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LTM4676A Datenblatt(PDF) 73 Page - Linear Technology |
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LTM4676A Datenblatt(HTML) 73 Page - Linear Technology |
73 / 136 page LTM4676A 73 4676afa For more information www.linear.com/LTM4676A applicaTions inForMaTion EMI PERFORMANCE The SWn pin provides access to the midpoint of the power MOSFETs in LTM4676A’s power stages. Connecting an optional series RC network from SWn to GND can dampen high frequency (~30MHz+) switch node ringing caused by parasitic inductances and capacitances in the switched-current paths. The RC network is called a snubber circuit because it dampens (or “snubs”) the resonance of the parasitics, at the expense of higher power loss. To use a snubber, choose first how much power to allocate to the task and how much PCB real estate is available to implement the snubber. For example, if PCB space al- lows a low inductance 1W resistor to be used—derated conservatively to 600mW (PSNUB)—then the capacitor in the snubber network (CSW) is computed by: CSW = PSNUB VINn(MAX)2 • fSW where VINn(MAX) is the maximum input voltage that the input to the power stage (VINn) will see in the application, and fSW is the DC/DC converter’s switching frequency of operation. CSW should be NPO, C0G or X7R-type (or better) material. The snubber resistor (RSW) value is then given by: RSW = 5nH CSW The snubber resistor should be low ESL and capable of withstanding the pulsed currents present in snubber cir- cuits. A value between 0.7Ω and 4.2Ω is normal. For ease of snubber implementation, integrated 2.2nF snubber capacitors connect to each of the LTM4676A’s channel switch nodes via a low inductance path. The electrically floating ends of these snubber capacitors are made available on the SNUBn pins of the LTM4676A. Us- ing the aforementioned guidance on snubber selection, a properly sized snubber resistor can be conveniently connected directly between SNUBn and GND. EMI performance of the LTM4676A (on DC1811B-B) with and without a snubber is compared and contrasted in Figure 32 and Figure 33. The snubber resistors applied to the SNUBn pins reduce EMI signal amplitude by several dBµV/m. Figure 32. Radiated Emissions Scan of the LTM4676A Producing 1VOUT at 26A, from 12VIN. DC1811B-B Hardware with Outputs Paralleled. No Snubbers Applied. fSW = 350kHz. Measured in a 10m Chamber. Peak Detect Method Figure 33. Radiated Emissions Scan of the LTM4676A Producing 1VOUT at 26A, from 12VIN. DC1811B-B Hardware with Outputs Paralleled. 1Ω (1/4W rated) Snubber Resistors Applied from SNUBn to GND. fSW = 350kHz. Measured in a 10m Chamber. Peak Detect Method FREQUENCY (MHz) 30 –10 0 10 30 40 50 70 4676A F33 20 60 422.4 1010 226.2 618.6 814.8 FREQUENCY (MHz) 30 –10 0 10 30 40 50 70 4676A F32 20 60 422.4 1010 226.2 618.6 814.8 |
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Ähnliche Beschreibung - LTM4676A |
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