[CW32L052R8T6 Evaluation Board Evaluation] 2. Hardware Evaluation (Supplement)
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This post was last edited by Haoyueguangxifeiziming on 2023-8-1 21:19
I am very sorry that I accidentally touched Markdown when adjusting the layout last week because I was not familiar with the editor. I fiddled with it here and there but could not switch back. Then I accidentally touched it again and the document was published before it was edited. Some content could not be edited. Because the timer seemed to start from the date of adding to the draft box instead of the date of publication, the modification timeout was displayed when I tried to modify it. I was afraid of other problems and did not dare to delete it. So I made a supplement here. I apologize to everyone  and thank the poster @ freebsder for accusing me of spamming! Even if it was an accidentally published fragment, it is actually a bad article. I am also very sure that his accusation is also a spur and urge for the quality of the article! ! I also hope that everyone will continue to criticize and correct me in the future and give me advice! !
(Supplement): According to the article "【CW32L052R8T6 Evaluation Board Evaluation】 2. Hardware Evaluation", it is speculated that the adjustable version of LDO is used, and the output is not set. Secondly, a power supply hidden danger is found on the board. A large capacitor is found in the upper right corner of the development board. The multimeter is used to measure 39.8uF. Due to the existence of other loop capacitors in the circuit, the measured value is generally larger than the actual value, so it is positioned as a 33uF capacitor.
There is no current-limiting resistor in series along the capacitor, and the power supply line is quite wide, about 20~30mil. It is speculated that there will be a hidden danger of high-amplitude instantaneous charging current when power is turned on (mainly depending on the load capacity of the power supply part). One is that it is very likely to cause the power supply system to trigger protection. If the power supply part is not protected, it is very easy to damage the power supply part; secondly, it is easy to affect the IC power-on reset and other systems under the same power supply link, affecting the operating conditions. It is strongly recommended to repair it.
When we use a large capacitor, the startup current will be much larger than the static current, which can easily cause overshoots that trigger protection or damage the energy supply components. When using an oscilloscope to capture the power-on waveform of capacitor C31, the current overshoot phenomenon was indeed captured, but it was immediately pulled down to a charging platform of ≈1V (this should be because the ST LDO has its own overcurrent protection, which is generally limited to 80% output. In reverse, it can be concluded that the LDO output is indeed 1.25V on site). After the capacitor is charged, it exits the protection mode and continues to raise the voltage to 1.44V.
Based on this discovery, the capacitor before the LDO was measured and found that C97 was 22uF (…do engineers have any special hobbies for large capacitors…)
However, due to the strong power supply of the USB Host, this charging curve is smooth without spikes. However, I personally recommend not using large capacitors above 22uF (MLCC) after the LDO. One capacitor is very load-intensive when powered on, and when powered off, if the voltage at the back end of the LDO is discharged slower than that at the front end, the voltage difference >0.7~1.2V may easily cause reverse damage to the LDO. If it really doesn't work, connecting two 10uF capacitors in parallel is also a solution.
If such a large capacitor is really needed, then a low-resistance 182 inductor can be connected in series to ensure the stability of the overall system. . Excessive instantaneous load will reduce the service life of the energy supply components and increase operational risks. By a strange coincidence of time, place and people, other components on the board cannot be reset normally. They are offline and cannot be called during use. Because it is too accidental, they are restored after the next reset. This phenomenon cannot be reproduced for a long time and the fault point cannot be locked. The hardware is busy but the problem cannot be found. Finally, they are forced to run away.
Solution:
(1) The 33uF capacitor is replaced with two 10uF capacitors in parallel (disadvantages: increasing the PCB layout area, and the energy storage may be insufficient when the back end needs current);
(2) Replace the LDO with one with a larger output capacity (disadvantages: only treat the symptoms but not the root cause, the hidden danger still exists, but it cannot be manifested under the power of the wildness);
(3) Use inductive buffering to reduce the risk of transient overshoot (disadvantages: increase cost and increase PCB layout area).
(4) Make the wiring in front of the large capacitor thinner and use it as a current limiter (bushi! Fake! Don’t take it seriously!)
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