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USB 2.0 Layout on 2 or 4 layer

Rajinder1268

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Hi all
I am currently in the process of laying out a schematic that has a USB 2.0 Micro USB connector, which will be connected to a portable device that has a USB connector. The portable device needs to only supply power and some initial communications to the board with the MIcro USB connector.

For the Micro USB board, is it best to lay this out as a 2 layer or 4 layer PCB (this is going in production)? A 2 layer would be cost effective, however with 1oZ copper and FR4 1.6mm size board, the track width and spacing of the D+/D- tracks might prove to be tricky. I am also concerned about passing EMC with a 2 layer board. Any advice would be appreciated.

Secondly, please could someone advise on the EMC tests that I would require as the unit is not self powered but receives its power from another source i.e. portable device.

Thanks in advance.
 

KlausST

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Hi,

in the end it´s the chracteristic impedance that you have to match.
But the characteristic impedance takes the full effect at full wavelength of the frequency of interest.
So with USB 2.0 we talk about 480Mbits/s.
The wavelength is around half a meter.

If your trace is shorter it has reduced effect, but still it has effect.
A rule of thumb says that you should care about characteristic impedance down to 1/10 of the wavelength. In your case about 5cm.
So my recommendation is to keep the traces short.

I surely don´t want to discourage you from careful PCB layout design ... but it won´t hurt if your characteristic impedance is not perfect for short traces.

Klaus
 

stenzer

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Hi,

usually there are also some protective components used between the connector and the MCU e.g. diodes [1] and series resistors. Consequently, to maintain the chracteristic impedance all along between connector and MCU is not completely possible. As Klaus suggested, keep your traces short.

[1] https://www.st.com/en/protection-devices/usblc6-2.html

BR
 

barry

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As far as what EMC tests are required, that totallybdepends on where you want to sell the product. In the US, there are different requirements (FCC) for commercial vs industrial applications, and you must be certified by an authorized facility. As i remember, the EU doesn't REQUIRE any tests(you can 'self-certify') but you DO have to meet the requirements, or you could be in big trouble.
 

Rajinder1268

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Hi,

usually there are also some protective components used between the connector and the MCU e.g. diodes [1] and series resistors. Consequently, to maintain the chracteristic impedance all along between connector and MCU is not completely possible. As Klaus suggested, keep your traces short.

[1] https://www.st.com/en/protection-devices/usblc6-2.html

BR
Hi,thanks for your reply. I am using a Semtech RClamp0503f for the ESD protection. This is then followed by RC combination on the D+ and D- lines I.e. 27R and 47pF for series termination and signal integrity before it goes into the FTDI chip.
--- Updated ---

As far as what EMC tests are required, that totallybdepends on where you want to sell the product. In the US, there are different requirements (FCC) for commercial vs industrial applications, and you must be certified by an authorized facility. As i remember, the EU doesn't REQUIRE any tests(you can 'self-certify') but you DO have to meet the requirements, or you could be in big trouble.
Selling it in the UK and USA. Even to self certify you still need to created a Technical File with all the tests that you have done. I am not sure which tests apart from ESD, emissions and transients.
--- Updated ---

Hi,

in the end it´s the chracteristic impedance that you have to match.
But the characteristic impedance takes the full effect at full wavelength of the frequency of interest.
So with USB 2.0 we talk about 480Mbits/s.
The wavelength is around half a meter.

If your trace is shorter it has reduced effect, but still it has effect.
A rule of thumb says that you should care about characteristic impedance down to 1/10 of the wavelength. In your case about 5cm.
So my recommendation is to keep the traces short.

I surely don´t want to discourage you from careful PCB layout design ... but it won´t hurt if your characteristic impedance is not perfect for short traces.

Klaus
I aim to keep the traces short. However on a 2 layer PCB to meet the 90 ohm impedance the track width required for D+/D- was about 1mm. Hence I am looking at a 4 layer PCB. I am not sure which way to go.
 

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