10.1.1 Compact Layout for EMI Reduction

Radiated EMI is generated by the high di/dt components in pulsing currents in switching converters. The larger area covered by the path of a pulsing current, the more EMI is generated. The key to minimize radiated EMI is to identify pulsing current path and minimize the area of the path. In Buck converters,the pulsing current path is from the V_IN side of the input capacitors to HS switch, to the LS switch, and then return to the ground of the input capacitors, as shown in Figure 97.

Figure 97. Buck Converter High Δi/Δt Path

High-frequency ceramic bypass capacitors at the input side provide primary path for the high di/dt components of the pulsing current. Placing ceramic bypass capacitor(s) as close as possible to the VIN and PGND pins is the key to EMI reduction.

The SW pin connecting to the inductor must be as short as possible, and just wide enough to carry the load current without excessive heating. Use short, thick traces or copper pours (shapes) for high current condution path to minimize parasitic resistance. The output capacitors must be place close to the V_OUT end of the inductor and closely grounded to PGND pin and exposed PAD.

Place the bypass capacitors on VCC and BIAS pins as close as possible to the pins respectively and closely grounded to PGND and the exposed PAD.

10.1.2 Ground Plane and Thermal Considerations

TI recommends using one of the middle layers as a solid ground plane. Ground plane provides shielding for sensitive circuits and traces. It also provides a quiet reference potential for the control circuitry. The AGND and PGND pins must be connected to the ground plane using vias right next to the bypass capacitors. PGND pins are connected to the source of the internal LS switch. They must be connected directly to the grounds of the input and output capacitors. The PGND net contains noise at switching frequency and may bounce due to load variations. Constrain PGND trace, as well as PVIN and SW traces, to one side of the ground plane. The other side of the ground plane contains much less noise and should be used for sensitive routes.

TI recommends providing adequate device heat sinking by utilizing the PAD of the device as the primary thermal path. Use a recommended 4 by 3 array of 10-mil thermal vias to connect the PAD to the system ground plane heat sink. The vias must be evenly distributed under the PAD. Use as much copper as possible, for system ground plane, on the top and bottom layers for the best heat dissipation. Use a four-layer board with the copper thickness for the four layers, starting from the top of, 2 oz / 1 oz / 1 oz / 2 oz. Four layer boards with enough copper thickness provides low current conduction impedance, proper shielding and lower thermal resistance.

The thermal characteristics of the LM43603-Q1 are specified using the parameter R_θJA, which characterize the junction temperature of silicon to the abient temperature in a specific system. Although the value of R_θJA is dependant on manhy variables, it still can be used to approximate the operating junction temperature of the device. To obtain an estimate of the device junction temperature, one may use Equation 27:

The maximum operating junction temperature of the LM43603-Q1 is 125°C. R_θJA is highly related to PCB size and layout, as well as enviromental factors such as heat sinking and air flow. Figure 98 shows measured results of R_θJA with different copper area on a 2-layer board and 4-layer board.

Figure 98. R_θJAvs Copper Area
2 oz Copper on Outer Layers and 1 oz Copper on Inner Layers

10.1.3 Feedback Resistors

To reduce noise sensitivity of the output voltage feedback path, it is important to place the resistor divider and C_FF close to the FB pin, rather than close to the load. The FB pin is the input to the error amplifier, so it is a high impedance node and very sensitive to noise. Placing the resistor divider and C_FF closer to the FB pin reduces the trace length of FB signal and reduces noise coupling. The output node is a low impedance node, so the trace from V_OUT to the resistor divider can be long if short path is not available.

If voltage accuracy at the load is important, make sure voltage sense is made at the load. Doing so corrects for voltage drops along the traces and provide the best output accuracy. The voltage sense trace from the load to the feedback resistor divider must be routed away from the SW node path and the inductor to avoid contaminating the feedback signal with switch noise, while also minimizing the trace length. This is most important when high value resistors are used to set the output voltage. TI recommends routing the voltage sense trace and place the resistor divider on a different layer than the inductor and SW node path, such that there is a ground plane in between the feedback trace and inductor/SW node polygon. This provides further shielding for the voltage feedback path from EMI noises.