In my design, the device is battery powered via the DCDC_BATT pin of iMX280. Instead of using the iMX280 charger, we used an external charger chip which has better power-path control. For this reason, the BATTERY pin of iMX280 is NOT connected to the battery and is left NOT connected. This approach has been checked with Freescale support and was told that it is not a problem.
In my design, when the battery is first connected to DCDC_BATT, the iMX280 should be in the OFF state (until the PSWITCH input is activated). However, what I observed is, iMX280 is in an unknown and unstable state with ~1.5V on the DCDC_VDDIO rail and ~0.8V on the DCDC_VDDA rail. There is also a leakage current from the battery (~12mA). The 24MHz crystal is also found oscillating occasionally in the OFF state and when it happens, it will draw extra ~3mA leakage current. When the PSWITCH input is activated, the iMX280 will be turned on and all the power rails will be normal. In other words, this unstable state only occurs in the iMX280 off state.
What I eventually found out is, this behavior is caused by the BATTERY pin being left un-connected. After I wired the BATTERY pin to DCDC_BATT, iMX280 will behave normally in the OFF state, i.e. 0V on all the power rails and no leakage current.
Has anyone encountered the same issue before???
For us, it's not clear from the documents (application note, reference manual or hardware guide).
Could you tell me where it is written, and in which document, how we have to connect DCDC_BAT and BATTERY pins when we don't use a battery?
There is also a patch (L2.6.35_10.12_SDK_5V_SUPPLY_PATCH.tar.gz) to use 5V only.
So could you tell me if we need to use the patch and/or the hardware modification when we don't use the battery?
Hi Dong Hui,
If the BATTERY pin is not used, it should be pulled up to VDD4P2 with a 1K resistor. Besides this, I would turn off the charger by means of the PWD_BATTCHRG bit (please see reference manual) just for safety.
Thanks for the reply! What I'd like to know is, if it is left un-connected, what would happen? Is Freescale aware of the behavior I observed?
Also, VDD4P2 is internally connected to DCDC_BATTERY, right?
Not exactly. The DCDC converter can be supplied from either the DCDC_BATTERY pin or the VDD4P2 internal regulator, but they are not connected together. For more reference please see figure 1 of the attached AN4199.
To be honest, I haven't heard about the leakage issue, so not sure if this is something that has been tested before. What I can say, is that this recommendation (pulling-up DCDC_BATT if unused with 1K resistor) is to avoid high current consumption in this pin, so most likely it could be related to your issue.