARZ in a supervisory function.  The maximum voltage that the SENSE pins can see are as follows:

SENSE1_MAX_INPUT = 1.1V + 5% = 1.155V (not sure if SENSE1 and SENSE3 are backwards and have asked for verification)

SENSE2_MAX_INPUT = 1.8V + 5% = 1.89V

SENSE3_MAX_INPUT = 2.5V + 5% = 2.625V

where VDD is 3.3V +/- 5%.  Now, with a reference voltage of 0.6V (Figure 2) and looking at the "Absolute Maximum" ratings for the "Sense" pins on page 6 of the data sheet, the equation given yields the following as the absolute maximum voltage allowed on each sense pin where VIT1 = 2.25V, VIT2 = 0.6V and VIT3 = 0.6V according to the ordering guide on page 11.

MAX_SENSE - [(VDD + 0.3V) * VIT] / VREF → from page 6

MAX_SENSE1 = [(VDD + 0.3V) * VIT1]/VREF1 = [(3.3V + 0.3V) * 2.25V] / 0.6V = 15V  so good here

MAX_SENSE2 = [(VDD + 0.3V) * VIT2]/VREF2 = [(3.3V + 0.3V) * 0.6V] / 0.6V = 3.6V so good here

MAX_SENSE3 = [(VDD + 0.3V) * VIT3]/VREF3 = [(3.3V + 0.3V) * 0.6V] / 0.6V = 3.6V so good here

The ADM13307 is used in an application where the main power goes away but the SENSE voltages they are monitoring stay on because of a SuperCap for 5-10ms.

QUESTION:  Is it safe to have voltages present on SENSE1, SENSE2 and SENSE3 when no voltage is applied to the VDD pin 8?

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•     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?

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•     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?

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