Layout Tips for Radiated EMI Reduction in Your Designs

hello my name is Dennis Lafayette cough I'm systems and applications engineer from the simple suture group at Texas Instruments and today I want to talk to you about the new LM for 3603 buck converter EMI and board layout let's go to the screen here and talk about buck converter Heidi ret loops the most important thing and switching regulators is looking at the high di/dt loops identifying where those sit on the board and making those as small as possible so for a buck converter the high di/dt loop is formed by the input capacitor the V in terminal the high side switch the low side switch and the ground terminal and keeping this loop as small as possible will make sure that the inductance formed by this loop is also small as possible why is this important because higher inductance in the presence of high di DT equals more noise and that noise is going to show up as high am i and also can affect the normal operation of the buck converter another topic we should look at is protecting sensitive nodes the feedback node is a sensitive node and has to be as small as possible the trace is connected to the feedback pin should be short and thin so they don't pick up any noise it's very often that we see this rule violated the feedback trace on many boards is long and thick and that is not good because it affects the performance of the regulator and it could also affect the emi shielding is also very important here we have a two boards two board examples one it has two layers and the other one has four layers the component placement is the same it's the same bomb components sit on top the routing is exactly the same the only difference is we have two additional shielding layers in between and from a test candle we did here in our three meter chamber we can see that adding additional ground layers in the middle can result in reduced EMI in this case we had five DB difference below the SIS per twenty two Class V line let's take a look at the four 3603 board layout we have the board here but it would be easier to look at the layout on the screen the LM four 3603 panel was designed so that you can have easy layout and EMI good EMI performance the whole family of converters has the same pin out so all the parts are pin-to-pin compatible so your board level will be compatible with any part from that family now let's look at the pin out the we we said that the high di DT loop in a buck converter is formed by the input capacitor the high side switch and the low side switch ground so in the four 3603 the VN pin and the P ground pins are right next to each other so that allows you to place the input capacitor as close as possible making this loop as small as possible and which decreases the inductance in this high di DT loop another important thing to point out is that switches on the opposite side so that you all you have to do is bring switch out and connect it straight to the inductor you don't have to send the switch node to other layers because that would generate more noise also the C boot pin is right next to switch which allows you to place the boot capacitor next to the switch pin make that also high di DT loops as small as possible the feedback node which is a sensitive node is all the way down in the corner a right hand side corner bottom corner and feedback is close to right next to a ground which provides some shielding and also allows you to place the feedback divider very close to the pin making the feedback node very small since all these signals are routed on top layer we can have unbroken large unbroken shape here on top for which helps for thermal and for also for shielding and we have we can have unbroken bottom ground plane for again for shielding and better thermal performance now let's go inside our three meter EMI chamber and prepare a board for a scan and now we're inside t is 3 meter chamber this is where we do our preliminary evaluation board testing before we go outside to your 10 meter certified facility this chamber is lined up with these ferrite tiles to provide the RF insulation and this is a very typical setup we have a 360 degree turn table the board would sit in the middle here powered up and here we have the antenna tower with the antenna attached to it and moving up and down to scan emissions at every height let's go and do some testing all right let's run a scan inside the chamber we have the board powered up and here we have a spectrum analyzer which is getting data from the antenna inside and on the screen we're plotting the results from this scan against the sister 22 Class B and Class A limit lines and we're almost done here scanning as you can see the LM 4 3603 is passing Class B specification for a 3 meter chamber and now I would like to show you the results we got from my 10 meter certified in my chamber facility this is 4 LM 4 3603 running with 12 volts input 3.3 volts out at 3 amps it is passing Class B specification and all the parts in this family were tested and passed the same Class B test and for more information go to simple switcher comm thank you for watching

Channel: Texas Instruments

Views: 35,872

Keywords: emi design optimization, board layout tips, simple switcher, synchronous regulator, lm4, gp_1886105, gp_1886107, gp_1884920, gp_1884921, gp_1886109, gp_1886111, gp_1884958, tl_10946, tl_10947, tl_10395, tl_11009, tl_11010, tl_10633, tl_9905

Id: HT3v71NoiKw

Length: 7min 12sec (432 seconds)

Published: Thu Sep 11 2014