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Original question:

[FAQ] BQ2575X FAQ Page

BQ25758: Startup Behavior and Gate Drive Control

Carissa Pocock
Prodigy 130 points
Part Number: BQ25758
Other Parts Discussed in Thread: BQ25756

Tool/software:

Hello-

I am interested in understanding more about the start-up behavior of this charger, and the gate drive timing limitations. 

Gate Drive:
What is the minimum controllable ON-TIME of the gate drive circuit? I understand that this is a function of other variables that are within my design control, but at some point, the gate driver itself will not be able to turn on/off fast enough. What is that minimum value or close to it? 


Start Up and Compensation:
Could you provide more details on the startup behavior of this charger, especially during compensation and then the transition to charging. I understand that at start up, the IC performs a compensation mode where it either alternates SW1 or SW2 net depending on the voltage on the output, Vbat. 

During compensation, when SW1 or SW2 is switching, does the charger attempt to monitor/limit current? Does the switching occur at a static or dynamic frequency? Does it monitor voltage? Is it looking for a "stable" signal of any kind?

One reason I am asking is I have an application where my battery is sometimes disconnected until a minimum voltage threshold near the battery voltage, is met. Which means, I have virtually no load on my charger and no voltage. I am using a 3.3uH inductor and a nominal 24V input. 

Since my voltage is ~0V at startup (my 200uF balanced across Rbatt sense with 100uF each are discharged), I expect compensation mode to toggle HIGH LEFT SIDE FET Q1 and as soon as Q1 is turned on, I will have conduction to my output through the body diode of Q3 (HIGH RIGHT SIDE FET). Since the voltage across my inductor is ~24V, if the charger does current limiting to Ibat =~150mA, that would only be a 21ns pulse.21ns seems relatively quick for a gate drive to hold. Would that be problematic for the charger?

If it doesn't current limit during compensation, does it monitor Vbatt? If Vout goes above the pre-charge threshold voltage in the middle of compensation during startup, would that be problematic? Would it start switching SW2 and stop SW1 switching? 

If it doesn't monitor either current or voltage, and compensation mode lasts 40ms, do you think that might be problematic? With 3.3uH and 24Vin, I would expect I would get to my maximum Vout (into 200uF load) within a couple of ms depending on my switching frequency. If there is no voltage or current monitor during this time, and it keeps switching SW1, I would expect Vout to just keep rising until it ~matches the input voltage.

One last question:
The datasheet recommends disabling the frequency modulation at low loads, but it is enabled at startup, which is when I have the low load condition, so I can't disable it. Is there anything to be aware of in terms of current monitoring, or perhaps switch node ringing during startup into low loads I should be aware of?

Thank you for your clarification.

  • 0
    TI__Guru**** 170196 points

    Hi Carissa,

    Regarding gate drive min controllable on time, its ~10us.  The converter monitors on time and starts pulse skipping when on time gets close to 10us.  This pulse skipping can happen with fsw =1.5Mhz and just before entering buck-boost mode.  Buck-boost mode is a digital feedback algorithm not analog.

    Regarding startup, with no battery (VSYS<~2V), input current limit is limited to 200mA until VSYS rises to 2V to ensure not starting into a short.  Then IINDPM register is implemented. See answer above for how to handle small on time. PFM to PWM transition is a function of the number of inductor current zero crossings within a short time frame.

    Regarding disabling PFM, it is recommended when higher accuracy IINDPM or ICHG current regulation is required.  The pulsing input and battery currents are difficult for the current sense amplifiers to filter for accurate measurement.  Disabling PFM does not prevent pulse skipping as described above.

    Regards,

    Jeff

     

  • 0 in reply to Jeff F
    Prodigy 130 points

    Jeff-

    My apologies. I am using the BQ25756, not the 8. Not sure how different the two of them are. I expect it to be in trickle charge until ~3V with ~150-200mA of current.

    Regardless, 10us seems much larger than I was expecting. Did you mean 10ns? The datasheet indicates that it can control the top right side (BOOST) FET to a control time of 225ns, and with a switching frequency of 600kHz, the total cycle time is only 1.67us. 

    Also, can you clarify more about HOW the current is limited? I know that current is limited AFTER compensation, but are you saying it is also limited during the compensation mode right right at startup? 

    thank you. 

  • 0 in reply to Carissa Pocock
    TI__Guru**** 170196 points

    Hi Carissa,

    Oops.  I saw BQ25798. And yes I meant 10ns.  I don't directly support the BQ2575x family. I will reassign to the correct apps engineer for the BQ2575x family.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    TI__Mastermind 32355 points

    Hello Carissa,

    Thanks for being patient with this.

    Gate Drive: The minimum ON time for LODRV and HIDRV is 100ns.

    Start Up and Compensation: The BQ25756 starts inductor detection whenever the BQ25756 starts charging. The BQ25756 will use the voltage of either VAC or VBAT for inductor detection.

    For your setting of 24V VAC to 0V VBAT, the charger works asynchronously while charging. Q2 never turns ON and current conducts through its body diode. Q3 is held high continuously.

    Carissa Pocock said:
    If Vout goes above the pre-charge threshold voltage in the middle of compensation during startup, would that be problematic?

    This would not be problematic.

    Carissa Pocock said:
    If it doesn't current limit during compensation, does it monitor Vbatt? If Vout goes above the pre-charge threshold voltage in the middle of compensation during startup, would that be problematic? Would it start switching SW2 and stop SW1 switching? 

    The BQ25756 doesn't charge while this compensation is occurring

    PFM:

    We recommend setting EN_PFM=0 for more accurate termination when the termination current is less than 2A. It will be fine to start up in low load and then disable PFM later.

    Let me know if you have any questions about this.

    Best Regards,
    Ethan Galloway