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BQ76952: About the device parameter range on the typical circuit diagram of bq76952

JH Zhang
Prodigy 210 points
Part Number: BQ76952
Other Parts Discussed in Thread: BQ76940

Hi Ti's experts，

I want to normalize the bq76952 design schematic device, and I want to know the 68 pages of the bq76952 data sheet document, about the typical circuit diagram parameters of the bq76952, does each of the above devices have relevant value ranges and device packaging recommendations, because I want Know the effect of fine-tuning values different from typical circuit diagrams.
For example
1. The 22nF capacitor between REGIN and VSS, is this capacitor only for filtering, can I change it to 200nF?
2. Can the 1uF capacitor between REG1 and VSS be changed to other values?
3. If the cell voltage sampling input is 220nF, will it be affected if it is changed to 100nF?
4. Do you need to add a 100pF capacitor when DFETOFF, CFETOFF and RST_SHUT are connected to the MCU pins? If not, what will happen?
5. TS1 temperature sampling will be 470pF on the NTC, so can I change the capacitance to 100pF?
6. The ALERT pin is connected to the MCU for signal processing. Is it necessary to add a 1M resistor to weakly pull down VSS? I see other designs like this
As you can see, I need a document similar to the schematic parameter range recommendation, can this be provided.

  • 0
    TI__Expert 8526 points

    Hi JH,

    In response to each of your questions:

    1. Please use the datasheet recommended capacitor value.  Having a different capacitor value can affect circuit transients and potentially affect the performance of the internal LDO.
    2. Again, please use the datasheet recommended value to preserve the integrity of the internal LDO performance.
    3. Yes, reducing the VC pin capacitors will lower the RC time constant, which will affect circuit transients.  If the RC time constant is too low, then the filter will not be able to effectively reduce transients during a short circuit event.  If you anticipate high transients in your system, please increase the resistor value to keep a high enough RC constant with a lower capacitor value.  This will reduce cell balancing current, though, so keep that in mind.
    4. These pins are not listed as needing capacitors, and they should be fine without them.  Having capacitors may help with noise filtering, which could be desirable for ADC measurements depending on how the pins are configured.
    5. Using a different capacitor value is okay.  The main concern is using a capacitor that is too large, which will affect the settling time when the thermistor is biased and measured.
    6. Can you elaborate more on how the pin is configured? If you would like a document with instructions on schematic creation, I recommend looking at section 16.2 Typical Applications in the datasheet and watching this video: Creating the Schematic for BQ769x2 Battery Monitors - YouTube.

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    Prodigy 210 points

    Hi Andria,

    1. Regarding point 6, I need to configure the ALERT pin to select active-high, use REG1 3.3V as the ALERT alarm function, the default is low level, and output high level after triggering the alarm, the high level signal will give To the MCU for detection processing, and at the same time drive a pull-down 5.1K (resistor placed between the G S pins) NMOS to open. The ALERT Pin Config in Settings is configured as 0x2a. Is there any problem with this?


    2. BQ76952、BQ76942 和 BQ769142 的引脚等效图 (ti.com.cn)
    Regarding the internal equivalent diagram of the ALERT pin on page 4 of this link document, after configuring ALERT in point 1, is the ALERT pin directly pulled up to REG1 after the two MOSs of CONTROL are turned on, or is it connected to CONTROL after 1M What about pulling to REG1?


    3. https://www.ti.com.cn/cn/lit/ds/symlink/bq76940.pdf?ts=1691919264753&ref_url=https%253A%252F%252Fcn.bing.com%252F
    On page 26 of the bq76940 document, it is written that ALERT is recommended to connect 500K~1M to VSS. Why is this? I refer to this so I use 1M pull-down to ground. Is this applicable to the bq76952 series?

    Hope to reply. Thanks.

