LTC7150 Stepdown regulator

[Ellu24]

I am trying to bring up a custom board using LS1046. I have designed the power supply similar to freeway board. I used PF8100EA as PMIC and LTC7150 as step down regulator and giving input power supply using programmable dc supply and connected using molex cable. I give 3.3V to PMIC and 12V to LTC7150 as input.

Initially, the CH1 and CH2 of the bench power supply is set to 3.3V, 1.8A and 12V, 0.8A respectively. When I increase the Amps at CH1 to 2.2A, the voltage at CH2 jumps to 24V when the device is connected. So, we set the CH1 and CH2 at 3.3V/1.8A and 12V/0.8A. We checked the output voltage of the stepdown regulator (which is an input voltage to the CPU VDD) and measured 0.63V(expected 0.9V) at inductor L6 and capacitor C443, as shown in the attachment. 

The input voltage at the capacitor C435 was 12V. When we increase the amps in the CH1 (3.3V) from 1.8A to 2.2A, the voltage at CH2 jumps from 12V to 24V. The voltage across the capacitor C435 is 24V and the voltage across the L6 inductor and capacitor C443 is 0.9 V.

When the 12V is disconnected (only 3.3V connected), we still measure 0.9V at the L6 inductor and C443 (CPU VDD line). We are not sure why that is, we expect no voltage at all.

I would like to know:

1) Why voltage jumps to 24 V when current increases.

2) Why 0.9V measured even 12V is disconnected?

Please help me to resolve this problem.

Thanks in advance

[guoweisun]

If you identify the issue from LTC7150 you can disconnect the CPU_VDD(0.9/1V)  to MUP side and only debug LTC7150 to check whether this issue has or not.

For my questions2 I need to know whether there is other part to generate power to CPU_VDD(0.9/1V) except LTC7150?

[Ellu24]

Thank you so much for your quick reply. As per your opinion, Is the low current rating inductor is responsible for the issues? I am not sure about it. If so, I think I have to rebuild the board with new inductor.

For question 2, We don't have another part to generate power to CPU_VDD (0.9/1V) except LTC7150.

Thanking you,

[Ellu24]

Thank you very much for the prompt reply. Please find my answers to your question:

1. I have tested 2 boards, and both have the same problem. I tested in our custom board.

2. Sorry, I didn't understand your question. Can you please clear the question?

3. I changed the power supply and tried, still no change in the issue.

I checked the voltage and current ratings of the capacitors and inductors respectively. I found all the components are matching with the freeway board except the inductor current rating. In freeway board and in LTC7150 datasheet, they are recommending an inductor with more than 20A current rating. But in my design, I used an inductor with 3.69 A. Was this low current inductor being responsible for these problems or is there any other reason for these? 

One more thing I would like to mention is that when I connect 3.3V only on bench supply (to PF8100), I can observe 1.8V across JTAG. At the same time, when I set 3.3V, 1.8A and 12V, 0.8A I can observe 1.5 V only across JTAG. Does this because of any voltage drop due to 12 V input.

Please help me to resolve these problems.

Thanks in advance. 

[guoweisun]

1：Did you test in EVB or your own board? how many boards do you test? same issue?

2: For the CPU_VDD(0.9/1V),do you have other channel in parallel with CPU_VDD?

3: can you change power supply to test? or use two power supplies to provide 3.3V and 12V.