Part Number:BQ40Z60
I'm experiencing difficulty with the documentation. Admittedly there is a lot to cover. The data sheet focuses more on the hardware details and the TRM focuses more on registers and configuration, but there are gaps between the two that aren't covered. Both documents sometimes read more like marketing pieces describing the extensive safety features (most of which I don't need).
The best part is section 9.2.2 of the data sheet, which gives design decisions and formulae. However it is incomplete. Here are some noticeable omissions:
1. ACP is connected to the external power through a resistor. No mention is given as to appropriate values or why one would pick a value or whether this could be used for threshold detection.
2. ACFET has a 10 M resistor S to G in the worked out example, but not in the simplified one. Presumably that prevents accidental turn on of the FET.
3. VFB (also called just FB some places). I get it that at maximum charge voltage it should have about 1.22 volts on it, but is there a hard coded comparator involved, or is it just a matter of getting it in range and then setting registers appropriately? I'm sure I will know the answer by the time I'm done, but I sort of need a board before I dive into the registers too far.
4. No mention is made of the gate resistor values for the non-SMPS FETs. The worked out example uses 5.1 k, and I suspect it isn't critical, but are resistors even needed at all? These are FETs.
5. Nothing is said about the BAT pin, other than it powers the IC unless the batteries are so low it can't (which wouldn't have been my preference. I'd have rather powered the IC from the adapter any time it was present). The example shows a 10 ohm resistor and a shottkey diode, with a note to replace the diode with another 10 ohm resistor in single cell applications (presumably because of voltage drop), but no mention as to why either is needed or how to determine the value.
6. There is a lot of information in various documents about charge balancing, ranging from using just the internal (200 ohm) FET resistance to 100 ohm series resistors or 1000 ohm or even 10K or forcing quick balancing with FETs (which adds complexity). But there really isn't much that says what is necessary in typical usage. So I have 5 AH cells that will charge in about 3 hours. Given the cell to cell variability I would typically encounter (or some tolerance spec) what formula would suggest an appropriate value.
7. Related to item 6, there are comments about filtering the feedback that comes through the VC pins. Obviously if there are no resistors (a tempting option for a minimalist solution) no filtering is possible. One source (for a different IC) suggests a time constant for such filtering, but the BQ40Z60 does not nor give any guidance. I'm assuming the 100 ohm resistors and 0.1 uf capacitors in the example are appropriate.
8. For both SHR and SR, there is mention of 0.01 ohms. There are also some ranges and limits mentioned. It appears that these differential sensing circuits are used in some cases scaled by register values, but may also be used in other cases with fixed limits, so the size of the resistor should be application specific. However, I can't find any specific information about such, or even appropriate scaling constants for the registers. I'm sure the latter exists, but it is buried beneath information on how to protect the cells from events that won't happen in this design. It would be helpful if a paragraph were devoted to the things these two sensors are used for and how to compute an appropriate resistor value and set related registers--all in one place.
9. The TS lines can be used to drive LEDs (with or without thermistors). I probably won't use thermistors in this design, as my charge and discharge currents are below 1C. Can I use the pins to directly drive LEDs? Or must I buffer them with FETs as shown in the example? That adds significant complexity to the design (you get the idea that I'm looking for a minimalist solution? That's why I chose a single do-everything IC). I couldn't find any information about the drive strength or even polarity of the pins as outputs for LEDs. I did manage to find some information in the TRM about configuring registers to support LEDs.
The material in 9.2.2 is excellent as far as it goes, but it really needs to extend to about every pin and component on the board. The example schematic is not the best help, either, as it is for the eval board, which incorporates almost every possible feature one could imagine, many of which are never talked about. There are even FETs in it for purposes I don't even know. Alternatively, maybe there just needs to be a third document, like the TRM, but focused on hardware design decisions. A tutorial that says here is how to use this IC in a minimal configuration (like the simplified schematic, but with enough parts to work), and here is how to add features, one at a time.
Thanks,
Wilton
