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BQ25570 to charge battery from Serial port

jerome d.
Expert 1255 points
Other Parts Discussed in Thread: BQ25570, TPS61220, BQ25100

Hi

I'm searching a solution to power a product from a serial port input @ 31kbps, serial port signal is a current loop of about 10mA.

My system power requirement is about 30mA @ 3V.
My idea is then to use a Li-Ion battery (about 200mAh), that could be charged from the serial port.
Serial signal could be from 2.5 to 5V when idle, going to zero while data are transmitted.

I plan to use the BQ25570, serial input connected to Vin, with a small input current of 10 to 20mA used to charge the single cell battery.

1) I do not find in the data sheet if the input charge current can be set as low as 10mA ?
2) If Vin is 5V with 20mA max, will the boost be auto-disabled ?

Any suggestion, comments are welcome 

thanks

  • 0
    TI__Guru*** 143950 points

    The bq25570 was designed to be configured as a boost converter.  Per the caution statement in the datasheet: If VIN_DC is higher than VSTOR and VSTOR is higher than VBAT_OV, the input VIN_DC is pulled to ground through a small resistance to stop further charging of the attached battery or capacitor. It is critical that if this case is expected, the impedance of the source attached to VIN_DC be higher than 20 Ω and not a low impedance source.  When not transmitting and V(MIDI)>4.2V, the pull down FET would effectively disable the boost but the battery will not charge.  Furthermore, since the bq25570 was designed to pulse charge the battery from a high impedance source, the battery charge current cannot be set to a constant current.

    I recommend that you use a boost converter like TPS61220 followed by a constant current step down charger like bq25100.

  • 0 in reply to Jeff F
    Expert 1255 points

    Hi

    Thanks for your fast answer.
    The bq25100 (and most of the chargers) requires an input voltage > 4.4V.
    The signal from the serial port is between 2.5V and 5V (when the serial port is idle), and zero volt while receiving data.

    You suggested to use a boost converter like tps61220, but, according to the input signal voltage range (2.5 to 5V), I think I need to use a buck/boost so the regulator output is configured to a voltage > 4.4V ?

    Also, setting the regulated output voltage of the regulator to something higher than 4.4V so the charger could charge the battery, that means I will need an additional converter to regulate my application Vcc (3V), from the charger output which is also about 4.4V

    At the end, I need a buck/boost (2.5-5V -> 4.5V) + charger + additional regulator (4.4V -> 3V)

    On more thing: I only have 10 to 20mA available from the serial port, and, on the bq25100, there is no power path management (battery separated from the system load), my system requires about 30mA.
    If there is no power path on the charger, how this could work since the charge current will be lower than required system current ? will the current requested by the system be provided by both the battery and charger when serial port is idle (> 0V) and provided by the battery only when serial port is active (Vin charger = 0V) ?

    Can I find a charger with built-in regulator, capable of charging batteries with a very low input current, and featuring power path ?

    thanks
    Jerome

  • 0 in reply to jerome d.
    TI__Guru*** 143950 points

    I am concerned that you will not be getting enough power from the port to charge a battery and provide 3.0V @ 30mA, especially if the port is only providing 2.5V at 10mA.

    Assuming the above is not an issue, you would need a buck/boost that has the input regulation feature like the bq25570's MPPT and our other buck charger's VINDPM feature.  Otherwise, the buck/boost will try to pull up to its current limit and collapse the port voltage. 

    Can you use a 5V rated, low leakage supercapacitor instead of a LiIon battery?  If so, then you can use the bq25570 with VBAT_OV=5V and buck VOUT = 3.0V.  With VBAT_OV >= VINmax, the bq25570 would be able to extract power from the port from 2.5V to 5.0V.  The next question would be how to configure the MPPT voltage since your MPP varies from 2.5V to 5.0V.