The PCB layout of any DC/DC converter is critical
to the optimal performance of the design. Bad PCB layout can disrupt the operation
of an otherwise good schematic design. Even if the converter regulates correctly,
bad PCB layout can mean the difference between a robust design and one that cannot
be mass produced. Furthermore, the EMI performance of the regulator is dependent on
the PCB layout, to a great extent. In a buck converter, the most critical PCB
feature is the loop formed by the input capacitor or input capacitors, and power
ground, as shown in Figure 10-1. This loop carries large transient currents that can cause large transient
voltages when reacting with the trace inductance. These unwanted transient voltages
will disrupt the proper operation of the converter. Because of this, the traces in
this loop must be wide and short, and the loop area as small as possible to reduce
the parasitic inductance. Figure 10-2 and Figure 10-3 show recommended layouts for the critical components of the LMR33630.
- Place the input capacitor or capacitors as
close as possible to the VIN and GND terminals. VIN and GND pins
are adjacent, simplifying the input capacitor placement. With the VQFN
package there are two VIN/PGND pairs on either side of the package. This
provides for a symmetrical layout and helps minimize switching noise and EMI
generation. A wide VIN plane must be used on a lower layer to connect both of
the VIN pairs together to the input supply; see Figure 10-3.
- Place bypass capacitor for VCC close to the VCC pin. This capacitor must be placed close to the device and routed with short, wide traces to the VCC and GND pins.
- Use wide traces for the CBOOT capacitor. Place CBOOT close to the device with short/wide traces to the BOOT and SW pins. For the VQFN package, it is important to route the SW connection under the device to the NC pin, and use this path to connect the BOOT capacitor to SW.
- Place the feedback divider as close as possible to the FB pin of the device. Place RFBB, RFBT, and CFF, if used, physically close to the device. The connections to FB and GND must be short and close to those pins on the device. The connection to VOUT can be somewhat longer. However, this latter trace must not be routed near any noise source (such as the SW node) that can capacitively couple into the feedback path of the regulator.
- Use at least one ground plane in one of the middle layers. This plane acts as a noise shield and also act as a heat dissipation path.
- Connect the thermal pad to the ground plane. The SOIC package has a thermal pad (PAD) connection that must be soldered down to the PCB ground plane. This pad acts as a heat-sink connection and an electrical ground connection for the regulator. The integrity of this solder connection has a direct bearing on the total effective RθJA of the application.
- Provide wide paths for VIN, VOUT, and GND. Making these paths as wide and direct as possible reduces any voltage drops on the input or output paths of the converter and maximizes efficiency.
- Provide enough PCB area for proper heat
sinking. As stated in Section 188.8.131.52, enough copper area must be used to ensure a low RθJA,
commensurate with the maximum load current and ambient temperature. Make the top
and bottom PCB layers with two-ounce copper; and no less than one ounce. With the SOIC package,
use an array of heat-sinking vias to connect the thermal pad (PAD) to the
ground plane on the bottom PCB layer. If the PCB design uses multiple
copper layers (recommended), thermal vias can also be connected to the inner
layer heat-spreading ground planes.
- Keep switch area small. Keep the copper area connecting the SW pin to the inductor as short and wide as possible. At the same time the total area of this node should be minimized to help reduce radiated EMI.
See the following PCB layout resources for additional important guidelines:
Figure 10-1 Current Loops with Fast Edges