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BQ25792: VSYS when powered only by battery. Is it possible to set it up higher than VBAT?

Alessandro Rogora
Prodigy 10 points
Part Number: BQ25792
Other Parts Discussed in Thread: BQ25611D

Hi and thanks for the reply in advance!

I am designing a device that need to be connected 24/h. It will be powered for a couple of hours per day and the other time must rely on a battery.

The line voltage is about 12V - 14V and the powered device needs a voltage of about 5V. I was experimenting with your IC, but I find very difficult to understand how the buck-booster converter (and in general the overall power rail works). I have read the datasheet but I do not found it clear at all...

I want to power it using a single LiPo battery in order to minimize uneven charge problems because the battery must last at least an year or two.

Please help me to understand how the settings on VSYSMIN will influence the output. As I read on other forum posts this is the minimal output voltage of the VSYS line: if I change its values it correctly change the VSYS voltage but as soon as i disconnect the VBUS the VSYS voltage drops at battery level. It is possible to boost it up also when the device is powered only by battery? I do not mind if it goes up a couple of hundreds of millivolts more than the VSYSMIN setting when charging, but I need to keep it around 5V.

Is it possible to do it using this device using only a single battery? I have read about the OTG, but I think that it works on the VBUS line when there is no voltage on it, am I right?

If it is not possible could you tell me if TI has an IC that meet my needs in its portfolio? 

Thanks

  • 0
    TI__Guru**** 166176 points

    Hi Alessandro,

    The NVDC power path topology of TI chargers sets V(SYS) = the higher of MINSYS or V(BAT)+few 100mV when input power is applied to VBUS.  When there is no power at VBUS, V(SYS)=V(BAT)-I(SYS)*Rdson-batfet.  In OTG/reverse mode, the converter works in reverse and provides a regulated voltage at VBUS.  For your single cell LiPo application, you will need a charger like followed by a boost converter that provides 5V.  If you can provide more info about your power requirements, I can recommend the best charger and a boost converter.  

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 10 points

    Hi Jeff,

    thanks for the reply. The device is a microcontroller (3.3V) + a simcom cellular module (3.4V - 4.2V). Currently my design consists in a buck regulator that converts the VSYS output to 4V and 3.3V. The problem is that the input of the device is very close to the regulator input voltage and overlaps it when the battery becomes charged so I opted to have an higher input voltage and regulate it.

    The needed current will fluctuate a lot when the cellular module is transmitting (0,1A when the device is idle to 2A when transmitting but is flattened by a couple of capacitors). One of the most important constraint is the volume of the device so the lesser component the better. I was searching for an all-in-one solution that has built-in all and can charge the battery from a broad voltage (12V is ok and 12V-24V is the best) and outputs a stable voltage (4V) that remains the same throughout the charging cycle and when there is no input voltage.

    I have chosen the bq25792 because I can limit the vbus input current (my client has some use-cases where he needs to limit the input current to 0,4A at maximum) and there are lots of informations that I can retrieve from the I2C registers that I can read about the battery status and the various voltages.

    A fully integrated solution that uses the least components will be the best!

    Thanks

    Alessandro

  • 0 in reply to Alessandro Rogora
    TI__Guru**** 166176 points

    Alessandro,

    To reiterate, none of our NVDC chargers were designed to output a constant voltage on SYS for the entire battery charge cycle and definitely not when no input voltage is applied.

    For 12-24V input and small size, BQ25792 is the best choice.  For only 12V input, BQ25611D is another choice but it does not allow higher MINSYS than VBATREG.  A boost converter like https://www.ti.com/product/TPS61253A might work for your 2A transient while transmitting.

    Regards,
    Jeff