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BQ27742-G1: BQ27742 not starting catch-22

John Newell
Intellectual 280 points
Part Number: BQ27742-G1
Other Parts Discussed in Thread: BQ25890, CSD87501L, CSD83325L, BQ27742EVM, TCA9406, EV2400

My equipment uses the raw battery (a 5300maH Li-Ion) to supply about 1mA to maintain a clock and RAM, even when the voltage falls below the UVP voltage. If the battery is at about 2V or less, then the BQ27742 is unresponsive, even when the PACKP is connected to a charger and has 4.2V present. The charger is a BQ25890 system.

Since the spec for VPWR of the  BQ27742 is a minimum of 2.45 ( Flash write disallowed) then with the recommended application circuit showing VPWR connected to the battery, how can it ever start?

I have seen reference to temporarily shorting out the FET's to 'kick start' the BQ27742, and indeed this works, but this is not achievable in the delivered unit.

The state diagram Fig 15 in slusbv9c suggests that if VBAT > VLVDET and PACKP has charger voltage, then the CHG FET will connect to the charger and the battery will charge in Low Voltage Charging mode. This does not seem to happen. Any suggestions? I appreciate that leaving a Li-ION battery for a prolonged period at less than about 2.5V is not a good idea, but it happens.

  • 0
    TI__Mastermind 41531 points

    Hi, Mr. Newell,

      The device TRM shows the lowest battery voltage that can be pre-charged (Low Voltage Charging) is 2400mV and lowest UV protection threshold is 2300mV. when VPWR of the  BQ27742 is lower than 2.45V ( Flash write disallowed), then the bq27742 communication with host is not guaranteed, could be unresponsive.

    Thanks!

    Phil

  • 0 in reply to Phil Yi40
    Intellectual 280 points

    Hi Phil,

    OK, I understand that the I2C may be unresponsive because the unit is in the Analog shutdown  state and is non-functional - so no I2C comms. However, re-reading the TRM Para 2.2.2.2.1.6 states that the CHG FET gate connects to the PACKP pin if the cell is above VLVDET - 1.7V max. I should then see the battery trickle charge. with 4.2V on the PACK+ and the suggested 2k+1k resistor which effectively connects to the gate of the CHG FET, then it is intended that the unit trickle charges - right? Perhaps my problem is that I am using a CSD87501L as the protector FET. VGS is effectively 4.2-2=2.2V and its ohmic resistance is too large so the battery does not charge, Ideally, as it discharges down to say 1.7V, then VGS rises to 2.5V, but still not enough to offset my small discharge current.

  • 0 in reply to John Newell
    Intellectual 280 points
    Having examined the data sheet of the CSD83325L device mentioned in the slusbv9c.pdf document, this has a value for RON specified at 2.5V, although your selection tool only allows a search for 4.5V spec'd devices. This device would probably work and be a solution. I think I can use it on the same board. With 4.2V charge and 2.4V required to start the BQ27742 then VGS of 1.8V is still above the approx 1.3V for this device to enter ohmic turn-on. The CSD87501L needs approx 2.2V so explains the problem ( I hope).

    I can also program the BQ25890 to output a higher charging voltage if it does not detect the BQ27742, to 'kick-start' the system. This might be enough to solve the problem in firmware. Setting the charge voltage to 4.608V should ensure that a battery discharged to below 2V will recover and then the BQ25890 charger can control the recovery until the BQ27742 enters normal mode.
  • 0 in reply to John Newell
    TI__Mastermind 41531 points
    Hi, Mr. Newell,

    Would you please share the schematics with us? so, we can figure out what's going on there.

    Thanks!
    Phil
  • 0 in reply to Phil Yi40
    Intellectual 280 points

    Hi Phil,

    Not so easy to do  this, but both the BQ25890 and BQ27742 schematics are per your published suggestions except for my choice of protection FET. Measured VGS on the charge FET with the maximum voltage from the BQ25890 of 4.608. Value is 3.6-VBAT = approx 1.8V. The 3.6V is lower than I expected so  there maybe two diode drops here. Either way, the protection FETs are open circuit with a low battery so will never charge. As mentioned, briefly bypassing the FETs allows the BQ25890 to start its precharge state and then the operation starts normally.

    The 7.2 Functional Block Diagram gives no clue as to how PACKP is connected to CHG in the Low Voltage Charging Mode. Presumably the Protector FSM is operational down to  a low VPWR.

    The BQ27742EVM also uses a 2.5V specified protection FET UPA2375T1P, so I would guess I should have been more careful in the choice of device. I will change one on our board design and report the result.

    John

  • 0 in reply to John Newell
    Intellectual 280 points

    For the benefit of other designers, Section 8.2.3.11 NFET Selection makes no mention of the requirement for a low VGS Threshold Voltage. I happened to pick a device which had the highest! Using this as a parameter in the selection guide makes it clear which device to use and which to avoid.

    Also, I discovered that the EV2400 will not drive the I2C lines high enough if  the on board pull-ups are connected to the VDD which is zero volts because the NFETs have disconnected the supply! More catch-22, which probably needs something like a TCA9406 to isolate and shift the external and internal I2C. Strictly, this is only necessary at the design and testing stage. In use, the unit will connect as soon as a charger is applied.

  • 0 in reply to John Newell
    Intellectual 280 points
    I can confirm that with a FET with lower threshold voltage, a fully discharged cell will recover through ohmic conduction until the BQ27742 starts ( provided the cell is above the low voltage charge threshold of 1.7V)