PMIC Power Down Behavior
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Good afternoon!
I am verifying a PCBA design with an OSD3358-512M-ICB C-SIP. It is my understanding that the power down sequence should be the reverse of the power up sequence. There are a handful of places where the power down sequence of our design does not match what I’d expect. I would be grateful for clarification on whether we should be concerned about these variations from the expected behavior. Thanks!
1. PMIC_PGOOD, PMIC_NWAKEUP, and PMIC_LDO_PGOOD drop-out behavior. During power-down, these three signals are briefly pulled low before decaying as expected. While not clear in these first scope capture, the drop-out behavior occurs simultaneously for all three signals. It almost looks like these external pins are briefly disconnected. In our design, these signals are connected exactly how they are in the OSM33558-C-SIP-RED reference design.
2. VDDSHV_3P3V and SYS_VDD1_3P3V begin to fall ~3.6 ms before VDD_CORE and VDD_MPU begin to fall. Based on the reverse power-up sequence, I would expect VDD_CORE and VDD_MPU to begin to fall about 1 ms before VDDSHV_3P3V and SYS_VDD1_3P3V. Our design for all of these signals match the design with the exception that VDDSHV2 is connected to SYS_VDD_1P8V instead of SYS_VDD3_3P3V like the rest of the VDDSHV pins.
Here are those scope captures. I tried the file attachment rather than copy paste. Hopefully that works better.
Hi Neeraj,
Upon further investigation of the controlled power down behavior, we are seeing a bump in the power down waveform for SYS_VDD3_3P3V. I’ve attached a scope capture. We’ve powered VDDSHV 1 & 3-6 with SYS_VDD3_3P3V, so SYS_VDD3_3P3V is labelled VDDSHV_3V3 in the scope capture below. In addition to the VDDSHV connections, SYS_VDD3_3P3V is also connected to a clamping circuit identical to the reference design for the C-SIP and an analog pin (to measure its voltage).
As I understand it, the VDDSHV voltages can be used as the pull-up for various GPIO pins. A similar bump is visible on a couple of inputs where the external driver (pull-up) was removed. I’ve attached a scope capture of one (OCT_EMMC_PG) below. I was able to suppress the bump with a 510 ohm resistor but not with a weaker one. These two factors indicate to me that this bump is driven.
Have you seen this behavior before? Is it concerning? Will it damage the device?
Thanks,
Simon
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