Hi Everyone,
I'm using a boost converter LMR62014, to generate the input voltage for the battery charging PMIC BQ24250. This configuration is chosen as the minimum input for the BQ24250 to function is 4.2V (given the VDPM value is updated in the respective register) and my source voltage is around 3.7V. For this reason I use the LMR62014 to step up the source voltage (VSYSTEM) to 4.5V as shown in my schematic below:
The problem is when I bring the BQ24250 out of HIZ mode, the voltage at the input crashes, causing the internal registers of the BQ24250 to be reset. When bringing my system up, I do the following steps:
- Enable HIZ mode on the BQ24250, no current drawn. At this stage the LMR62014 is disabled and the input to the BQ24250 is sitting just below VSYSTEM at around 3.6V (due to D4)
- Enable the watchdog to fire every 10s - this ensures the device remains in host mode
- Enable the LMR62014 - this brings the input to the BQ24250 up to 4.5V (stable)
- Adjust the BQ24250 VDPM setting to 4.2V - The impact of this is verified in register 5 (0x04) of the BQ24250 as the device is no longer in the VDPM regulation loop
- At this point all registers are checked and verified
- Bring the BQ24250 out of HIZ mode and set the current limit to 100mA - at this point all registers appear to reset.
A trace of the VSYSTEM and input to the BQ24250 shows that VSYSTEM is stable at 3.7V (CH1 - Yellow), and the input to the BQ24250 (CH2 - Blue) is crashing from around 4.5V to around 2.5V as soon as the device is taken out of HIZ mode and allowed to draw current. From this we can conclude that bringing the BQ24250 out of HIZ mode is crashing the LMR62014. I have tried increasing the input capacitance, to damping the transient load from bringing the BQ24250 out of HIZ mode - however this doesn't resolve the problem.
So following this, I have a two questions.
- What is the in-rush current when the BQ24250 is taken out of HIZ mode?
- Could the switching noise from the LMR62014 be causing problems for the BQ24250 (or vice versa)?
Any advice on this issue would be really appreciated! In particular if the team at TI E2E could take the evaluation modules for these two devices and try replicate this scenario, that would be awesome!! ;)
All the best!