LTM2886 datasheet(26/38 Pages) LINER

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LTM2886

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For more information www.linear.com/LTM2886

APPLICATIONS INFORMATION

Figure 17. LTM2886 Low EMI Demo Board Layout

conduction. The drawback of using ground planes,

where EMI is of concern, is the creation of a dipole

antenna structure which can radiate differential volt-

agesformedbetweenGNDandGND2.Ifgroundplanes

are used it is recommended to minimize their area,

and use contiguous planes as any openings or splits

can exacerbate RF emissions.

• For large ground planes a small capacitance (≤330pF)

from GND to GND2, either discrete or embedded

within the substrate, provides a low impedance cur-

rent return path for the module parasitic capacitance,

minimizinganyhighfrequencydifferentialvoltagesand

substantially reducing radiated emissions. Discrete

capacitance will not be as effective due to parasitic

ESL. In addition, voltage rating, leakage, and clear-

ance must be considered for component selection.

Embedding the capacitance within the PCB substrate

provides a near ideal capacitor and eliminates com-

ponent selection issues; however, the PCB must be

four layers. Care must be exercised in applying either

technique to ensure the voltage rating of the barrier

is not compromised.

• In applications without an embedded PCB substrate

capacitance a slot may be added between the logic

side and isolated side device pins. The slot extends

the creepage path between terminals on the PCB side,

andmayreduceleakagecausedbyPCBcontamination.

The slot should be placed in the middle of the device

and extend beyond the package perimeter.

The PCB layout in Figures 17 and 18 shows the low EMI

demo board for the LTM2886. The demo board uses a

combination of EMI mitigation techniques, including both

embedded PCB bridge capacitance and discrete GND to

GND2 capacitors. Two safety rated type Y2 capacitors are

used in series, manufactured by MuRata, part number

GA342QR7GF471KW01L. The embedded capacitor ef-

fectively suppresses emissions above 400MHz, whereas

the discrete capacitors are more effective below 400MHz.

EMI performance is shown in Figure 19, measured using

a Gigahertz Transverse Electromagnetic (GTEM) cell and

method detailed in IEC 61000-4-20, Testing and Measure-

ment Techniques – Emission and Immunity Testing in

Transverse Electromagnetic Waveguides.

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