BQ76PL455A: Choosing Cables and understanding cabling spec

Hi Brian,

Sorry  for the delayed response, I have been on Holiday and  just got back into the  office.

These types of questions are the toughest to answer for  the pl455, in my opinion/experience. What is needed is often determined by the amount of noise the system will see. It really comes down to the system noise profile (ie from an inverter), where cables are located (keep daisy chain away from high voltage/ noisy GNDs), etc.

To give a frank and truthful answer (although probably not the most helpful) is that the system designer should look at it with  a scope, and  will  likely need to tweak things  to get the best  performance. We give a relatively good baseline recommended app circuit, but there is no way we can account for the various noise  profiles, etc.An extra layer of complication comes in the fact  that these  can  be treated as transmission lines.

I will  attempt  to answer your questions inline:

I wanted to understand the daisy-chain specification a little bit more. I just want confirmation that the length imposed in equation 13 in datasheet is between two IC’s, not the total length of bus length of all the IC’s tied together.  I suspect it’s the former, based on the first sentence of section 8.1.2.2.3, but just want confirmation in my understanding.

DW: This is between any two ICs

2. Are shielded cables recommended?  My understanding is that shielded cables aren’t recommended because of the added capacitance.  Is this true?

DW: You certainly could use  shielded cables. Again, it largely depends if the system is noisy or quiet. We have seen  customers  get away with  unshielded cables. You  are correct about the added capacitance.This  will just reduce the maximum usable cable length, since there will be more attenuation in the signal. To combat this  a 2nF isolation cap could be used.

3. If we were to use a shielded cable, which capacitive value would we choose to use for the formula?  The cable to shield capacitance, conductor to conductor?  Overall capacitance?

DW: This link gives a useable formula similar to the  one in our datasheet: http://www.standard-wire.com/cable_design_equations.html

4. If we were to use a shielded cable, which potential do we tie the shield to?

DW: Theoretically, i don't think it will have  a huge  effect (I could  be wrong). I think the best route would be to tie  the shield to the "low" module GND. Just make  sure  the  shield isn't tied to GND at both the high and low module. If this happens you  will short  the low module out.

5. If we’re going to stack the BQ’s up to a 1000V using the up/down connection, how would we define the cable insulation?

DW: We  recommend 2X the pack  voltage, so 2kV

6. In figures 33 and 34 in datasheet, how were the capacitances of the dual common mode choke setup determined?  In the EVM user-guide it says that its 40pf.  The datasheets don’t really mention a capacitance.  We’re using the recommended parts, but would like to understand this interface more.

DW: My guess is that this info was  taken using an LCR meter.

7. Does the cable have to be 120 ohm impedance?  Or can it be others.  I understand the recommendation is to use CAN Cabling.  Which would mean we could use RS485 cabling as well?

DW: It  does not necessarily have to be 120 ohms... keep in mind that the drivers are 50 ohm, and the receiver is  high impedance.