BQ24780S: some questions about the datasheet of BQ24780S

Hey Alice,

For 1) The charger is in a light load condition, which makes it much more difficult when the current ripple content is lower. The ripple is lower under those conditions.

For 2) This is the bias current pulled by the SRx differential amplifier. It will dictate what the offset between the SRP and SRN pin will be.

For 3) This is the way to effectively test the current limit of the REGN regulator. It is not a real application case; its only to show the current capability of the regulator. .

For 4) The REGN regulator must be able to provide more current when in charging mode as it is also used to drive the internal buck power stage gate drivers. When not in charge mode, the buck power stage is not operating. 

For 5) The only valid values are 20/10, 10/20, or 20/20. Other values are not accounted for.

For 6) This is the leakage current when that voltage is applied to those pins individually. 

Regards,

Joel H

Hi Joel:

   Thanks for your answers. I still have doubts about your answer.

For 1) The charger is in a light load condition, which makes it much more difficult when the current ripple content is lower. The ripple is lower under those conditions.

    As shown in the figure, Only when the current is 256mA does the accuracy depend on the voltage, Does that mean that the accuracy is independent of the voltage when the current is 512 /192mA. Why is the 256mA different from others.

For 5) The only valid values are 20/10, 10/20, or 20/20. Other values are not accounted for.

     We have to use 5m ohm/10m ohm sense resistor for 230W power so we have to get POMN Gain. So if it's convenient can you give me the value? Thanks a lot.

For 6) This is the leakage current when that voltage is applied to those pins individually.

    Why is the voltage 7v when his operating voltage is 6.5v,just the test conditions?

7. Why does the efficiency not increase with the increase of voltage? Maximum efficiency of voltage is 11.1V instead of 14.8/7.4V.

Thanks!

Alice

Hi Joel:

   Please reply to my question at your convenience.I have some additional questions.

For 3) This is the way to effectively test the current limit of the REGN regulator.It is not a real application case; its only to show the current capability of the regulator. .

          What are the values of I (REGN_LIM) and I (REGN_LIM) for VREGN=6V?

//

8.Some questions about inductor short,MOSFET short protection.

  8.1  What's the difference between inductor short and MOSFET short protection?

  8.2  Is protection triggered only if both high mos and low mos are above the threshold?

  8.3  After seven short circuit events, the charger is latched off. Why not trigger it the first time?

  8.4  During boost function, the low-side MOSFET short circuit protection threshold is used for cycle-by-cycle current limiting, charger does not latch up. How to understand?

Thanks!

Alice

Hey Alice,

For 1) 256mA is a transition point. You can see the higher current 512mA setting is +/-10% accurate, which is better than all 256mA cases (either +/-16% or 20%). On the other hand, the lower 192mA current setting is similar in accuracy to the lower voltage setting of the 256mA setting of +/-20%. As such, you can infer that across all voltages, the 192mA case and lower will have an accuracy similar or worse in accuracy to the 4.2V of the 256mA. And all this is still due to the magnitude of the feedback signal (VRSR).

For 5) For the PMON gain, you can use either value; either the 0.5uA/W setting or the 2uA/W setting because RAC and RSR are skewed by a factor of 2x. Note that the PMON pin voltage is clamped at 3.3V, so 230W should not produce a value higher than 3.3V on PMON.

For 6) This test condition is based on the absolute maximum voltage rating of these pins, not the recommended operating voltage.

For 7) I will have to look into this further, but I would venture to reason that this related to increased switching losses. For the most part, I don't image the conduction losses to be higher, but the switching loss and reverse recovery loss would increase. Keep in mind that this is a controller, so your efficiency will vary based on your MOSFET and inductor selection. This efficiency data was collected on the EVM.

For 3) This value is not specified in the datasheet, but you can infer that @ 6V, the ilimit will be similar to the 0V test condition.

For 8.1-2) Please see the following diagram:

For 8.3) This could be a sudden spike in current, and does not necessarily assume converter is continually providing more than the peak current. In general, MOSFETs can handle pulse current at currents much higher than their continuous current rating. 

For 8.4) In buck mode, after 7 occurrences of this event in 90sec, the ACFET/RBFET will  be latched off. However, in boost mode, this will not latch off the input FETs. Boost mode on this charger is purely supplemental current to the load. 

Regards,

Joel H

Hi Joel:

For 5) For the PMON gain, you can use either value; either the 0.5uA/W setting or the 2uA/W setting.( 5m ohm/10m ohm sense resistor)

       I found that Liangyu_Ge@asus.com asked the same question of resistance 5/5 in September,the following figure is the answer.Should the gain be 0.25uA/W or 1uA/W if I Refer to the following answer when I use 5m ohm/10m ohm?

For 7) I will have to look into this further, but I would venture to reason that this related to increased switching losses

     Please don't forget this question. By the way, Should the maximum efficiency of voltage be 11.1V even if this related to increased switching losses?

  Looking forward to your reply.Thanks!

 Alice

Please refer to the latest question, Dec 12

Hey Alice,

For 5) We recommend these values when the RAC and RSR resistors are mismatched by a factor of 2. 

For 7) There are increased switching losses, but I believe you are correct that the 14.8V data should be above the 11.1V data based solely on calculations.This may be an editorial error in the datasheet.

Regards,

Joel H

Hi Joel,

     Thanks so much for your comment. I will confirm with you through email.