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LM3402, LM3402HV

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SNVS450E – SEPTEMBER 2006 – REVISED MAY 2013

Layout Considerations

The performance of any switching converter depends as much upon the layout of the PCB as the component

selection. The following guidelines will help the user design a circuit with maximum rejection of outside EMI and

minimum generation of unwanted EMI.

COMPACT LAYOUT

Parasitic inductance can be reduced by keeping the power path components close together and keeping the

area of the loops that high currents travel small. Short, thick traces or copper pours (shapes) are best. In

particular, the switch node (where L1, D1, and the SW pin connect) should be just large enough to connect all

three components without excessive heating from the current it carries. The LM3402/02HV operates in two

distinct cycles whose high current paths are shown in Figure 24:

Figure 31. Buck Converter Current Loops

The dark grey, inner loop represents the high current path during the MOSFET on-time. The light grey, outer loop

represents the high current path during the off-time.

GROUND PLANE AND SHAPE ROUTING

The diagram of Figure 24 is also useful for analyzing the flow of continuous current vs. the flow of pulsating

currents. The circuit paths with current flow during both the on-time and off-time are considered to be continuous

current, while those that carry current during the on-time or off-time only are pulsating currents. Preference in

routing should be given to the pulsating current paths, as these are the portions of the circuit most likely to emit

EMI. The ground plane of a PCB is a conductor and return path, and it is susceptible to noise injection just as

any other circuit path. The continuous current paths on the ground net can be routed on the system ground plane

with less risk of injecting noise into other circuits. The path between the input source and the input capacitor and

the path between the recirculating diode and the LEDs/current sense resistor are examples of continuous current

paths. In contrast, the path between the recirculating diode and the input capacitor carries a large pulsating

current. This path should be routed with a short, thick shape, preferably on the component side of the PCB.

Multiple vias in parallel should be used right at the pad of the input capacitor to connect the component side

shapes to the ground plane. A second pulsating current loop that is often ignored is the gate drive loop formed

by the SW and BOOT pins and capacitor CB. To minimize this loop at the EMI it generates, keep CB close to the

SW and BOOT pins.

CURRENT SENSING

to the CS and GND pins of the IC.

REMOTE LED ARRAYS

In some applications the LED or LED array can be far away (several inches or more) from the LM3402/02HV, or

on a separate PCB connected by a wiring harness. When an output capacitor is used and the LED array is large

or separated from the rest of the converter, the output capacitor should be placed close to the LEDs to reduce

the effects of parasitic inductance on the AC impedance of the capacitor. The current sense resistor should

remain on the same PCB, close to the LM3402/02HV.

Copyright © 2006–2013, Texas Instruments Incorporated

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