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MIC24420 Datasheet(PDF) 18 Page  Micrel Semiconductor 

MIC24420 Datasheet(HTML) 18 Page  Micrel Semiconductor 
18 / 34 page Micrel, Inc. MIC24420/MIC24421 June 2012 18 M9999062012C E.G. Where RCS = 250Ω, CCS = 82pF Snubber A snubber is used to damp out high frequency ringing caused by parasitic inductance and capacitance in the buck converter circuit. Figure 10 shows a simplified schematic of one of the buck converter phases. Stray capacitance consists mostly of the output capacitance (COSS) of the two MOSFET’s. The stray inductance is mostly package and etch inductance. The arrows show the resonant current path when the highside MOSFET turns on. This ringing causes stress on the semiconductors in the circuit as well as increased EMI. Figure 10. Output Parasitics One method of reducing the ringing is to use a resistor to lower the Q of the resonant circuit. The circuit in Figure 11 shows an RC network connected between the switch node and ground. Capacitor CS is used to block DC and minimize the power dissipation in the resistor. This capacitor value should be between 5 and 10 times the parasitic capacitance of the MOSFET COSS. A capacitor that is too small will have high impedance and prevent the resistor from damping the ringing. A capacitor that is too large causes unnecessary power dissipation in the resistor, which lowers efficiency. The snubber components should be placed as close as possible to the lowside MOSFET and/or external Schottky diode since it contributes to most of the stray capacitance. Placing the snubber too far from the MOSFET or using traces that are too long or too thin adds inductance to the snubber and diminishes its effectiveness. Proper snubber design requires the parasitic inductance and capacitance be known. A method of determining these values and calculating the damping resistor value is outlined below. 1. Measure the ringing frequency at the switch node which is determined by parasitic LP and CP. Define this frequency as f1. 2. Add a capacitor CS (normally at least 3 times as big as the COSS of the FET) from the switch node to ground and measure the new ringing frequency. Define this new (lower) frequency as f2. LP and CP can now be solved using the values of f1, f2 and CS. 3. Add a resistor RS in series with CS to generate critical damping. Step 1: First measure the ringing frequency on the switch node voltage when the highside MOSFET turns on. This ringing is characterized by the equation: P P 1 C L 2π 1 f ⋅ = Where: CP and LP are the parasitic capacitance and inductance Step 2: Add a capacitor, CS, in parallel with the synchronous MOSFET, Q2. The capacitor value should be approximately 3 times the COSS of Q2. Measure the frequency of the switch node ringing, f2. ) C (C L 2π 1 f P S P 2 + ⋅ = Define f’ as: 2 1 f f f' = Combining the equations for f1, f2 and f’ to derive CP, the parasitic capacitance 1 ) (f 2 C C 2 ' S P − ⋅ = LP is solved by rearranging the equation for f1. () 2 1 P 2 P ) (f C 2π 1 L ⋅ ⋅ = Step 3: Calculate the damping resistor. Critical damping occurs at Q=1 1 C C L R 1 Q P S P S = + = Solving for RS P S P S C C L R + = Figure 11 shows the snubber in the circuit and the 
