I have a board that uses the 5V input to the i.MX28 PMU for the power supply and just requires a small NiMH backup battery to keep the RTC alive. The NiMH is trickle charged from the VDD4P2 rail when the device is powered on to keep it topped up.
I am a little confused about the connections for the PMU battery and DCDC batt pins. When I started my design the AN4199 document from Freescale did not cover this so I laid the board out based on the descriptions of the connections. Since obtaining my prototypes Freescale have updated AN4199 to add more detail on systems with just a backup battery and now I am not so sure of what I have done!
If you look at the diagram of the PMU in AN4199 it shows that the BATTERY pin is the output from the internal charger, and the DCDC_BATT pin is the input to the DC-DC from the battery.
In the datasheet for the i.MX28 it states that the BATTERY pin provides current to charge the battery, and the DCDC_BATT pin provides power into the iMX28 PMU from a battery. This agrees with the diagram.
Now, assuming that you are not using the internal battery charger naturally you would think to leave the battery pin disconnected. However, in the data sheet it states: "Application should include a Schottky diode between BATT and VDD4P2.". Which I have done.
I am feeding my battery backup into the DCDC_BATT pin and as from the above I believed this was the input to the board for the battery. Even if just used for RTC backup.
I have since found out that the RTC backup battery should be fed into the pin labelled BATTERY (which is described as the charger output in the documentation?!) and not DCDC_BATT.
My schematic is below, thankfully I added R24 which is a big 1206 pad in case I needed to make a mod!
In this schematic I have the diode the datasheet asks for from BATTERY to VDD4P2. I am trickle charging the NiMH via D5 and R65 from VDD4P2.
D8 is to prevent the possibility of charging the battery back from DCDC_BATT (I guess nothing should be output from this pin anyway). R67 is there to essentially protect the battery should the board try and boot from it. The Vdrop accross R67 is negligible under backup mode but would kill the supply if the board tried to boot.
The problem I have is that I have now found out that BATTERY is not just the charger output as described in the documentation. It is also the input for the RTC backup. Apparently it is also the pin on which the internal battery monitor measures the battery voltage too. Can anyone confirm this for me?
I can mod my circuit above as I only have a small prototype run and have the necessary pads brought out from the device.
I have seen the power scheme suggested in the revised AN4199. This connects the battery to the BATTERY pin. It then has resistors from DCDC_BATT and BATTERY to VDD4P2.
Can anyone from Freescale who actually knows the internal architecture offer any advice on my circuit above?