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LP3921: Does PWR_ON and PS_HOLD affect charging?

JLA
Intellectual 2475 points
Part Number: LP3921

Hello, 

I have 2 questions regarding the LP3921:

Q1: 

One thing is not clear in the datasheet - does PWR_ON and PS_HOLD affect charging, or only LDO / audio amp? Will charging start (and continue) also without PWR_ON and PS_HOLD asserted? 

 Q2: 

Can you supply a reference design or typical connection diagram

Thanks & regards

Jorgen

  • 0
    TI__Genius 10460 points

    Hi Jorgen,

    Please note this is a legacy device with limited information available. The customer should validate the device behavior in the context of their application. 

    Q1: Based on the datasheet description, CHG_IN and PS_HOLD are required for charging. The charger enters pre-charge mode once the correct voltage at CHG_IN is detected. Additionally, Figure 3 indicates PS_HOLD must be asserted within 1200 ms of detecting CHG_IN. PWR_ON provides an alternate method to power up, and should not be required if CHG_IN is present. 

    Q2: As this is a legacy device there is no confirmed reference design or schematic available beyond the functional block diagram shown in the datasheet. 

    Thanks,

    Gerard

  • 0 in reply to Gerard Copeland
    Prodigy 10 points

    Hi Gerard, 

    (Thanks Jörgen for helping me find the correct forum). 

    I am indeed troubleshooting an existing design, this is why I need to get clarity on this. 

    The image you refer to does not mention charging, just LDO outputs, which is why I ask. Does it really also affect charging? If it does, how is PRE-CHARGE supposed to work, when battery voltage is below a valid MCU voltage, MCU cannot assert PS_HOLD before it has sufficient voltage? 

    Note that in this design there is no power path from VBUS to the MCU, MCU is powered directly on the battery. This is why I asked on the reference design - is this a recommended power tree design with this device? 

    I do observe that de-asserting PS-HOLD during normal charging (Vbat = 4V) does not stop charging, just shuts down LDO's. Is this not expected behaviour? 

    Thanks a lot,
    /Johannes

  • 0 in reply to Johannes Book
    TI__Genius 10460 points

    Hi Johannes,

    Based on the charger function description and state diagram (Figure 6), charging will occur with a valid CHG_IN voltage unless disabled by the serial interface. With this in mind your observation when de-asserting PS_HOLD is expected behavior. 

    After some more digging, here is the most extensive schematic available, where the COP8 MCU that controls PS_HOLD is powered off of 3.3 V regulated from VBUS: USB_comp_REVa.pdf

    De-asserting PS_HOLD may not affect charging, but asserting PS_HOLD during the initial application of CHG_IN is required by the device per the PS_HOLD note in Figure 3. While it looks like this does not turn on or off the charger, it may affect device functionality in a different way. As such, it is not recommended to deviate from this sequence.

    Thanks,

    Gerard