LTC3871

Dear sir,

I'm considering to use LTC3871 to design a powerbank. The battery side is 14S and input/out interface is a USB type C interface.  Here I have some questions:

1. The example in datasheet is a buck battery charger. Does LTC3871 support boost battery charger ?This means, the type C interface connects to Vlow and the battery connects to Vhigh. When the powerbank works in charge mode, the LTC3871 boost the type C input voltage to charge the battery. When the powerbank works in discharge mode, the LTC3871 buck the battery voltage to the type C interface.

2. What needs the external MCU to control when LTC3871 charge the battery? I don't find any information about battery charge control in the datasheet. For example, how does LTC3871 control the trickle charge current, CC/CV, end of charge, and so on....

3. The lowest voltage of type C interface is 5V. How about the efficiency when 5V boost to 58V?

Thanks! 

Best Regards

David

  • 0
    •  Analog Employees 
    on Sep 21, 2020 9:45 PM 5 months ago

    Hello David,

    Answers to your questions are below:

    1. The LTC3871 can operate as a boost charger, but an external MCU or other external components will be required to drive the SETCUR and BUCK pins. The SETCUR pin controls the low side current while the BUCK pin sets the direction of power flow. When the BUCK pin is tied to V5 (output of 5V LDO on LTC3871), it operates as a buck. When the BUCK pin is tied to GND, it operates as a boost.

      To give you more information, the LTC3871 is a bi-directional controller designed for 48V / 12V battery systems. When it operates as a buck, it delivers energy from VHIGH (48V bus) to VLOW (12V bus) and when operating as a boost, it delivers energy from VLOW to VHIGH.
      1. When operating as a buck, the converter can operate either as a CV or a CC converter. When the current flowing out of VLOW (ILOW_BUCK) is below the level programmed by the SETCUR pin, it operates as a CV converter and VLOW is regulated. When ILOW_BUCK rises above the value programmed by the SETCUR pin, the CC loop takes over and regulates ILOW_BUCK to the value set by the SETCUR pin.
      2. When operating as a boost, the converter can operate either as a CV or a CC converter. When the current flowing into VLOW (ILOW_BOOST) is below the level programmed by the SETCUR pin, it operates as a CV converter and VHIGH is regulated. When ILOW_BOOST rises above the value programmed by the SETCUR pin, the CC loop takes over and regulates ILOW_BOOST to the value set by the SETCUR pin. Note, when operating as a boost, only the current flowing into VLOW is regulated, not the current flowing out of VHIGH.


    2. The MCU or external circuits will need to control the trickle charge, end of charge and other battery charger functions through the SETCUR and BUCK pins. Other useful pins include the OVHIGH, UVHIGH, OVLOW, IMON & RUN pins. The data sheet provides details on each. The transition from CV to CC operation and CC to CV operation is controlled by the LTC3871 as described above.


    3. The maximum output current from a USB PD interface with a 5V output is 3A. Therefore when operating as a boost with an output of 58V, VHIGH can only supply about 0.2A - is this enough for your system? The efficiency depends on component selection, switching frequency and PCB layout so it is hard to pin-point. A ball park figure for 5V to 58V @ 0.2A without knowing the specifics is about 90% with an external source on the EXTVCC pin. Also keep in mind the following:
      1. When operating as a boost the maximum duty cycle is 92%. (This duty cycle is the on-time of the BG pin divided by the switching period.) Therefore, the output of the boost (VHIGH) can not go much further than 58V when operating from an input (VLOW) of 5V. 
      2. The maximum power available from a USB PD 3.0 interface is 15W @ 5V and 100W @ 20V. At these power levels a single phase solution could likely provide the highest efficiency due to lower gate drive  losses.
      3. The LTC3871 is designed for systems with a low side voltage of 12V. For better current sensing accuracy at VLOW = 5V, consider using the LTC7871. It is a 6-phase version of the LTC7871 designed for use with the LTC7060 which is our latest gate driver. For power levels of 100W and less, a single phase could be used. The LTC7871 is designed for low side voltages ranging from 1.2V to 60V. Another alternative is the LT8228 which is a single phase bi-directional controller with input and output protection which can also be paralleled.
      4. Input and output protection will likely be required to protect the battery and the USB port from faults. Also check agency requirements.

    Best regards,

    Mike