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Figure 1: A typical application circuit of an Austin Lynx II DC/DC converter module is depicted with external capacitors.
To reduce high-frequency switching noise at the input and output of the module, 0.1 μF (0603) and 1.0 μF (0603) small-package ceramic capacitors should be placed at the input and output pins of the module.
Figure 2 illustrates the physical layout for the Austin Lynx II module shown in Figure 1. This example shows important guidelines that must be followed when designing the module on a multilayered PCB. For simplicity, all three power traces (input, output, and ground) are assumed to be on the top layer of the PCB, where the Austin Lynx II module is placed.
The most important guideline here is to extend the ground plane (green color in Figure 2) to the area underneath the module. GE’s guideline does not recommended using this space for routing signal traces unless they are in inner layers underneath the ground plane. The VOUT and ground planes are placed close together to minimize interconnect inductance on the output side. Output capacitors (COUT) are connected as close to the output/ground pins as possible to provide the most effective output filtering. Similarly, on the input side, interconnect inductance is minimized by placing the VIN and ground planes close together with input capacitors placed as close to the input/ground pins as possible.
Figure 2: Simplified printed circuit board (PCB) layout for Austin Lynx II converter module.
Recommended PCB layout
In short, the Austin Lynx II DC/DC converter is so placed that it minimizes loop area and noise coupling. The signal traces should not be routed underneath the module, unless they are sandwiched between ground planes, to avoid noise coupling. Similarly, to prevent any coupling, no component should be placed under the module. As shown in Figure 2, only the ground plane is placed under the module.
For ease of repair and removal of the surface-mountable DC/DC module from the PCB, a clearance of 4.0 mm (0.16 in.) around the outline of the module is recommended. Aside from clearance, it also isolates adjacent components from exposure to heat during the removal process.
What is more, the GE guideline also recommends using copper planes for routing power traces, including the input, output, and ground connections. Traditionally, in multilayer PCBs, top and bottom layers are primarily used for routing signals. This leads to the inner layers being used for ground, input, and output. With non-isolated converter modules, since the input voltage is often used to feed multiple modules, one layer is normally assigned to it. The output can either be another layer or part of the same layer.
However, in the case of surface-mountable modules, multiple vias are needed to carry the current from the top layer to the inner power planes. The rule of thumb here is to have 3 A per via, and the recommended via size is 22 mils (0.022 in. or 560 μm) plated-through hole. For control pins, one via per pin is deemed to be sufficient. The vias also should be located in the direction of current flow for optimum performance. One or two vias per capacitor connection are recommended for bulk capacitors.
For signal traces, the company recommends a trace width of 7 to 10 mils (180 to 250 μm).
Figure 3 shows a layout of the Austin Lynx II module showing vias located near the output, input, and ground pins for carrying current to the inner layers.
Figure 3: Recommended PCB layout for Austin Lynx II converter module shows placement of vias.
Although the internal test results are not presented in the application note referenced,¹ the maker says that following the guidelines presented here will minimize noise and EMI problems associated with mounting POL DC/DC converter modules on multilayer PCBs.