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BQ25886EVM: Maximum output current on no battery connection

Satoshi
Guru 19645 points
Part Number: BQ25886EVM
Other Parts Discussed in Thread: BQ25886

I evaluating bq25886EVM on no battery connection, setting is nearly default and different points is below.

⇒ICHGSET: 1.5A, D+ and D- shorted (DCP), ILIM: two pattern for 1.5A and 2.5A, Battery: no connection

Waveform are two case below;

※ch1 (Yellow): Vsys, ch2 (Green): Vsw, ch3 (Blue): Iin, ch4 (Red): Isw

①ILIM: 1.5A ⇒ Isys set Light load to under 600mA become PFM operation. when Isys exceed 600mA, reach the current limit and unstable.

②ILIM: 2.5A ⇒ Isys set Light load to under 600mA become PFM operation. when Isys exceed 600mA, bq25886 become PWM operation and Vsys drop by input current clamp.

【Question】

・About the case of ①, Condition for ILIM set 1.5A; when Isys exceed about 600mA, probably reach the current limit and unstable operation.

　Can't there condition operate over than Isys: 600mA by bq25886 and EVM specification, correct? 

　Or is there any measure point? 

・About the case of ②, Please let me know about the conversion trigger of PWM and PFM.

Best regards,

Satoshi

  • 0
    TI__Guru**** 166866 points

    Hi Satoshi-san,

    With no battery, is charge enabled?  If so, it is possible that V(BAT) and/or V(SYS) are hitting OVP, which causes the converter to shutdown.  What are the peak values of the SYS (yellow) trace? Going in and out of OVP shutdown looks like PFM but is not the same.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Guru 19645 points

    HI Jeff

    Thank you for reply,

    These condition is charge enabled, because require to fix charge enable by customer board condition.

    Peak Vsys is about 9.1V_max.

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Guru**** 166866 points

    Hi Satoshi-san,

    If VSYS is peaking at 9.1V, then what is VBAT doing?  VBAT_OVPmin rising is 102.5% of VBATREG so for VBATREG=8.4V than is only 8.61V.  Can you retest and measure VBAT?  If my theory is correct, then as long as charge is enabled and with minimum capacitance on BAT pin, you will see this behavior. Can you test by preventing charge by pulling TS pin to ground (i.e. forcing TS_HOT)?  To prevent, you will have to increase the capacitance on BAT.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Guru 19645 points

    Hi Jeff

    I understood, I will retest with measure VBAT waveform (and without Isys because probe have 4ch_max) next week.

    Please wait next week.

    If I remember right, VBAT voltage is the nearly as the VSYS on Isys: 0A~0.55A.

    Over than 0.58A, VBAT became rapidly decrease and either reached UVLO.

    By the way, is PWM operation (Isys: 0.58A~) not abnormal, correct?

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Guru**** 166866 points

    Hi Satoshi-san,

    Ok.  PFM is not unexpected at light load.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Guru 19645 points

    Hi Jeff

    I measured based on VBAT output voltage, please see attached below; 

    I looked VBAT overvoltage on TS pin to 10kΩ condition.

    【Additional information】

    ・When TS pin tied to GND, switching was stable (0A~ILIM) but VBAT occur about 2V. 

    ・When TS pin to 10kΩ (default), low current is unstable and about 500mA ~ 900mA range became more bad unstable (unmeasurable). 

    ・On the case of add 22μF capacitor on VBAT, VBAT was not change and peak current was a little increased.  

    【Question】

    ・Is these operation reached OVP, correct?

    ・Why VBAT voltage waved and reach OVP on no battery connection?

    ・Is there any idea for suppress VBAT voltage wave and OVP?

    　(Additional VBAT capacitor is not effect) 

    ・When TS pin to GND, is these operation normal?

    ※ch1 (Yellow): VBAT, ch2 (Green): Vsw, ch3 (Blue): Iin, ch4 (Red): Isw

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Guru**** 166866 points

    Satoshi-san,

    When TS fault is induced (i.e. TS=GND), battery charging stops so VBAT should stay at a low voltage. 

    PFM is expected at light load.  SYS output should be stable at light load and TS fault.

    To stop battery charging when no battery is connected, induce TS fault, which would likely happen if a real battery pack with embedded thermistor is removed.

    Regards,

    Jeff

  • 0 in reply to Satoshi
    TI__Genius 10385 points

    Satoshi-san,

    TS to GND waveforms look normal.

    What is your VBATREG setting? I think Jeff is right and you are most likely seeing the converter switch off due to BATOVP.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Guru 19645 points

    Hi Jeff, Ricardo

    Thank you for reply,

    VBATREG setting is 8.2V.

    I understood the operation of TS to GND condition, and no battery with TS normal is not so much the real case.

    About no battery with TS: 10kΩ, is this condition unexpected or not recommend?

    When possible, I want to understand the operating principle why above condition continue to reach OVP, for explain to customer.

    I guess that charging VBAT capacitor induce VBAT voltage increase and reach the OVP, correct? 

    Best regards,

    Satoshi

  • 0 in reply to Satoshi
    TI__Genius 10385 points

    Satoshi-san,

    Based on VBATREG being 8.2V, you are definitely seeing the converter shut off because of BATOVP when TS: 10k. When the converter is shut off, the VBAT capacitor supplies SYS  until the recharge threshold is hit.

    Another thing to note is when your ILIM is 1.5A and ICHGSET is 1.5A you will always be in IINDPM. You will sometimes be in IINDPM when your ILIM is 2.5A as well. 

    Both of these combined influence the behavior you see and your ever increasing system load has an impact.

    Thanks,

    Ricardo

  • 0 in reply to Ricardo
    Guru 19645 points

    Hi Ricardo, Jeff

    Thank you for information.

    Our question points was cleared.

    Best regards,

    Satoshi