EMC Aware

Switch Node Ringing

• Switch node ringing can be a significant source of EMI.
• Requires a resonant circuit and activation energy at the resonant frequency

Activation Energy

• Higher switching frequency reduces passive component size
• Faster edges increase efficiency
• Both increase activation energy

Resonant Circuit

• LLoop includes inductance from the track, via, leads, bond wires, ESL
• All should be minimised

Slow Down The Edges

• Adding a resistor in series with the boot capacitor slows the rise time
• The slower edge reduces the efficiency
• Recommend 0R0 RBOOT and only add resistance if required
• Gate drive resistors also possible for controllers with external FETs

Use A Snubber

• Snubber absorbs energy, damping the resonance of parasitic elements
• Reduces efficiency
• Snubber placement can compromise the layout

Add a High-Frequency Capacitor

Select CHF with 𝒇𝒁𝒎𝒊𝒏 > 𝒇𝒓𝒊𝒏𝒈𝒊𝒏𝒈

Input Capacitor Position

• Every millimetre makes a difference! Minimize the area of the loop from
  CIN to VIN and PGND back to CIN. Return current directly to the source.
• HF Capacitor closest to the VIN pin

Smaller, Cooler, Quieter

• Smaller means higher switching frequency
• Efficiency (cooler) requires the fastest edges
• Tightly packed circuits may be more susceptible to EMI

Hotrod™ Package

• Pin assignment and footprint optimised for layout
• Butterfly layout reduces inductance through parallel paths (& magnetic field cancellation)
• Flip chip on copper column eliminates bond wire inductance
• Reduces series resistance
• Enhanced thermal path

Capacitor On Leadframe

• High frequency capacitors integrated into package on the lead frame

Summary

• Switch node ringing can be a significant source of EMI
• Activation energy is increased by higher switching frequency and faster edges
• Resonant frequency is a product of the COSS and LLOOP.
• Circuit/PCB solutions: bootstrap resistor, snubber, HF capacitor, CIN position.
• Package innovations for high fswitch: HotRodTM, flip chip on Cu column, the capacitor on a lead frame.