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BQ25700A: noise, heating and problem with ADC

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Replies: 7

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Part Number: BQ25700A

Good day.

We had questions related to the use of the chip bq25700a. The schematic is shown in Figure 1, in an enlarged format in Figures 2-4.

The parameters we used shown in Figures 5-7. The Figures 8-12 show the data from Battery Management Studio v1.3.80 and indication of an external power source.

We encountered the following problems:

1)    With using the charger we hear a significant noise, which becomes quieter or louder depending on the position of the device and charging current.

2)    We suppose to use the device for charging with a current of 3 A. However, even at a current of 2 A, a significant heating of the chip and transistor VT11 is observed. Within 5 minutes of our temperature of 100 ºC.

3)    If the resistor R27 is set to 20 mΩ instead of 10 mΩ, input current is not adjusted correctly. The board consumes twice less current than is set in the 0x3F register. Setting the resistor parameter in the 0x30 register does not result. This problem is illustrated in Figures 8-9. With installing a resistor of 10 mΩ problems are not observed.

4)    Charge and input currents from ADC in Battery Management Studio are not true. With using 20 mΩ input resistor the input current shows correctly, but when using 10 mΩ this does not happen, but the charging current is still incorrect. This problem is shown in Figures 10-12. At the same time, the charger correctly regulates the current (with 10 mΩ resistors), but the data in the Battery Management Studio is not shown correctly.

 

Please comment on this situation and give appropriate recommendations regarding the using the chip.

 

Thanking in advance.

  Figure 1Schematic


Figure 2Schematic (enlarged)

Figure 3Schematic (enlarged)

Figure 4Schematic (enlarged)
 

Figure 5Registers in BMS
Figure 6 – Input current register and real current
 
Figure 7 – Charge current register and input current
Figure 8 – On the board RSNS_RAC is 20 mΩ, in BMS RSNS_RAC is 10 mΩ
Figure 9 – On the board and in BMS RSNS_RAC is 20 mΩ

Figure 10 – Wrong ADC data

Figure 11 – Charge current from ADC
 

Figure 12 – Input current from ADC
Figure 13 – PCB Top Layer
 

Figure 14 – PCB Bottom Layer

 

 

Figure 1Schematic

  • Hi there,

    We are reviewing the information you provided. We expect to get back to you by 08/24.

    Regards,

    Eric

    Texas Instruments

    Battery Charger Solutions

  • In reply to ezhao:

    Hello,

    I was looking at your schematic on the compensation on the COMP2 pin is not designed correctly. It is difficult to tell what unit your caps on the schematic have (pF, nF) as these are not shown, but assuming you have followed the EVM correctly, here I note the following:

    The 10k resistor (R56) should be in series with the 680pF (C41), and the 15pF (C39) should be in parallel with those two. Your schematic has the inverse.

    This is likely causing instability in the converter and my be the reason all of your other questions' behaviors have manifested.


    Please correct this first and see if it improves the performance.


    Regards,
    Joel H
  • In reply to Joel Hernandez II:

    Hello,

    Correcton of this mistake has not given result. Chip and MOSFET are heating yet.

    Using of 20 mOhm resistor still leads to wrong ADC data and incorrect input current regulation.

    Also, board is working quietly and correctly in buck-mode, but not in boost-mode.

  • In reply to user4962001:

    Hello,

    Can you provide a waveform of the SYS voltage. BAT voltage, BAT current, and either VBUS input voltage or input current when in both buck and boost mode.

    I want to see if your design is unstable.

    Regards,

    Joel H

  • In reply to Joel Hernandez II:

    Hello,

    Charge Current 2A, Input Current 3.25A. Input Voltage: boost mode – 12 V; buck mode – 18V.

    IADPT(4 green) VSYS (3 purple) IBAT (1 yellow) VBUS (2 blue)

    10 mOhm RAC, Boost:

    10 mOhm RAC, Buck:

    20 mOhm RAC, Boost:

    20 mOhm RAC, Buck

  • In reply to user4962001:

    Hello,

    I am reviewing all of your data to investigate this issue further.


    Regards,
    Joel H
  • In reply to Joel Hernandez II:

    The waveform shows that the charger did not switching. The Vsys is from the adaptor but from the battery voltage. There is ~500mA loading on the system. It is really tough to debugging the board remotely. We would suggest compare the operation with a BQ25700AEVM, which is a four-layer board, with most of the two inner layers as ground for noise reduction and thermal improvement. The two-layer board is noise sensitive due to no shielding layer and also has challenges for thermal management due to limited copper area. The EVM guideline is here www.ti.com/.../slua808.pdf

    Regards,

    Eric

    Texas Instruments

    Battery Charger Solutions

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