DCDC converter troubles

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DCDC converter troubles

905 Views
jaymonkman
Contributor I

I'm having difficulty with the DC/DC converter on the RT1050.

I have 2 test cases. For both of them, the processor never turns on the DCDC converter or turns it off shortly after power on, then the device starts drawing excessive current - over 1A.

I've attached the schematics and scope traces.

The scope traces are with a delay on driving PSWITCH (R117 is 30K) and with no delay (R117 is 0 ohms)

When R117 is 0 ohms (dcdc-nodelay.png), the DCDC converter comes on for 5 seconds, then turns off. After it turns off, the power consumption goes up dramatically - over 1A - and the rt1050 gets very hot. The 5 seconds is very consistent. It rarely varies by more than 0.05s.

When R117 is 30k (dcdc-delay.png), the DCDC converter never comes on and immediately draws excessive current.

When I measure DCDC_SENSE while the converter is on, it is 1.17v.

 

Any idea what is causing this behavior?

 

The scope traces are:

ChannelSignalProbe point - name in schematic
1 - yellowDCDC_INTP84 - MCU_DCDC_IN_3V3
2 - light blueVDD_SOC_INC36 - DCDC_OUT
3 - pinkDCDC_LPL5
4 - dark bluePMIC_ON_REQTP63 - PMIC_ON_REQ
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carlosflr
Contributor II

Hello,

I was having problems also with the internal DCDC of an RT1052 and wanted to share a solution that worked for me.

I designed a custom board following the EVK schematic. Occasionally it failed to power up. Upon examination I found out that the problem was that the internal DCDC was not providing the 1.1V core voltage. When reading the documentation I found out that DCDC_PSWITCH must be delayed (at least) 1ms *after* DCDC_IN is stable. However, this delay is implemented in the EVK (and my board) with an RC supplied from DCDC_IN. I noticed that some times the delay was not enough. For example, DCDC_IN doesn't go all the way down to 0 on a power reset event (it stays at 1.3V in my board) which reduces the effective delay.

The solution was to wire the power good signal (PG) from the *external* DCDC switcher to the capacitor in the DCDC_PSWITCH RC circuit. This forces the voltage in the capacitor near 0V until DCDC_IN is within 10% of the regulated voltage (3.3V), then PG goes high with a weak pull-up (it's open drain) which allows the RC delay to start. To clarify, the resistor in the RC circuit is still wired to DCDC_IN. This resistor is 30K vs 500K or so of the weak pull-up so it's the major contributor to the delay.

I hope this helps.

Regards,

Carlos Lopez

 

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jaymonkman
Contributor I

dcdc-pmic_on.png has the waveforms with PMIC_ON tied to DCDC_IN

SNVS_IN is the same as DCDC_IN - they both come from the same source.

dcdc-delay-pswitch.png is the same as dcdc-delay.png, except channel 3 (pink) is PSWITCH and the time scale is different so you can see the rise time.

I didn't include a waveform of pswitch for the no delay case, as it is the same as DCDC_IN in that case.

 

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jeremyzhou
NXP TechSupport
NXP TechSupport

Hi,
Thank you for your interest in NXP Semiconductor products and for the opportunity to serve you.
After having a brief review of the dcdc-nodelay.png and dcdc-delay.png, I think the issue is caused by PMIC_ON_REG which should output a reliable high-level voltage after powering up SNVS, however, it doesn't.
So you'd better check the PCB about the partial circuit.
Have a great day,
TIC

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jeremyzhou
NXP TechSupport
NXP TechSupport

Hi,

Thanks for your reply.
After having a brief review of wave figures and schematic, from the schematic, an external PMIC is used to support power, so I think you need to supply the DCDC_IN by DCDC_3V3 instead of the MCU_DCDC_IN_3V3, in brief, I'd like to suggest you'd better change the power supply design by referring to the MIMXRT1050 EVK board.
Have a great day,
TIC

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jaymonkman
Contributor I

Hi @jeremyzhou 

I removed R221 and supplied MCU_DCDC_IN_3V3 from DCDC_3V3 (TP51). dcdc-change-dcdc_in.png shows the waveforms.

