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BQ76942: Questions about a system present shutdown solution / Soft-shutdown

Part Number: BQ76942

Hello,

We would like to implement a system present function with bq76942. A kind of system present function with a key. The battery pack is not removable but a key is supposed to activate the battery (FETs) and if the key is not inserted the battery should be in shutdown state. The required registers are permanently programmed via OTP.

The following link suggests a solution which I have tested and find very good. But the tests revealed certain behavior that gave rise to some questions:

BQ76942: Create a 'System present' function using TS2 and RST_Shut - Power management forum - Power management - TI E2E support forums

Here is a short schematic that shows the same case that i tested:

Let's assume the following case:


The key is inserted all the time. (System present signal or Key in signal pulled to GND). Therefore battery is connected until an undervoltage event occurs which triggers a shutdown and deactivates the FETs. But because of the circuit, TS2 is pulled low and therefore the device will only go into soft-shutdown mode until someone removes the key and thus TS2 can pull the enable signal to 3-5v and the device will transition into shutdown mode.

Question 1: 

Is soft shutdown generally an unwanted situation or is it acceptable for the device to be in soft shutdown for a certain period of time? How big is the power consumption in the soft shutdown state in the case that the key is not removed (the same would apply if the pack is not removed as in the example with the attached link)?

Question 2:

In order to get around the problem, in my opinion there would have to be an analog circuit which does a short pull down when inserting the key on TS2, but the TS2 can otherwise float freely. Unfortunately, I couldn't find such a reference design with pack present/key anywhere. Is there a solution you recommend? For my understanding, that would also mean that in a certain way you have to get a pull-up voltage from the battery, which you would like to avoid because of the current draw...

Thank you for your help in advance

  • Hi Lukas,

    1. Soft shutdown is a very undesirable situation. It shouldn't pose any risk to the device itself, however most of the device's functions are disabled until the device is recovered from the state. I recommend reading this FAQ, which provides more detail on soft shutdown: (+) [FAQ] BQ76952: What is the Soft SHUTDOWN state? - Power management forum - Power management - TI E2E support forums.

    We haven't characterized the IQ of the device while it is in soft shutdown.

    2. We don't have any reference designs for this, but you may consider using both a key and a push button (or something with a similar functionality) to create a simple analog circuit to achieve a pulse low on TS2.

    Here's a simple example of something that could work. (Wake Button should be a push button)

    Inserting the key would allow the push button to pull TS2 low and wake the device. Once the key is removed, RST_SHUT will pull up to REG1 and trigger the device to enter SHUTDOWN mode.

    Regards,

    Max Verboncoeur

  • Thank you for the answers. 

    Regarding the circuit, I would switch to a slightly different solution. In our product it is not possible to have an accessible extra wake up button on TS2 next to the key. That's why I'm going to use a analog circuit which creates a short pull down on the TS2 pin when the key is inserted and so the battery pack can always be woken up when the key is inserted.

    Thank you also for the information provided regarding the soft shutdown. 

    Lets say if I don't rely on the safety functions and the device is in soft shutdown mode due to the previously described behavior, is it acceptable for the device to be in this state for a while as described in the original question? Or does this have to be avoided at all costs?

  • Hi Lukas,

    I recommend implementing the circuit that pulses the TS2 pin low.

    You can read more details on the consequences of soft-shutdown in the FAQ I linked in my prior message, but the device is practically disabled while in soft-shutdown until it is reset (the FETs are off, the communications are off, the LDOs are off, etc.).

    Because of this, I don't recommend allowing the device to enter soft-shutdown, however there shouldn't be any risk of the device being damaged because it is in that state, so if the behavior described in the FAQ is acceptable to you and your application, then it should be okay to use it.

    Regards,

    Max Verboncoeur