LM5145 datasheet(55/69 Pages) TI

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• 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.

• Follow any layout considerations of the MOSFETs as recommended by the MOSFET manufacturer, including

pad geometry and solder paste stencil design.

• The SW pin connects to the switch node of the power conversion stage and acts as the return path for the

high-side gate driver. The parasitic inductance inherent to loop 1 in Figure 12-1 and the output capacitance

(COSS) of both power MOSFETs form a resonant circuit that induces high frequency (greater than 50 MHz)

ringing at the SW node. The voltage peak of this ringing, if not controlled, can be significantly higher than

the input voltage. Make sure that the peak ringing amplitude does not exceed the absolute maximum rating

limit for the SW pin. In many cases, a series resistor and capacitor snubber network connected from the SW

node to GND damps the ringing and decreases the peak amplitude. Provide provisions for snubber network

components in the PCB layout. If testing reveals that the ringing amplitude at the SW pin is excessive, then

include snubber components as needed.

12.1.2 Gate-Drive Layout

The LM25143 high-side and low-side gate drivers incorporate short propagation delays, adaptive dead-time

control, and low-impedance output stages capable of delivering large peak currents with very fast rise and

fall times to facilitate rapid turn-on and turn-off transitions of the power MOSFETs. Very high di/dt can cause

unacceptable ringing if the trace lengths and impedances are not well controlled.

Minimization of stray or parasitic gate loop inductance is key to optimizing gate drive switching performance,

whether it be series gate inductance that resonates with MOSFET gate capacitance or common source

inductance (common to gate and power loops) that provides a negative feedback component opposing the

gate drive command, thereby increasing MOSFET switching times. The following loops are important:

• Loop 2: high-side MOSFET, Q1. During the high-side MOSFET turn-on, high current flows from the bootstrap

(boot) capacitor through the gate driver and high-side MOSFET, and back to the negative terminal of the boot

capacitor through the SW connection. Conversely, to turn off the high-side MOSFET, high current flows from

the gate of the high-side MOSFET through the gate driver and SW, and back to the source of the high-side

MOSFET through the SW trace. Refer to loop 2 of Figure 12-1.

• Loop 3: low-side MOSFET, Q2. During the low-side MOSFET turn-on, high current flows from the VCC

decoupling capacitor through the gate driver and low-side MOSFET, and back to the negative terminal of the

capacitor through ground. Conversely, to turn off the low-side MOSFET, high current flows from the gate of

the low-side MOSFET through the gate driver and GND, and back to the source of the low-side MOSFET

through ground. Refer to loop 3 of Figure 12-1.

TI strongly recommends following circuit layout guidelines when designing with high-speed MOSFET gate drive

circuits.

• Connections from gate driver outputs, HO1/2, HOL1/2, LO1/2, and LOL1/2 to the respective gates of the

high-side or low-side MOSFETs must be as short as possible to reduce series parasitic inductance. Be aware

that peak gate drive currents can be as high as 4.25 A. Use 0.65-mm (25 mils) or wider traces. Use via or

vias, if necessary, of at least 0.5-mm (20 mils) diameter along these traces. Route HO and SW gate traces as

a differential pair from the LM25143 to the high-side MOSFET, taking advantage of flux cancellation.

• Minimize the current loop path from the VCC and HB pins through their respective capacitors as these

provide the high instantaneous current, up to 4.25 A, to charge the MOSFET gate capacitances. Specifically,

locate the bootstrap capacitor, CBST, close to the HB and SW pins of the LM25143 to minimize the area of

loop 2 associated with the high-side driver. Similarly, locate the VCC capacitor, CVCC, close to the VCC and

PGND pins of the LM25143 to minimize the area of loop 3 associated with the low-side driver.

12.1.3 PWM Controller Layout

With the provision to locate the controller as close as possible to the power MOSFETs to minimize gate driver

trace runs, the components related to the analog and feedback signals as well as current sensing are considered

in the following:

• Separate power and signal traces, and use a ground plane to provide noise shielding.

• Place all sensitive analog traces and components related to COMP1/2, FB1/2, CS1/2, SS1/2, RES, and RT

away from high-voltage switching nodes such as SW1/2, HO1/2, LO1/2, or HB1/2 to avoid mutual coupling.

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