The high frequency and large switching currents of the LP8758-EA make the choice of layout important. Good power supply results only occur when care is given to proper design and layout. Layout affects noise pickup and generation and can cause a good design to perform with less-than-expected results. With a range of output currents from milliamps to 4 A per converter core, good power supply layout is much more difficult than most general PCB design. The following steps should be used as a reference to ensure the device is stable and maintains proper voltage and current regulation across its intended operating voltage and current range.
- Place CIN as close as possible to the VIN_Bx pin and the PGND_Bxx pin. Route the VIN trace wide and thick to avoid IR drops. The trace between the positive node of the input capacitor and the LP8758-EA VIN_Bx pin(s), as well as the trace between the input capacitor's negative node and power PGND_Bxx pin(s), must be kept as short as possible. The input capacitance provides a low-impedance voltage source for the switching converter. The inductance of the connection is the most important parameter of a local decoupling capacitor — parasitic inductance on these traces must be kept as tiny as possible for proper device operation.
- The output filter, consisting of Lx and COUTx, converts the switching signal at SW_Bx to the noiseless output voltage. It must be placed as close as possible to the device keeping the switch node small, for best EMI behavior. Route the traces between the output capacitors of the device and the load (or input capacitors of the load) direct and wide to avoid losses due to the IR drop.
- Input for analog blocks (VANA and AGND) must be isolated from noisy signals. Connect VANA directly to a quiet system voltage node and AGND to a quiet ground point where no IR drop occurs. Place the decoupling capacitor as close to the VANA pin as possible. VANA must be connected to the same power node as VIN_Bx pins.
- If the load supports remote voltage sensing, connect the feedback pins FB_Bx of the device to the respective sense pins on the load. The sense lines are susceptible to noise. They must be kept away from noisy signals such as PGND_Bxx, VIN_Bx, and SW_Bx, as well as high bandwidth signals such as the I2C. Avoid both capacitive as well as inductive coupling by keeping the sense lines short and direct. Run the lines in a quiet layer. Isolate them from noisy signals by a voltage or ground plane if possible.
- PGND_Bxx, VIN_Bx and SW_Bx must be routed on thick layers. They must not surround inner signal layers which are not able to withstand interference from noisy PGND_Bxx, VIN_Bx and SW_Bx.
Due to the small package of this converter and the overall small solution size, the thermal performance of the PCB layout is important. Many system-dependent issues such as thermal coupling, airflow, added heat sinks and convection surfaces, and the presence of other heat-generating components affect the power dissipation limits of a given component. Proper PCB layout, focusing on thermal performance, results in lower die temperatures. Wide power traces come with the ability to sink dissipated heat. This can be improved further on multi-layer PCB designs with vias to different planes. This results in reduced junction-to-ambient (RθJA) and junction-to-board (RθJB) thermal resistances and thereby reduces the device junction temperature, TJ. Performing a careful system-level 2D or full 3D dynamic thermal analysis at the beginning product design process is strongly recommended, using a thermal modeling analysis software.