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Non-Isolated DC/DC Forum
Input cap placement
When placing an input capacitor to a switcher, should I place it as close to the switcher or as close to the power input connector? Do I want to get rid of the noise as soon as possible? or try to minimize the input current loop?
Any help is appreciated!
Power StageA synchronous BUCK power stage has two primary current loops. The input current loop carries high ACdiscontinuous current while the output current loop carries high DC continuous current. The input current loopincludes the input capacitors, the main switching MOSFET, the inductor, the output capacitors and the groundpath back to the input capacitors. To maintain the loop as small as possible, it is generally good practice to placesome ceramic capacitance directly between the drain of the main switching MOSFET and the source of thesynchronous rectifier (SR) through a power ground plane directly under the MOSFETs. The output current loopincludes the SR MOSFET, the inductor, the output capacitors, and the ground return between the outputcapacitors and the source of the SR MOSFET. As with the input current loop, the ground return between theoutput capacitor ground and the source of the SR MOSFET should be routed under the inductor and SRMOSFET to minimize the power loop area. The SW node area should be as small as possible to reduce theparasitic capacitance and minimize the radiated emissions. The gate drive loop impedance(HDRV-gate-source-SW and LDRV-gate-source- GND) should be kept to as low as possible. The HDRV andLDRV connections should widen to 20 mils as soon as possible out from the device pin.
In general you want the input bypass capacitor as close to the IC as possible. You want to minimize the loop distancce from the positive side of the cap to the Vin and from the negative side of the cap to GND. This is for the ceramic bypass capcitor. You may also have an electrolytic type bulk de-coupling capcitor. Its placement is not as critical and may be placed at the input power connector. It will depend on how far the comnnector is from the converter IC. If it is more than a couple inches you may want a bulk capcitor right at the connector and another close to the converter, as well as the input bypass capacitor which should be as close as possible to the IC as mentioned above.
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