  • 0 in reply to JH Zhang
    TI__Expert 8526 points

    Hi JH,

    1. It should be okay to have the ALERT pin be active HIGH with Reg1 3.3V, but could you provide a drawing or schematic showing the circuitry connected to the pin just so I can have a clear understanding?  The description is a bit vague.  Configuring Settings:Configuration:ALERT Pin Config to 0x2A fits your application and should be okay.
    2. The current settings you have will route current through the path without the 1MOhm resistor.  The 1MOhm resistor path is only used when the ALERT pin is configured for a weak pull-up to REG1, which is not recommended when you are using REG1 for an output HIGH drive (as you are).
    3. The addition of a 1MOhm resistor external to the BQ76940 is for debounce purposes since the part has no internal debounce support.  This should not be necessary on the BQ76952.  Please do not take circuitry suggestions from the BQ769x0 family and apply it to the BQ769x2 family.  These are very different parts.

    Best,

    Andria

  • 0 in reply to Andria McIntyre
    Prodigy 210 points

    Hi Andria,

    1. As shown in the figure below, the ALERT Pin Config is configured as 0x2a, and the default low level triggers an alarm output high level to the MCU.
    2. How to configure the ALERT Pin Config to use the internal weak pull-up to REG1 through a 1M resistor?
    3. If the first point configures the ALERT Pin Config to 0x2a, as shown in the figure below, is it necessary to remove the 1M resistor and 330pF, which is not needed on the bq76952, is it so?

  • 0 in reply to Andria McIntyre
    Prodigy 210 points

    Hi Andria,

    Regarding the fourth point at the beginning, I need to connect DFETOFF, CFETOFF and RST_SHUT to the MCU for control.
    1. I will configure the DEFTOFF pin as the DEFTOFF function, and configure the CFETOFF as the CFETOFF function, as shown in the register in the figure below. Does it need to add a 100pF capacitor filter?
    2. I need to use the RST_SHUT pin as a shutdown pin, and the MCU outputs a high level, as shown in the figure below, do I need C5 and R3, because I see that the two RCs are added in the typical circuit diagram, what is the function?
    Hope to reply. Thanks.

  • 0 in reply to JH Zhang
    TI__Expert 8526 points

    Hi JH,

    Setting the ALERT pin registers to 0x2A will make the pin active HIGH.  This means if an ALERT signal is sent from the BQ76952 to the MCU in the case of an undesirable event, it will send a HIGH signal.  The default state of the pin when there is no ALERT is LOW.

    If you want to use the weak pull-up of REG1, then you will need to configure the ALERT pin differently from 0x2A.  Currently, the active HIGH setting prevents you from enabling the weak pull-up of REG1 because your output HIGH drive uses the REG1 LDO.  Please see section 13.3.2.11 in the TRM for more information (BQ76952 Technical Reference Manual (Rev. B) (ti.com)).

    The configuration you have shown should be okay.  To my knowledge, the BQ76952 does not need debounce support, like the BQ769x0 family of parts required, but having a 1MOhm weak pull-down and a filtering capacitor on the BQ76952 ALERT pin is fine.  Please note, though, that there are many circuitry recommendations for the BQ769x0 family that do not apply to the BQ769x2 family, so it is important to follow the appropriate documentation.

    The capacitors are for filtering, and you can decide if your system requires this.  Leaving them in should not harm the pins' functionality.  If you want the CFETOFF and DFETOFF pins to have CFETOFF and DFETOFF properties, then you will for sure need to ensure that PIN_FXN1 = 1.  If bits 0 and 1 both have a value of 0, then CFETOFF is configured for "SPI_CS or unused," and DFETOFF is configured for "unused."  Please see sections 13.3.2.9 and 13.3.2.10 in the TRM.

    To SHUTDOWN the BQ76952 using the RST_SHUT pin, you will have to hold the pin HIGH for > 1s.  The 10kOhm resistor to GND that is shown in the EVM is probably to ensure that the RST_SHUT pin stays LOW when it is not actively being asserted HIGH.  The capacitor is most likely for filtering.

    Best,

    Andria