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Original question:

BQ25505: External Power Mux Topology and System Isolation

BQ25505: Hot plug - cold start challenges

user4839477
Intellectual 275 points
Part Number: BQ25505
Other Parts Discussed in Thread: CC2530, ALLIGATOR

Fighting the hot plug issue.  Awaiting PCP redesign with PFET load isolation driven by VBat_Sec_inverted.  But in meantime trying to debug other items.  Cannot get BQ25505 to close internal PFET between VSTOR and Bat_Sec.  Load is CC2530 and 10ma indicator LED.  When load is removed charging works as designed.  However, cannot connect load before battery, after battery, with solar panels, without solar panels, etc.

I assume connection bounce with alligator clip may be creating havoc at PFET VSTOR-Bat_Secondary path, but simply cannot get load attached.  Once load is attached in any combination of solar panel/battery/load order, the device will not come out of cold start mode.

Assistance is greatly appreciated.

Jim

  • 0
    TI__Guru**** 166626 points

    Hi Jim,

    VBAT_SEC voltage must stay above VBAT_UV voltage or the internal PFET turns off.  The extact timing is explained in datasheet excerpt below.

    Assuming the voltages on VSTOR and VBAT_SEC are both below 100mV, when a charged storage element is attached (that is, hot-plugged) to VBAT_SEC, the IC

    1. first turns on the internal PFET between the VSTOR and VBAT_SEC pins for tBAT_HOT_PLUG (45ms) in order to charge VSTOR to VSTOR_CHGEN then turns off the PFET to prevent the battery from overdischarge,

    2. then performs an initialization pulse on VRDIV to reset the feedback voltages,

    3. then disables the charger for 32 ms (typical) to allow the VIN_DC voltage to rise to the harvester's opencircuit voltage which will be used as the input voltage regulation reference voltage until the next MPPT sampling cycle and

    4. lastly performs its first feedback sampling using VRDIV, approximately 64 ms after the initialization pulse.

    If the VSTOR pin voltage remains above the internal under voltage threshold (VBAT_UV) for the additional 64 ms after the VRDIV initialization pulse (following the 45-ms PFET on time), the internal PFET turns back on and the main boost charger begins to charge the storage element assuming there is sufficient power available from the harvester at the VIN_DC pin.

    You may need more bulk capacitance and/or small decoupling capacitors at VBAT_SEC and VSTOR to handle hot plug voltage dips.

    Regards,

    Jeff