i.MX RT1061 DCDC converter not working

[Daniël_Kunst]

We designed a board around a MIMXRT1061CVL5B. Today we finally received the boards. After first checks, I found that the MCU's internal DCDC converter is not turning on. I tried several patches, but whatever I try, it's not turning on. I also measured the switching node, it's not even trying to turn on.

In the reference manual I found: 

18.4.1 Turn on DCDC
The below conditions must be met to enable the DCDC:
• Power snvs_1p0 and clk32k in snvs_1p0 are available
• DCDC_IN powered up
• After DCDC_IN powered up, delay about 1ms reset time to enable DCDC
• PSWITCH and PMIC_ON_REQ are both high to enable DCDC

The sequence is:
1. Power snvs1p0 and clk32k
2. PMIC_ON_REQ = 1
3. DCDC_IN powered up
4. Delay 1ms reset time
5. PSWITCH = 1
6. DCDC is now enabled

OR:
1. Power snvs_1p0 and clk32k
2. DCDC_IN powered up
3. Delay 1ms reset time
4. PSWITCH = 1
5. PMIC_ON_REQ = 1
6. DCDC is now enabled

Only if when PSWITCH and PMIC_ON_REQ are both high, DCDC can be enabled,
PMIC_ON_REQ can be before or after DCDC_IN， just to assure delay about 1ms reset
time to enable DCDC after DCDC_IN power up,

Everything is powered by the same 3V3D rail. So SNVS_IN will come up at the same time as DCDC_IN. The LDO's are working (I measure ~2.5V at VDD_HIGH and ~1.1V at NVCC_PLL). 

PMIC_ON_REQ measured 1, but it's not connected to anything on our board except a test pad. Clk32k should be available, but where is that coming from? I measured the ramp-up of the 3V3D, it takes 2.5ms to go from 0 to 3.22V. We used the RC combination from the reference design (30k / 220n), but in combination with the ramp-up time from the 3V3D I think it was to fast. I also tested with 30k 440n, but this also did not work. What are we doing wrong? I attached some screenshots from the MCU power sheets of the schematics. Thanks in advance!

[Daniël_Kunst · Accepted solution]

So I solved it today. Apparently the 3V3D line didn't come up at once. It went to approximately 1.4V, stayed there for ~70ms and then continued to 3.22V. The DCDC_PSWITCH RC network just wasn't sufficient here. I changed it to 100k / 1u and now it turns on as intended.

Yellow  = DCDC_IN (3V3D)
Green  = PMIC_ON_REQ
Pink     = SNVS
Blue     = DCDC_PSWITCH

With 30k / 440n RC

With 100k / 1u RC

I do have to look in the weird 3V3 start-up. This board is a redesign from a previous one and I don't know if that one had the same behaviour...

I also think the PMIC_ON_REQ signal comes up pretty early. SNVS is not even stable then.

[Miguel04]

Hi @Daniël_Kunst 

I'm glad you found the workaround for your issue, for any future designs you make remember to use the specifications.

Have a nice day, Miguel.

[Miguel04]

Hi @Daniël_Kunst 

Please look into this document for you to review the hardware design of your custom board.

i.MX RT Hardware Development Guide for the MIMXRT1050/MIMXRT1060 Processor 

Here you will find all the recomendations and specifications needed.

Best Regards, Miguel.