SNVA964 June   2020 LP8866-Q1

 

  1.   LP8866-Q1 Typical Design Guidelines to Achieve CISPR 25 Class 5 Conducted and Radiated Emissions Compliance
    1.     Trademarks
    2. 1 Typical Reference Design
      1. 1.1 Design Overview
      2. 1.2 Test Condition
      3. 1.3 CISPR 25 Measurement Results
    3. 2 EMI Mitigation Techniques Discussion
      1. 2.1 Spread Spectrum
      2. 2.2 Gate Resistor
      3. 2.3 LED Current Sink Capacitors
      4. 2.4 Layout Consideration
      5. 2.5 Inductor Selection
      6. 2.6 Boost Output Ferrite Bead
      7. 2.7 Common Mode Filter
    4. 3 Summary

2.3 LED Current Sink Capacitors

Since the LED driver board is connected to the LED load with a 20-cm long wire harness, these wires can act as efficient antennas in the region of a few hundred MHz. It is of high importance to place bypass capacitors close to the connector on the LED driver board to prevent the wires from radiating noise. A 1-nF capacitor is recommended to be placed at each LP8866-Q1 OUTx pin to ground. Figure 11 and Figure 12 show a substantial improvement in EMI with these capacitors.

LP8866-Q1 snva964-re-out-cap.gif
(a) With 1-nF capacitors
from current sinks to ground		 (b) Without 1-nF capacitors
from current sinks to ground
Figure 11. Radiated Emissions: LED Current Sink Capacitors
LP8866-Q1 snva964-ce-out-cap.gif
(a) With 1-nF capacitors
from current sinks to ground		 (b) Without 1-nF capacitors
from current sinks to ground
Figure 12. Conducted Emissions: LED Current Sink Capacitors