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LMS33460: Power ON/OFF circuit with under voltage protection

Part Number: LMS33460
Other Parts Discussed in Thread: TM4C123GH6PM, TPS3890

Hi,

I have put together the push button circuit explained in TI's SCEA048B application report with an under voltage protection using TI's LMS33460 chip. I ended up with the circuit attached.

The idea is to have the LMS33460 under voltage detector monitoring the +PACK line from the battery so it pulls down the regulator enable pin to power down the device in case the battery voltage falls bellow 3v, but I have several doubts:

1. After doing the schematic I realized that I should use an under voltage detector that triggers at 3.4v instead of 3v, because the 3.3v LDO regulator has a maximum dropout voltage of 250mV and the TM4C123GH6PM has a minimum operating voltage of 3.15v, so 3.15v + 0.25v = 3.4v.. under this battery voltage the microcontroller will not have enough voltage supply. But, since I am using a 3.7v lithium battery, this would only give 3.7-3.4 = 300mV of margin for the battery to discharge.. I don't know much about lithium batteries but that doesn't seem to be a lot... is that a problem?

2. I haven't found the input resistance of the TLV70033's Enable pin in the datasheet, so how can I know the maximum pull down resistor (R33) to minimize consumption and still pull down the voltage to turn off the regulator. Anyway, this pull down resistor is not needed anymore after connecting the enable line to the flip-flop, since its output is not an open drain and it will command the line high or low, right?.

3. Do you think this would work?  : )

4. Is there a better way for protecting the battery?

Thanks

  • Hi Javi,

    lithium batteries often already contain a protection circuit to keep the load currents at safe limits. Often, there's also an undervoltage and overvoltage protection, which disconnects the battery from the load or from the charger. This in order to prevent overheating, fire and explosion. So, eventually, your protection measures aren't needed at all?

    Kai
  • Hi Kai, thanks for the answer. You are right, but it seems to be quite difficult to find lithium batteries' datasheets with useful information like the kind of protection circuitry included or charge/discharge graphs.. I'm just getting information about nominal voltage, capacity and size. Nothing else.
    I have a good candidate for my project, 3.7v 1200mAh (it is actually the one that TI sells with the batpakmkII boosterpack) but I don't find any datasheet (maybe the people from TI can help with that?)
  • Hi Javi,

    maybe this helps:

    uk.farnell.com/.../2699214

    www.ti.com/lit/ug/tidub31/tidub31.pdf

    Kai
  • Javi,

    I don't think this will work. The LMS33460 is an open-drain device so there is a pull-up resistor required to a pull-up supply voltage, and the output is then connected to the EN pin. I think you can accomplish the push-button circuit and the detector circuit all with one device. I would recommend the TPS3890 for example. Using the /MR pin, you can connect a push-button directly to this pin. And the TPS3890 has an adjustable version which allows you to set the threshold using external resistor divider at the SENSE pin. The delay can be set using a single capacitor for any delay between 40us to 30s making the design very flexible.

    The only reason you should use the push-button circuit from that Application Note is if you need the push-button to latch the output. Is this a intended requirement? If not, you can use a single device to accomplish your undervoltage battery monitoring.

    -Michael
  • Hi Michael, thanks for the answer. 

    I actually wanted an IC with an open-drain output, like the LMS33460 so, when the voltage is ok, the LMS33460's output is disconnected from the Enable line and the push-button circuit commands the TLV70033's enable pin just by itself. When the voltage is under the threshold the LMS33460's output pulls down the line and the TLV70033 is disable whatever the push-button circuit tries to do. I mean, the pull-up supply voltage you mention would be supplied by the SN74LVC flip-flop output, right?

    So the circuit would only turn on the device if the voltage is OK. The possible states would be:

    REGULATOR STATE
    Flip-Flop \ Battery Voltage OK NOK
    LOW DISABLED  DISABLED
    HIGH ENABLED DISABLED

    This way it would work, right?

    Anyway, I decided to remove the under voltage control and use a battery with the protection circuitry integrated on it, as Kai suggested. I think it will be easier and safer.

    Regardless the voltage control, I want to use the push-button circuit to turn on the device with a short button pulsation and to turn it off with a long one. I think that is better than a single switch.

    I put R37 and R33 voltage divisor because flip-flop's maximum low output voltage is 0.55v and regulator's maximum low input voltage is 0.4v

    Does all this make sense to you? 

    Thanks!

    Javi

  • Javi,

    Yes this makes sense and looks good. The push-button circuit is only required for the short press/long press on/off functionality which it sounds like you want. And R33 is not typically recommended but in your case, if it helps divide down the voltage logic levels then it is OK. This should work fine. Please test and feel free to share the results. Let me know if you need any additional support.

    -Michael