We have selected the AD8280 as a secondary safety monitor in our BMS for a LiFePO4 stack.
In working on the design I have run into a few apparent internal inconsistencies in the data sheet that I would like to have resolved before going into production. They mostly revolve around conflicts between Applications Information and the Absolute Maximum Ratings.
First (and most important) point:
In Protection Components and Pull-Up/Pull-Down Resistors (page 23), third paragraph, "[it is] recommended that a diode be used between the top cell in the stack (anode) and VTOP (cathode) of the top part". This diode is present in the evaluation board schematics (UG-175) as CR1/CR7. In the evaluation board this part is a DL4148. At the typical VTOP current for the AD8280 the forward drop of this diode is 0.6V.
This is where it gets sticky: according to the Absolute Maximum Ratings (table 2), all of the cell inputs (VIN1...VIN6) must be at or below VTOP+0.3V. As VIN6 is tied to the same cell terminal as the diode that connects to VTOP, it would appear that this AbsMax rating is violated when the Applications Information advice (and the evaluation board schematic) is followed.
Bottom line: Is the AD8280 still working as guaranteed with this diode present? I am most worried about VIN6 monitoring, as I could foresee a situation where conduction of internal ESD structures would increase the current into VIN6, which might cause VIN6 voltage to be underreported.
This being a safety circuit I'd rather not guess (or rely on "the eval board / our prototype seems to work").
Second, related point:
In Enabling and disabling the AD8280 (page 20), first paragraph: "The AD8280 can be disabled or put into a standby mode by bringing the ENBI pin to logic low, lowering the quiescent current of the AD8280 from a maximum of 2.0 mA to 1.0 μA and dropping the LDO and reference output to 0 V. Bringing the ENBI pin to a logic high takes the part out of standby mode and enables it." That's all very well, but the Absolute Maximum Ratings (table 2) specify the voltage range for ENBI as -0.3V to LDO+0.3V. So if the LDO is off (0V) when the part is disabled, how do I enable the part if I'm not allowed to pull ENBI higher than 0.3V? (Related question: does this mean that you always need an external logic supply to turn the part on, as a pull-up from LDO to ENBI will have no effect?).
Am I missing someting here?
Message was edited by: Jan-Derk Bakker (typo fix)
Regarding your question about the abs max of the ENBI pin: The LDO designation is meant to indicate a static voltage of 5.1V (the typical LDO voltage) with the variation of 4.85V to 5.35V for any given part. This is not meant to indicate an absolute max of 0.3 when the LDO is disabled. I agree that the way it is stated in the datasheet is too ambiguous and have initiated a change.
I understand your first question, but do not have an answer. As you state, it works on the evaluation board, but further explanation is needed. I will consult with the design team and provide feedback.
Thank you for your reply on the first part of the second issue. Any news on the diode in series with VTOP?
For the time being we've decided to include the diode in our design; I will be getting populated proto boards early next week. We still have the option to jumper the diode with a 0R resistor if needed, but it would be most helpful to get this resolved before we go into production.
I must say that I am a bit concerned that this question has been marked "Assumed Answered". After over two months none of the main points have been addressed. The only conceivable place for this chip is in safety-critical sections of Battery Management Systems, one would think that ambiguities like the ones I ran into would be addressed more swiftly and conclusively.
This is not meant to sound bitter, but if the Engineer Zone of Analog Devices Worldwide Technical Support is not a place where one can expect these questions to be answered, then what is? Go back to the nineties and call the local FAE?
I was able to verify that the AD8280 table needs to be corrected to accommodate the condition that you highlighted.
The corrected version will say that VIN6 will need to be in a range of VTOP-0.3V to VTOP+1.0V.
The other VIN inputs will remain unchanged.
We are working on the datasheet modification now.