Some Points to Keep in Mind During Layout

Some Points to Keep in Mind During Layout 

Note One inch of trace switching, say 1 A of instantaneous current in a transition time of 30 ns, gives a spike of 0.7 V. For 3 A, and two inches of trace, the induced voltage tries to be 4 V  

Note It is almost impossible to see the noise spikes. First of all, various parasitics help limit/absorb them somewhat (though they can still retain the capability to cause controller upset ). Further, the moment we put in an oscilloscope probe, the 10 to 20 pF of probe capacitance can also absorb the spikes, and we would probably see nothing significant. In addition, probes pick up 

Whichever traces have ONLY Switch-ON arrows OR only Switch-OFF arrows have a very high dl/dt during the switch transition, and are therefore critical. They need to be made wide and short  

A paralleled ceramic capacitor can also help, provided it does not cause loop instability issues (especially in voltage mode control - see Chapter 7). 

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