Input decoupling

High-Frequency Effects and the Importance of Input Decoupling... 

However, as you will see a little later, it is not a bad idea to always include this 0.1 pF input decoupling capacitor. The reasons may be different on different occasions, and for different types of switchers, but this component is generally always nice to have. 

Figure 2-12 Improving Input Decoupling by Stacking Capacitors...

In a Buck we see that it is very important to have good input decoupling, and also to minimize the trace lengths between the ceramic capacitor and the IC. We have discussed that issue previously. But we now see that it is very important to also minimize the length of the trace section connecting the SW pin to the common node of L and D. This is one of the first things I try to check out when I troubleshoot a Buck switcher board handed over to me. 

Note that bad input decoupling was not the primary reason for the failure here, because that was known to only cause the first current limit to be breached on occasion. This breach of... 

For all topologies, that is in fact a key requirement—that the control IC be powered off a clean rail. It is just that in a Buck, the input decoupling capacitor for the power stage and the decoupling capacitor for the control are often the same component. Though in some cases, it may be necessary, even for a Buck controller IC, to add a small RC going to its supply pin (typically a 10Q resistor and an additional 0.1 pF ceramic capacitor). 

Note that in the Buck and the Buck-Boost, the input capacitor is included in the critical path. That implies we need very good input decoupling in these topologies (for the power section). So, besides the necessary bulk capacitor for the power stage (typically a tantalum or aluminum electrolytic of large capacitance), we should also place a small ceramic capacitor (about 0.1 to IpF) directly between the quiet end of the switch (i.e., at the supply side) and the ground—and also as close as possible to the switch. 

Figure 6-12 Input Decoupling Capacitor Placed Very Close to the IC Between VIN and GND Pins...

Check the clock. In DC-DC converters you can simply look carefully at the SW node. It should be stable otherwise please read up some more on PCB routing and input decoupling in previous chapters, and then return to this point. When designing AC-DC converters, I used to look very hard at the signal coming out of the IC meant for driving the Gate of the switch (Pin called OUT on the 3842). 

However, before you start, you should have read the previous chapters and therefore actively ruled out PCB design issues, input decoupling issues, and also junk IC issues. You should also have asked the Twelve Questions from Figure 8-1 and assured yourself you are not obviously falling into any of the all-too-familiar traps. 

We should remember that the control circuitry usually needs good local decoupling of its own. And for that we need to provide a small ceramic capacitor very close to the IC. Clearly, especially when dealing with switchers, the decouphng ceramic for the power stage can often do double-duty as the decoupling capacitor of the control too (note that this applies to the Buck-Boost and the Buck only, since a power input decoupling capacitor is only required for them). 

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