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BQ25120: Removing the IN voltage causes voltage regulation on SW to change from the set voltage of 1.8V to 3.5V

Bruce Thompson2
Prodigy 50 points
Part Number: BQ25120
Other Parts Discussed in Thread: , EV2400

I am using the bq25120aevm development board.  I have a lithium ion battery connected  to J7 and a usb 5v charger connected to J3.   I have the chip set up to output 1.8V on the SW pin.  When I remove the USB charger from J3 with the battery still connected I expect 1.8V to still be present on the SW pin or Vsys.  Instead the chip switches and outputs the battery voltage.  Do I have one of the registers set wrong.  I have not been able to determine what the problem is.

Here are my register settings.

* Created: Sat Jan 30 21:38:46 EST 2021
*
* Format: Register Name  tab Character,\t  Hexadecimal register value.
* Device: bq25120
* BQZ Container: Charger_1_00-bq25120.bqz
*
Status and ShipMode Control    41
Faults and Fault Masks    00
TS Control and Status Mask    88
Fast Charge Control    E8
Termination/Pre-Charge and I2C Address    0E
Battery Voltage Control    78
SYS VOUT Control    AA
Load Switch and LDO Control    FC
Pushbutton Control    68
ILIM and Battery UVLO Control    3E
Voltage Based Battery Monitor    3C
VIN_DPM and Timers    42

  • 0
    TI__Mastermind 23170 points

    Hi Bruce,

    By default, the device is configured to have 1.8V on the SYS pin and even on IN voltage removal, should re-enable the SYS to 1.8V within the next second. 

    Do you see this behavior if you don't change any registers? Are you connecting the EVM to anything?

  • 0 in reply to Anthony P Pham
    Prodigy 50 points

    Hi Anthony,

    I am still having issues.  Let me try to articulate the issue more clear than I original did.  For all of the following tests all jumpers on the board the way they came.  I did not change any of the jumpers

    1. I initially connect a charged lithium ion battery to the BQ25120EVM that is at approximately 4.0V

    2. I observe Vsys and it is 1.8V

    3. I connect a charger to the BQ25120EVM on J3 microUSB connector

    4. I observe Vsys and it is still at 1.8V

    5. I connect the EV2400 to my computer first than I connect it to the BQ25120EVM

    6. I observe Vsys and it is still at 1.8V

    7. I remove the charger from J3

    8. I observe Vsys and it is still at 1.8v

    9. I reconnected the charger to J3

    10. I program register Load Switch and LDO control, first to 1.8V, then I check the enable box

    11. I observe the voltage on Vsys and it is still 1.8V and the voltage on the LDO is 1.8V

    12. I now disconnect the charger on J3, Vsys increases to approximately 3.5V and the voltage on the LDO pin goes to 0V after about 1 second.

    I expected the voltage on Vsys and LDO to stay at 1.8V

  • 0 in reply to Bruce Thompson2
    Prodigy 50 points

    Hi Anthony,

    I was able to determine my issue.  The problem was that I had a cracked etch on the load to Vsys so there was no load.  Once I fixed that and a load was present the issue went away.  I am now ok with the issue above.

    After fixing that issue I noticed that after 10-15 minutes with no activity on the I2C interface the LS/VLDO out put is disabled.  What is causing the LS/VLDO output to be disabled?  Is there something I can disable?  I have read the document but have not found a way to disable it.

  • 0 in reply to Bruce Thompson2
    TI__Mastermind 23170 points

    Hi Bruce,

    Glad the 1.8V to BAT voltage was solved!

    Bruce Thompson2 said:
    After fixing that issue I noticed that after 10-15 minutes with no activity on the I2C interface the LS/VLDO out put is disabled.  What is causing the LS/VLDO output to be disabled?

    This may be due to the I2C watchdog timer. Generally, the device will be reset after 50s if not in high impedance mode. After an I2C transaction, a 50s timer begins. If no transactions occur to reset it before it expires, then all registers will reset to default (except for MRRESET_VIN and MRREC). The only way to disable it would be to go into high impedance mode (which is a battery only mode with CD state low).