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battery backup application

Mohammad Sweileh
Prodigy 100 points
Other Parts Discussed in Thread: TPS61230, BQ24250

I have a customer that is looking at a battery backup application and I’d appreciate your advice on suitable parts.

The idea is basically to generate a battery-backup 5 VDC source using one or two (in parallel) Li-poly cells. The cells they have in mind have a charge current of 800 mA and a maximum charge voltage of 4.35 V. The power source is a 5V 3A wall-wart supply.

They want the 5 VDC output to continue to be available running off the Li cells if the user disconnects the power supply. The load current may peak to 2.5 A or so but typically will be much lower. Essentially it’s a small uninterruptible power supply.

Ideally, they’d like to find a PMIC/charger device that integrates the charger and output regulator in one device, but they haven’t been able to find anything like that yet.

There is no host/controller available, so the charger has to be programmable via resistors. It is desirable to be able to control how far the Li cell gets discharged, cutting off the output when
the cell gets to 3V or so. Do we have anything like that? Separate charger and regulator devices are an acceptable alternative.

  • 0
    TI__Mastermind 25800 points
    Hey Mohammad,

    How much load current would they need to support at 5V if the power supply is disconnected?


    Thanks,
    Joel H
  • 0 in reply to Joel Hernandez II
    TI__Prodigy 100 points

    Hi Joel,

    The maximum load is about 2.2 A. 

    They are to using separate regulator and charger devices. They have been looking at the TPS61230 for the former and another vendor’s part for the charger. One thing they want to avoid, is having the charger drain the Li-poly cell down below 3 V. The topology they arrived at is to have the regulator always in the power path, whether the source is connected or not. The source and the battery are diode-connected which ensures that the charger sense circuit doesn’t see the system load when the source is connected.

    A plus would be the ability to support two cells in parallel for more capacity.

    Regards,

    Moe

  • 0 in reply to Mohammad Sweileh
    TI__Mastermind 25800 points

    Hey Moe,

    Take a look at the BQ2589x family of devices. These parts are single series cell battery chargers with an integrated boost converter for On-The-Go (OTG) applications.

    That will give you a 3.1A capable 5V output as back up. Do note that the device needs about 30ms to transition to boost mode when set, so some type of intermediate hold-up may be needed to keep the voltage for the time being. 

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    TI__Prodigy 100 points
    Customer can't use the BQ2589x because it requires an I2C-connected host to set it up. They've designed in the TPS61230 which is a very good fit for thier needs. Do we have a similar device that can support more output current, say 3 A? That
    would give them some margin in case the requirements change.
  • 0 in reply to Mohammad Sweileh
    TI__Mastermind 25800 points

    I apologize, I misunderstood the requirements. 

    So if they have designed in the TPS61230, are you only looking for a battery charger? Or are you still looking for the integrated charger + regulator?

    If you are looking for only a charger, then the 3A current is only limited by the boost converter and the maximum discharge current of the cells. The charger will only factor into charging the battery. 

    Regards,

    Joel H

  • 0 in reply to Joel Hernandez II
    TI__Prodigy 100 points

    Separate charger and regulator devices are an acceptable alternative.

  • 0 in reply to Mohammad Sweileh
    TI__Mastermind 25800 points

    Hey Mohammad,

    All of these are charger. 

    Take a look at the BQ2419x and BQ2429x families. These can be operated as standalone devices, just like the BQ2589x family of parts even though they say I2C controlled. However, their charging parameters are autonomously configured to default values when they are powered up. 

    The BQ24250 is another good part if you want the ability to externally program the charge current via resistor.

    Most of they parts have max charge voltages up around 4.2V as a default setting. 

    The other consideration that may not meet your requirement is that when no adapter is present (i.e. powering Vsys from Vbus only), the device will leave the BATFET (switch connected between Vsys and Vbat) on until the battery reaches a depletion or under-voltage threshold spec'd in most of the datasheets between 2.4 and 2.6V. You were looking for 3V cutoff earlier. 

    Hope this helps,

    Regards,

    Joel H

    Regards,

    Joel H