LM5145 datasheet(55/73 Pages) TI2

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8.4 Layout

8.4.1 Layout Guidelines

Proper PCB design and layout is important in a high-current, fast-switching circuit (with high current and voltage

slew rates) to achieve a robust and reliable design. As expected, certain issues must be considered before

designing a PCB layout using the LM25137. The high-frequency power loop of a buck regulator power stage is

denoted by loop 1 in the shaded area of Figure 8-25. The topological architecture of a buck regulator means

that particularly high di/dt current flows in the components of loop 1, and reducing the parasitic inductance of this

loop by minimizing the effective loop area becomes mandatory. Also important are the gate drive loops of the

high-side and low-side MOSFETs, denoted by 2 and 3, respectively, in Figure 8-25.

CBOOT

VCC

PGND

VIN

VOUT

GND

VCC

Low-side

gate driver

High-side

gate driver

CVCC

CBOOT

CIN

COUT

High frequency

power loop

Figure 8-25. DC/DC Regulator Ground System With Power Stage and Gate Drive Circuit Switching Loops

8.4.1.1 Power Stage Layout

1. Input capacitors, output capacitors, and MOSFETs are the constituent components of the power stage of a

buck regulator and are typically placed on the top side of the PCB (solder side). The benefits of convective

heat transfer are maximized because of leveraging any system-level airflow. In a two-sided PCB layout,

small-signal components are typically placed on the bottom side (component side). Insert at least one inner

plane, connected to ground, to shield and isolate the small-signal traces from noisy power traces and lines.

2. The DC/DC regulator has several high-current loops. Minimize the area of these loops to suppress

generated switching noise and optimize switching performance.

• Loop 1: The most important loop area to minimize is the path from the input capacitor or capacitors

through the high- and low-side MOSFETs, and back to the capacitor or capacitors through the ground

connection. Connect the input capacitor or capacitors negative terminal close to the source of the low-

side MOSFET (at ground). Similarly, connect the input capacitor or capacitors positive terminal close to

the drain of the high-side MOSFET (at VIN). Refer to "loop 1" in Figure 8-25.

• Another loop, not as critical as loop 1, is the path from the low-side MOSFET through the inductor and

output capacitor or capacitors, and back to source of the low-side MOSFET through ground. Connect the

source of the low-side MOSFET and negative terminal of the output capacitor or capacitors at ground as

close as possible.

3. The PCB trace defined as SW node, which connects to the source of the high-side (control) MOSFET, the

drain of the low-side (synchronous) MOSFET and the high-voltage side of the inductor, must be short and

wide. However, the SW connection is a source of injected EMI and thus must not be too large.

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