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LP3918: LP3918 Operaation Mode

Roberto Rodriguez
Intellectual 800 points
Part Number: LP3918

Dear Support,

I am using the LP3918, and I want to understand few things I could not find in the datasheet.

1. In our application, we intend to use a LP3918 to charge a LiPo battery and power up its internal LDOs.  As the battery is being charged, the LDO's get some of that current to power up.  I also need to power up a buck/boost converter in my system to produce 5V directly from the battery.  What would happen if I am charging the battery, LDO's are ON, and I enable a buck/boost converter directly from the battery node (which is connected to the PMIC)?

2. Say my LIPO battery protection circuit activated and the battery terminals are at 0V.   CHG_IN would have say 5V, I press the switch connected as in Figure 3 of the datasheet, what happens? 

3.  Is there a application note where I connect USB and my device starts charging its battery immediately (the battery can be 0V, or >3V) without MCU intervention to configure the PMIC to the correct LDO voltage, and other configurations?  Is there a way to mimic power path control?

Your help will be greatly appreciated.

  • 0
    TI__Prodigy 450 points
    Hi Roberto,

    regarding your questions:

    1. Can you please give more details on the currents that are being pulled. The voltage stability on the input of the LP3918 is important to determine whether your charging voltage can have a drop.

    2. Can you please specify what the battery protection circuit is. At what point are the battery terminals 0V? Is the battery disconnected? Please refer to the datasheet Device Power up and Shutdown Timing. Can you find the answer to your question here?

    3. There is no separate application note. In the datasheet fig. 5 on page 18 will show you the power up sequence, when USB is connected. There is no internal load switches / power path, since all inputs are directly from VBAT.


    Please come back with us, if there are any more questions.
    Kevin
  • 0 in reply to Kevin Den Toom
    Intellectual 800 points
    Hello Kevin,

    1. Current draw is from a USB port, or stable 5V power supply. Typically we can draw up to 460mA from the bus, and out of that total draw about 120mA is used by the processor, so the battery can get up to 340mA charging current. The LDO1 is not used, LDO2 draws 100mA, and LDO7 about 20mA. Other LDO's not used.

    2. A typical battery protection for LiPo battery such a www.ablicinc.com/.../S8261_E.pdf. The point was to convey that when the LiPo reaches below 3.0V, the circuit literally disconnects the battery terminals via FETs. All LiPo batteries do this. So we call this a dead battery, and it is a protection mechanism to prevent the battery to further discharge below 2.7V. If the battery goes below that, it will not recover.

    3. A colleague of mine using this chip will pose an specific question. It has to do with LiPo and LP3918 underlockout voltage.
  • 0 in reply to Kevin Den Toom
    Prodigy 70 points

    Hello Kevin,

    I work with Robert on the same product.  Here is what we are observing:

    When we leave our device 'on' and the voltage on the battery (and Vin1,Vin2) drops below about 2.9 V, the device shuts off.  We expect that since the battery is low.  The problem is that when we plug in a charger (5V on CHG_IN)  the battery will not charge.  There is 0 current going into the battery and the voltage stays at 2.9V.  I would have expected a minimum of 50 ma and a voltage at the battery to be 4.2 V.  Is the device in 'lockout' since the battery is under 3V? or is the chip 'lost' somehow. How can recover from this? Also, If we unplug the battery and plug it back in, it will start to charge.  

    Thank you,

    Kevin Gin

  • 0 in reply to Kevin Gin
    TI__Prodigy 450 points

    Hi Roberto and Kevin,

    I have assigned your question to our component expert. Please give us some time to answer accurately.

    You may want to look into our battery gauges as well. It can give you the opportunity to get more out of your LiPo battery over lifetime.

    Also for further training on battery charging and battery management you can refer to our support sites:

    Kind Regards

    Kevin den Toom

  • 0 in reply to Kevin Den Toom
    TI__Genius 10885 points
    If I understand correctly, the loads on CHG_IN might be preventing the 50mA precharge from reaching 3V, and the PMIC could be timing out and flagging a bad_batt state.

    When bad_batt is detected, the battery needs to be removed and re-applied, or the charge needs to be unplugged. Is the PS_HOLD pin tied high, or is there any way to remove loads loads consuming current from the CHG_IN pin?

    Additionally, it would be interesting to see if the TOUT_Doubling bit can help here, but I do not have an EVM available to test this specific feature.

    Best Regards,
    Rick S.
  • 0 in reply to Kevin Den Toom
    Intellectual 800 points

    Dear Kevin,

    Thank you.

    I look forward to his reply.

    Regards

    RR

  • 0 in reply to Richard Solarski
    Prodigy 70 points

    We are providing 5V on CHG_IN.  No current is being drawn from the power source from the PMIC or any other parts of the circuit. 

    If the battery dips below 3V and there is NO 5V on CHG_IN, will the 'bad_batt' be flagged?  And will this cause the PMIC to not provide the 50ma?  This is what looks like is happening. How do we clear this bit? is removing the battery the only way? 

    Thank You 

    Kevin

  • 0 in reply to Kevin Gin
    TI__Genius 10885 points
    Bad_batt is read only, is only flagged when the chargers timeout, and can only be cleared by removing the battery. This should not be flagged during a UVLO event, does this happen every time the device operates to UVLO or is this an intermittent issue? Is it possible to verify the status of Bad_batt before re-creating this failure?

    Best Regards,
    Rick S.
  • 0 in reply to Richard Solarski
    Prodigy 70 points

    We are not sure if it is a BAT_BATT being flagged.  We cannot read it since the LDOs shut off.  

    It is consistent.  When the lipo reaches 2.9, the LDOs shut off.  When power is applied to the CHG_IN nothing happens.  We expect that the battery should start charging.  The PMIC will not do anything until the battery is disconnected and reconnected.

    Thank You,

    Kevin 

  • 0 in reply to Richard Solarski
    Prodigy 70 points
    I should add that we are not charging at the time the PMIC enters this state. We are running from the battery.

    Also, just wondering if there is any I2C command that can put the PMIC in this state?
  • 0 in reply to Kevin Gin
    TI__Genius 10885 points
    Accidentally asserting Set_LDOmode might cause this, as the device will not expect a battery and thus will not pre-charge. This bit can also technically clear the bad_batt bit, but should not be used when a battery is present.

    Would it be possible to grab a register log before the device reaches UVLO?

    Best Regards,
    Rick S.
  • 0 in reply to Richard Solarski
    TI__Genius 10885 points
    Hi Kevin,

    Was it possible to confirm the register values before the failure state?

    Best Regards,
    Rick S.