BQ77207: I am building a UUV, and I need help understanding how to integrate this part with my design

Hi Camille,

In response to each of your questions:

• What do Rin, Cin, Rvd, and Cvd mean?
  • Rin is the input filter resistance for each cell in your battery pack.  This resistor is used in conjunction with a capacitor (Cin) to create a low pass filter.  This will filter out noise when the BQ77207 is measuring the voltage across each cell.  The same principle applies to Rvd and Cvd.  They form a low pass filter to filter out noise at the Vdd pin. 
  • Here is a link you may find helpful if you'd like to learn about LPFs: What Is a Low Pass Filter? A Tutorial on the Basics of Passive RC Filters - Technical Articles (allaboutcircuits.com).
• What is the Active High 6V option?
  • This is referring to Cout and Dout.  Active high means that when the Cout or Dout pins are ON, they will output a HIGH voltage.  In this case, that is 6 V.  This 6 V will be applied to the gate of the N-MOSFET corresponding to the pin, turning on the MOSFET and either blowing a fuse or opening protection MOSFETs.
• And can I use a fuse instead of the back-to-back fets?
  • You can use a FUSE or FETs.  It depends on your system needs. 
  • FETs should be used when you want to recover a fault.  Here's an example: voltage gets too high, and an OV (overvoltage) fault triggers.  The COUT pin will output a HIGH signal (6 V) and close the NFET, thus opening the charge protection PFET.  This will prevent the battery pack from charging and prevent voltage from increasing.  Once the OV fault is cleared, the COUT pin will stop sending a HIGH signal, and the charge protection PFET will close again, allowing charging current to flow.
  • A FUSE should be used if you don't want to recover the fault.  Once the fuse blows due to a fault, you will have to replace the fuse before the circuit will function again.  Usually, a FUSE is used for secondary protection.
• Will I still need the Routs if I use a fuse?
  • I am confused by this question.  If you are asking about Rcout and Rdout, these resistors are necessary because they are pulldown resistors that help keep the MOSFET in the desired state.  They should be used for FET and FUSE configurations.
  • Here is a website that explains pull-up and pull-down resistors: What is Pull Up and Pull Down Resistor and Where to Use them? (circuitdigest.com).
• If I do use the Fets, how do I know the specifications for them?
  • If you want to use the MOSFETs, you will find specs and performance information about them on the MOSFET's datasheet.  The BQ77207 EVMUG has a schematic showing an implementation of the BQ77207 with a BOM showing components.  I will link the EVMUG here: BQ77207EVM User's Guide (ti.com).
• Is this the correct battery management IC for my project?
  • I cannot give you a definitive answer since I do not know all the details of your project, but the BQ77207 is designed to protect up to 7S cells (38.5 V), and you will be using a 6S configuration (22.2 V), so you are within the voltage specs of the part.  

Best,

Andria

Hi Andria,

Thank you for this very thorough answer. I have some follow-ups. The EVM User guide is a helpful resource, but the Cout and Dout Pins are not configured to a fuse or MOSFET protection, so it did not answer my question about the specifications for the MOSFETS.

What is the values for the  QDSG and the QCHG? I see they are P-MOSFETS, but what parts should I use for them? What is a good value for Roff ,RDout,RCout? The NMOSes on Dout and Cout, what should their values be?