DLPS052 October 2015 DLPA3000
As an example of a proper layout, one of the buck converters layout is shown in Figure 31. It shows the routing and placing of the components around the DLPA3000 for optimal performance. The output voltage of the converters used by the DLPA3000 is set through a register. The DLPA3000 uses the feedback pin to compare the output voltage with an internal setpoint.
For a proper layout, short traces are required and power grounds should be separated from each other. This avoids ground shift problems, which can occur due to interference of the ground currents of different buck converters. High currents are flowing through the inductor (L9) and the output capacitors (C46, C47). Therefore, it is important to keep the traces to and from inductor and capacitors as short as possible to avoid losses due to trace resistance. It is strongly recommended to use high quality capacitors with a low ESR value to keep the losses in the capacitors as low as possible, and to keep the voltage ripple on the output acceptable.
In order to prevent problems with switching high currents at high frequencies, the layout is very critical and snubber networks are advisable. The switching frequency can vary from several hundreds of kHz to frequencies in the MHz range. Keep in mind that it takes only nanoseconds to switch currents from zero to several amperes, which is equivalent to even much higher frequencies. Those switching moments will cause EMI problems if not properly handled, especially when ringing occurs on the edges, which can have higher amplitude and frequency as the switching voltage itself. To prevent this ringing, the DLPA3000 buck converters all need a snubber network consisting of a resistor and a capacitor in series implemented on the board to reduce this unwanted behavior. In this case, the snubber network is placed on the bottom-side of the PCB (thus not visible here) and connected to the trace of L9 routing to the switch node.
In order to clarify what plays a role when laying out a buck converter, this paragraph explains the connections and placing of the parts around the buck converter connected to the pins 50 through 54. The supply voltage is connected to pin 52, which is laid out on a mid-layer (purple-colored) and is connected to this pin using 3 vias to ensure a stable and low-resistance connection is made. The decoupling is done by capacitor C43 and C44, visible on the bottom-right of Figure 31, and the connection to the supply and the ground layer is done using multiple vias. The ground connection on pin 54 is also done using multiple vias to the ground layer, which is visible as the blue areas in Figure 31. By using different layers, it is possible to create low-resistive paths. Ideally, the ground connection of the output capacitors and the ground connection of the part (pin 54) should be close together. The layout connects both points together using a wide trace on the bottom layer (blue colored area) which is also suitable to bring both connections together. All buck converters in the layout have the same layout structure and use a separated ground trace to their respective ground connection on the part. All these ground connections are connected together on the ground plane below the DLPA3000 itself. Figure 31 shows the position of the converter inductor and its accompanying capacitors (L9 & C46, C47) positioned as near as possible to the pins 51 and 53 using traces as thick as possible. The ground connections of these capacitors is done using multiple vias to the ground layer to ensure a low resistance path.