Layout Optimisation And Package Enhancements To Minimise Switch Node Ringing And Associated Radiated EMI In A DC/DC Buck Converter

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

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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
Hotrodβ„’ 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
  • 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.

Dr Jim Perkins – Senior Member Technical Staff – Texas Instruments

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