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Original question:

BQ25886: BATTERY RECHARGE PROBLEM WHEN CONNECTING VBUS

BQ25886: DESIGN OPTIMIZATION

Alessandro Rigoli
Prodigy 210 points
Part Number: BQ25886
Other Parts Discussed in Thread: DRV8874,

Good Morning,

I've already received a detailed and satisfactory support concerning the use of your part number for the electronic board of the device that my company is developing.

I only have a couple questions left concerning the optimization of the design:

1. The battery used inside our device has a max discharge current of 5A (nominal operating current, limited by the safety board) and a peak discharge current of 10A (discharge cutoff current). We connect the battery to the battery charger thus to the system through BAT/SYS pins. SYS pin supplies digital power (through a 3V3 linear regulator) and directly provides power to a motor driver that drives a brushed DC motor. Such motor may draw up to 12A in stall condition (hence also in fast startup transients). However, BCD motor driver is limited to chop currents higher than 6A. Therefore, the intrinsic limit of the battery charger of providing 5A rms current and peak current (even if for a very short duration of 1us) of 9A should not constitute a problem for the functioning of our device. Morevover, the big bulk capacitor on the motor driver (470uF) helps the battery-battery charger system dealing with inrush currents peaks. The obtained results confirm such hypothesis, as the device operates correctly with accetable voltage ripples on VSYS during motor operations (note that we actually need to provide fast startups of the motor from 0V to max Voltage).

However, if we may need to increment the motor current, we thought about connecting the battery directly to the motor driver, and using the SYS pin only to supply digital power (or not using the SYS out pin at all).

My question is: what are the most critical disadvantages in such direct connection, thus not exploiting the Narrow VDC Architecture of the battery charger? Is it feasible? Moreover, concerning the motor driver DRV8874, since we want to increase the maximum current above 6A, could you suggest a similar part number with higher current limitations? (since DRV8874 has a peak output current of 6A and above such value overcurrent protection may trigger)

2. Protection diodes on BAT and SYS.

I saw on the schematics of BQ25886 Evaluation Module that you add a Schottky Diode between BAT and SYS nets. Is it only used to protect the internal BATFET from voltage spikes when connecting the battery?

Since we are driving a DC motor, it may occurs that during breaking some current is fed back into the battery charger. Does this reverse current constitute a problem for the battery charger? Can some internal protections be triggered thus disconnecting the battery from the system? (this is unwanted, since in normal functioning of the device we use only the battery to supply the system, no supply is connected to VBUS, and in any case VBUS+converter would not be able to provide sufficient power to the system, we would always operate in supplement mode). May be necessary to add a diode on VSYS net to avoid reverse polarity (current can only exit from SYS pin, not enter)? This would reinforce the hypothesis of 1. to connect the battery directly to the motor driver.

3. Maximum VBUS input current limit is clamped to ca.1.98A  using 560Ohm resistor (we use a 5V-10W/2A wall-adapter to recharge the battery). Moreover, ICO algorithm is enabled by shorting D+ and D- lines. Maximum charge current is set to ca.1.23A using 4.7k resistor.

My question is: given the maximum VBUS input power (VBUS voltage, VBUS current) hence the maximum converter output power (charge voltage, charge current), is the charge current primarily limited by the maximum output power of the converter? To be more clear, is there the risk of overloading the adapter also when charging only (SYS draws very little current) due to a high set max charge current? (extreme case example: max VBUS input current set to 1A, max charge current set to 2A,  adpater+converter is clearly not able to provide such current). Should be better to further lower the max charge current considering the max output power of the converter?

Thank you so much. Kind regards.

  • +1
    TI__Genius 10385 points

    Alessandro,

    Alessandro Rigoli said:
    Is it only used to protect the internal BATFET from voltage spikes when connecting the battery?

    It is actually to provide a more efficient path than the BATFET's body diode.

    Alessandro Rigoli said:
    My question is: what are the most critical disadvantages in such direct connection, thus not exploiting the Narrow VDC Architecture of the battery charger? Is it feasible?

    The key feature you are missing out on is the regulated minimum system voltage. In the event that your battery is deeply discharged or absent, your motor driver will not operate? Is this okay for your application?

    Alessandro Rigoli said:
    Moreover, concerning the motor driver DRV8874, since we want to increase the maximum current above 6A, could you suggest a similar part number with higher current limitations?

    I suggest creating a new thread about the DRV8874 so it can get to the motor drivers team.

    Alessandro Rigoli said:
    Does this reverse current constitute a problem for the battery charger? Can some internal protections be triggered thus disconnecting the battery from the system? (this is unwanted, since in normal functioning of the device we use only the battery to supply the system, no supply is connected to VBUS, and in any case VBUS+converter would not be able to provide sufficient power to the system, we would always operate in supplement mode).

    It most likely would not be an issue, but without some concrete transient values I am hesistant to rule it out.

    Alessandro Rigoli said:
    May be necessary to add a diode on VSYS net to avoid reverse polarity (current can only exit from SYS pin, not enter)? This would reinforce the hypothesis of 1. to connect the battery directly to the motor driver.

    I like this idea. Especially since the SYS load will be relatively small so the diode conduction is not a big deal. I recommend this approach. 

