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Original question:

BQ25504: Not coming out of cold start as expected

BQ25504: Not coming out of cold start as expected (update)

Karl Johnson
Prodigy 225 points
Part Number: BQ25504

In relation to my question under https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/1247228/bq25504-not-coming-out-of-cold-start-as-expected/4716916

We have now adjusted VSTOR_CHGEN to be 2.2V which is the minimum allowed for the supercaps and it has made no difference to the operation. We still get stuck in cold start until a very bright light is applied to the solar cell despite the supercaps being fully charged to 3.6V. We don't use VBAT_OK to disable the load, preferring to monitor the supply ourselves and go into a sleep mode when it drops too low. Do we need to do this? What is purpose of coldstart? Is there a device in this range that doesn't have cold start?

Many thanks

  • 0
    TI__Guru*** 146745 points

    Hi Karl,

    Cold start is intended to charge up the storage element at VSTOR>1.9V so that the more efficient boost converter turns on.  If there is any resistive load at VSTOR or in rare cases if the storage element has high leakage, cold start likely won't be able to get the voltage up.  Alternatively, if you insert a charged storage element that is > 1.9V (but only when VSTOR and VBAT are < 100mV and no input is applied), you can bypass soft start.

    Regards,

    Jeff

  • 0
    TI__Genius 12035 points

    Hi Karl,

    Karl Johnson said:
    We have now adjusted VSTOR_CHGEN to be 2.2V

    Based on the previous thread, BAT_UV was set to 2.2V? (VSTOR_CHGEN is not adjustable). This may provide help if you use the BAT_OK voltage in order to disconnect the system load, is this the same voltage threshold you use to disconnect your system load?

    Karl Johnson said:
    What is purpose of coldstart?

    Cold start is an unregulated boost controller that is less efficient than the main boost controller. The purpose of cold start is to charge VSTOR above VSTOR_CHGEN.

    The main boost charger used during regular operation to boost VIN voltage in order to charge VBAT and provide power to VSTOR requires VSTOR > VSTOR_CHGEN, otherwise it is not able to operate. Therefore the cold start circuit allows the BQ25504 to charge VSTOR until the main boost controller can take over.

    This is required if your initial BAT and VSTOR voltage are below VSTOR_CHGEN and it is therefore present in similar energy harvesters.

    Karl Johnson said:
    We don't use VBAT_OK to disable the load, preferring to monitor the supply ourselves and go into a sleep mode when it drops too low. Do we need to do this?

    Theoretically, if you can monitor VBAT and VSTOR voltage and disable supplies when voltage drops too low then I suppose that is the same functionality as the VBAT_OK signal. If that is the case, what VSTOR or VBAT voltage do you use to disconnect the load?

    Best Regards,

    Juan Ospina