PMIC_ON (dark blue) goes high, but I never get any output on DCDC_OUT (light blue). Also, when this happens, the chip gets very hot.

Thanks,
Jay

 

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jeremyzhou
NXP TechSupport
NXP TechSupport

Hi,
Thanks for your reply.
According to your reply, after circuit change, the DCDC_IN should be supplied by the external DCDC chip: U52, which is controlled by the PMIC_ON, however, in the dcdc-change-dcdc_in.png, the DCDC_IN (Yellow ) turns high in advance of PMIC_ON, it doesn't make any sense.
And the PMIC_ON should be expected to turn high prior to DCDC_IN.
Have a great day,
TIC

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jaymonkman
Contributor I

I've attached a new diagram - dcdc-change-dcdc_in2.png

Channel

Pin nameProbe point
1 - yellowDCDC_INTP51
2 - light bluePMIC_ON_REQTP63
3 - pinkDCDC_OUTR496
4 - dark bluePSWITCHR117

 

You keep suggesting that I redesign my board to match the EVK. I'd do that if I were in a position to redesign the board. This design is based off a different, known working board and was done according to a previous version of the RM that did not include as much documentation about the DCDC converter as the current one. For now, I'm stuck with trying to make this one work.

It appears that you are concentrating on the test case I have where the DCDC converter never starts to work. I do have a test case where it starts and everything works fine for 5 seconds until the DCDC converter shuts down. That was described in my first post. Maybe it would be worth investigating why the DCDC converter shuts down?

Thanks.

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jeremyzhou
NXP TechSupport
NXP TechSupport

Hi,
Sorry for the reply late.
Actually, I'd like to suggest you make sure your power supply circuit should be consistent with the below diagram, the EVK board is a good sample for the diagram and it's a simple way to adjust your board by referring to the EVK board.

jeremyzhou_0-1604979165977.png

 


Back to your question, it's too hard to investigate why the DCDC converter shuts down via just reviewing the schematic and DCDC testing result actually, to figure the issue out, I'd like to suggest you should test on a new board which only contains minimal system, it can eliminate the interferes from other partial circuits, after that, add the partial circuits one by one until replicate the issue, through this test process, I think you can figure the root cause.
Hope it helps.
Have a great day,
TIC

 

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jaymonkman
Contributor I

@jeremyzhouAny more advice on this issue?

 

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jaymonkman
Contributor I

Hi,

The yellow trace is actually VLDO_3V3. Before I removed R221 and changed the source of DCDC_IN, they were the same. Now that the resistor is gone, and the probe is on TP84, it's actually VLDO_3V3

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jeremyzhou
NXP TechSupport
NXP TechSupport

Hi,
Thanks for your reply.
So please share the trace of DCDC_IN, and I'd highly recommend you to adapt your customize board by referring to the MIMXRT1050 EVK board.

Have a great day,
TIC

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jaymonkman
Contributor I

@jeremyzhouI don't understand what you are saying. On both waveforms, you can see PMIC_ON_REQ go high, then low. If I drive PMIC_ON_REQ externally, it acts the same as the case with no delay on PSWITCH.

It looks to me, that something is causing the DCDC converter to shutdown and at the same time, the rt1050 stops driving PMIC_ON_REQ.

Thanks.

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jeremyzhou
NXP TechSupport
NXP TechSupport

Hi,
Thanks for your reply.
Regarding "If I drive PMIC_ON_REQ externally, it acts the same as the case with no delay on PSWITCH", can you introduce the details of testing and show me the scope traces?
And I'll appreciate it if you can provide more scope trace of other related voltage supply pins, such as DCDC_PSWITCH, SNVS_IN, etc.
Have a great day,
TIC

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