    Alessandro Rigoli said:
    My question is: given the maximum VBUS input power (VBUS voltage, VBUS current) hence the maximum converter output power (charge voltage, charge current), is the charge current primarily limited by the maximum output power of the converter?

    The charger will prioritize the SYS load and reduce charging current if enough power is not available. 

    Alessandro Rigoli said:
    Should be better to further lower the max charge current considering the max output power of the converter?

    I would say it is okay. IINDPM, ICO, and VINDPM will work to manage the power so that the adapter does not crash. You should set the maximum charging current to what your battery pack manufacturer recommends. The BQ25886 will charge at the maximum charge current if possible, but if not it will reduce the current as necessary to keep the system fed and the adapter from crashing.

    Best Regards,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 210 points

    Good Morning Ricardo,

    thank you for your answers.

    Ricardo said:
    Alessandro Rigoli said:
    Is it only used to protect the internal BATFET from voltage spikes when connecting the battery?

    It is actually to provide a more efficient path than the BATFET's body diode.

    In this way it wouldn't even be necessary to directly connect battery to system, as such Schottky diode would offer a parallel path for the battery to provide the additional current required by the system if battery charger's max discharge current is reached (saturated BATFET).

    Speaking of this, when will the BATET's body diode actually be forward biased? I understood that when VBUS is supplying the system, if system load increases, then the battery charger will go into supplement mode, with BATFET ON, in which battery is suppliying the system. I also understood that when VBUS is not present and only battery is connected, then BATFET will be still ON, and battery supplies the system. Therefore, does the BATFET's body diode only takes action when BATFET is saturated (max 5A RMS discharging current) and system load still requires higher currents? (in such case the system voltage would start dropping hence the BATFET body diode will become forward biased). Could you please confirm that?

    Ricardo said:
    Alessandro Rigoli said:
    My question is: what are the most critical disadvantages in such direct connection, thus not exploiting the Narrow VDC Architecture of the battery charger? Is it feasible?

    The key feature you are missing out on is the regulated minimum system voltage. In the event that your battery is deeply discharged or absent, your motor driver will not operate? Is this okay for your application?

    The battery will always supply the system. There's no problem if the battery gets deeply discharged, as both the battery pack itself and the battery charger (if I correctly understood) will isolate the system from the battery. VBUS is connected only to recharge the battery. For the standard intended use of the device, motor driver does not operate when VBUS is connected.

    That's the reason why I was thinking about not using SYS pin (or using it only to supply digital power) and directly connect battery to system, as converter will only be used to recharge the battery and not to supply the system. I needed an advice on the pros and cons of such choice.

    Ricardo said:
    Alessandro Rigoli said:
    Does this reverse current constitute a problem for the battery charger? Can some internal protections be triggered thus disconnecting the battery from the system? (this is unwanted, since in normal functioning of the device we use only the battery to supply the system, no supply is connected to VBUS, and in any case VBUS+converter would not be able to provide sufficient power to the system, we would always operate in supplement mode).

    It most likely would not be an issue, but without some concrete transient values I am hesistant to rule it out.

    We performed some measurements of current consumptions of the battery. During device operation, there are positive current spikes (discharging) of ca.6A (RMS current is 2.5A), and reverse current spikes (current coming from SYS pin, exiting from BAT pin, entering into the battery) of ca.10ms, which repeat at 5Hz frequency.

    Should I worry abot this current entering the battery without being regulated (as during charging)?

       

    Thank you so much. Kind regards.

  • +1 in reply to Alessandro Rigoli
    TI__Genius 10385 points

    Alessandro,

    Alessandro Rigoli said:
    Speaking of this, when will the BATET's body diode actually be forward biased? I understood that when VBUS is supplying the system, if system load increases, then the battery charger will go into supplement mode, with BATFET ON, in which battery is suppliying the system. I also understood that when VBUS is not present and only battery is connected, then BATFET will be still ON, and battery supplies the system. Therefore, does the BATFET's body diode only takes action when BATFET is saturated (max 5A RMS discharging current) and system load still requires higher currents? (in such case the system voltage would start dropping hence the BATFET body diode will become forward biased). Could you please confirm that

    You understood this perfectly. The BATFET body diode is only active in supplement mode. When the adapter is not present, the BATFET fully conducts to feed SYS, so no body diode conduction occurs.

    Alessandro Rigoli said:
    That's the reason why I was thinking about not using SYS pin (or using it only to supply digital power) and directly connect battery to system, as converter will only be used to recharge the battery and not to supply the system. I needed an advice on the pros and cons of such choice.

    The only benefits would occur when the adapter is present. If that is not an expected use case for your application, then save yourself the diode and have everything downstream of BAT.

    Alessandro Rigoli said:

    Should I worry abot this current entering the battery without being regulated (as during charging)?

    This is concerning. You may want to consider an ideal diode between the inductive load and the battery or some kind of flyback diode configuration. The unregulated negative current can cause safety issues with the battery. The DRV can help you arrive to the best possible solution.

    Best Regards,

    Ricardo

  • 0 in reply to Ricardo
    Prodigy 210 points

    Dear Ricardo,

    thank you so much for the perfect explanations. 

    I'll contact one of your colleagues concerning the motor driver and battery reverse current protection. 

    Best regards.