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Original question:

BQ32000: backup supply current concern

BQ32000: Vcc Initialization Pulse

SAm Seabrook
Prodigy 40 points
Part Number: BQ32000

Hi,

I am currently investigating an RTC circuit using this IC and was concerned that the chip is not entering its low current draw state on power down. From my understanding, Vcc must pass below Vref to change the power source to Vback. To then initialize Vback for low current consumption (1.2uA), a >1ms pulse is required from Vcc.

Below is the Vcc pin when the power is pulled to the product.

I can see there is a 2.3 V pulse for around 4ms, after the power is pulled. Is this enough to initialize the chip?

(from data sheet: The currents measured after issuing a pulse on VCC. The pulse amplitude 0-VCC; pulse width min 1 ms)

If this is not the case how would we go about providing a 3.3 V, >1ms pulse, after the power has been pulled from the device (Vcc provided from the main power rails). And how much time is needed between Vcc falling below Vref and the pulse. 

Thanks for the help!

Below is the schematic (note: the pull-up resistors for the I2C channels are not pictured) 

  • 0
    TI__Guru** 106990 points

    Hi Sam,

    Please note the 1ms initialization is needed right after the backup voltage is applied after a total power loss. That is, given there is no Vcc and Vback, we need to provide voltage to Vback first, then provide voltage to Vcc. 

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Thanks Noel,

    So for my understanding of the schematic above, in a total power loss situation Vcc will fall to ground. The battery is wired to Vback and therefore it will be  providing voltage to the chip instantaneously, at which point Vcc will need to be pulsed for >1ms at a voltage of 3-3.6 V in order to set Vback into its low current draw state. Please let me know is this is valid series of events in order to initialize the chip. If not please recommend the best way to set the chip into its low current draw state from the implementation in the schematic above.

    Thank you

  • 0 in reply to SAm Seabrook
    TI__Mastermind 22210 points

    Sam,

    Much of the team is out of office through January 1st. Please expect a response by January 2nd.

    Thanks,

    Kadeem