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How To Find The Real Problem and Solve It With MCP

01/10/2018 5:16 AM

I have designed a PCB which has LM1117 (for 3.3V), Micro USB connector, MCP2200 usb-to-uart bridge, 12 MHz SMD crystal and few passive devices.
We have been observing the PCB for some weeks and observed that the PCB doesn't work everytime. When I connect to any system, sometimes system throws an error "USB Device not recognized" (in windows) or "Device read/write descriptor failed" (in Linux) and then it doesn't work for hours and start working after sometime. I have reviewed my circuit and didn't have any solution to solve this issue. After resoldering the board and replacing old components several times, we have ruled out soldering/component issue. I think somehow the crystal is not able to start or there could be some race condition between voltage at Vcc and RST pin but I am not sure how to find the real problem and solve it with MCP.
The schematic of MCP2200 is shown below:

https://i.stack.imgur.com/EjanL.png

The layout of my PCB: (MCP2200's Pad is in green and left side of the picture with USB connector at top left corner)

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#1

Re: How to find the real problem and solve it with MCP

01/10/2018 5:34 AM

PCB layout round crystal and power supply decoupling in that area can be critical.
That's where I'd start if I wasn't retired
Intermittent problems are always a pig. Good luck
Del

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#2

Re: How to find the real problem and solve it with MCP

01/10/2018 7:47 AM

I believe the linear regulator shuts down due to over temperature.

From the datasheet: "The LM1117 regulators have internal thermal shutdown to protect the device from over-heating. Under all possible operating conditions, the junction temperature of the LM1117 must be within the range of 0°C to 125°C."

If you are in the south of India you might either need air conditioning or military specification electronics, which offer a higher temperature range, apart from the heat sink.

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#3

Re: How To Find The Real Problem and Solve It With MCP

01/10/2018 1:40 PM

If your regulator is overheating, you can drop the input voltage with a zener diodes or resistors in series from your power supply to the regulator.

Using a zener will allow you to run your regulator at a lower voltage thereby throwing off less wasted voltage as heat.

I get +5 VDC from a +24 VDC circuit by using a 12 V zener and a 7805 regulator without overheating.

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#4
In reply to #3

Re: How To Find The Real Problem and Solve It With MCP

01/10/2018 6:04 PM

Hi NotUrOrdinaryJoe,

Is this what you mean by your last sentence?

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#7
In reply to #4

Re: How To Find The Real Problem and Solve It With MCP

01/11/2018 6:50 AM

I think he means 12v zener where your R1 so it is in series with the input to the regulator (doesn't need a resistor).
I've used that trick before to get rid of excess input voltage and save some dissipation.
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#5

Re: How To Find The Real Problem and Solve It With MCP

01/10/2018 10:45 PM

No. I would put a 6v2 and then another 6v2 zener in series and have each zener dissipate a little bit of the wasted energy so the regulator does not have do all the releasing of heat.
Keep everything away from each other to prevent heat build-up in a confined area of the board.
If you are trying to drop 24v to 3v3, you can expect a lot of heat.

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#11
In reply to #5

Re: How To Find The Real Problem and Solve It With MCP

01/12/2018 12:25 PM

Del is right. Two Zener diodes costs twice as much. For the little bit of power that is needed, I don't think there is too much concern over heat distribution.

However, I do use a 1 Watt to 5 Watt Zener depending on how much power I need at 5 Volts DC. One does need to understand how much current is required at the lower voltage.

This technique is called "using a DC shunt" and most of the time it is OK. It is a little bit like placing a reversed battery in series to reduce the voltage where it needs to be lower. Of course, we don't use batteries like that, but it is good to know that the DC shunt works like a reversed battery (mostly).

The only place where I'm pretty sure that one should not use this technique is for amplifier circuits. The noise from electrons getting through the Zener diode PN junction can be an unwanted influence.

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#6

Re: How To Find The Real Problem and Solve It With MCP

01/11/2018 12:01 AM

Consider checking with an oscilloscope to see where the expected signal is missing.

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#8

Re: How To Find The Real Problem and Solve It With MCP

01/11/2018 9:40 AM

You are supplying 3.3V to your Vdd and Vusb pins. Can it be that whatever is connected to your GP pins is actually expecting to receive or deliver 5V?

The data sheet recommends: "The VUSB pin is required to have an external bypass capacitor. It is recommended that the capacitor be a ceramic capacitor between 0.22 μF. and 0.47 μF," so your bypass capacitor is perhaps too small.

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#9

Re: How To Find The Real Problem and Solve It With MCP

01/11/2018 9:47 AM

Cursory glance at the responses so far, your use of essentially the reference voltage as supply voltage is not good practice, if that is indeed the case. It results in not good transitions on digital sometimes, and too much wasted heat.

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#12
In reply to #9

Re: How To Find The Real Problem and Solve It With MCP

01/12/2018 12:35 PM

The best solution might be a DC to DC converter. Some heat will be wasted either way. Total heat loss is something less than 1 Watt or else the Zener burns up.

So the shunt does waste a little heat by keeping it out of the regulator, but it also limits the current for the regulator plus the load. Total available current for this trick is about 80 mA. If more is needed, a Zener of higher wattage is required.

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#10

Re: How To Find The Real Problem and Solve It With MCP

01/11/2018 9:50 AM

Complete lack of response from OP...
Del

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