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bq2403x: powering OUT from USB or AC with battery absent: more "elegant" way of driving CE high?

Vincent Rheaume
Prodigy 25 points

I need a clarification / some reassurance regarding bq2403x and its standby feature (CE pin). Note: I do not own an EVM. 

I have a use case where there may be no battery connected; in which case I would like USB or AC to be powering the system through the OUT pin (basically, I want the system to use any available power source). For this to work, the datasheet seems to suggest that CE needs to be HIGH:

"The bqTINY III-series enters the low-power standby mode when the CE input is low with either AC or USB present. In this suspend mode, internal power FETs Q1 and Q3 (see Figure 4) are turned off [...]"

However, a prior forum post seems to mention that the system would always be powered, even if CE state is LOW: 

"You are correct in that the BQ24035 will not "totally" shut down when the CE pin disables the chip. It was designed to intelligently manage the power sources that are available so that there would always be at least one source of power available to the system. "

The reason I'm asking is I would like a more elegant way to get power at OUT, rather than using two diodes to generate a HIGH for CE (as in the EVM), when a battery is not present (if a battery were present, it'd power OUT no matter the state of CE). I have no need for ever pulling CE to LOW, so I figured I'd tie it to OUT; however if there's no OUT voltage because CE is low and there's no battery, this may not work (as the datasheet suggests, see first quote above) (chicken and egg problem); however, the forum post (see second quote above) makes me think that it'd work. So which is it, can I avoid the two diodes that are in the EVM? Are there any counter-indications to tying CE, PSEL and ISET2 to the system side (OUT)? Why isn't the EVM doing this instead of adding diodes to the BOM? Did I miss this information from the datasheet? 

Thanks in advance for any insight! 

  • 0
    TI__Mastermind 30120 points

    Hi,

     Please order an EVM, it helps a lot in your evaluation of the IC. In the EVM, we put CE high to VIN, you can always use other rails in your system to put CE high as long as you meet the minimum logic high level.

     

    Thanks

    Tahar

  • 0
    TI__Intellectual 1830 points

    Hi,

    About your question, as you know, the series of this device is designed to make there is at least one source to supply the system (output) with AC/ USB input or battery.

    When you connect the output to make it for the CE pin, it works only at such condition:  when power up, your battery must be present. As your system runs continuously, it means the output will always be high. During the time when the battery is supplying to the system, before the battery voltage drops down to reset the system, the AC/USB input should be connected. After that, as you describe, your battery may not be present all the time, it doesn’t matter as the output now is supplied by the AC/USB. And the always high output will keeps the CE pin high to enable the chip.

    Of course, if you have other external signal/processor to change the CE pin accordingly, it doesn’t need your battery to be present all the time.

    Would suggest you order one EVM to better vilify.

    Thanks.