MIC24420 datasheet(18/34 Pages) MICREL | 2.5A Dual Output PWM Synchronous Buck Regulator IC

18 / 34 page

Micrel, Inc.

MIC24420/MIC24421

June 2012

M9999-062012-C

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 high-side MOSFET

turns

on.

This

ringing

causes

stress

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 low-side 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 high-side MOSFET turns

on. This ringing is characterized by the equation:

2π

⋅

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.

)

2π

⋅

Define f’ as:

Combining the equations for f1, f2 and f’ to derive CP, the

parasitic capacitance

)

−

⋅

LP is solved by re-arranging the equation for f1.

()

)

2π

⋅

Step 3:

Calculate the damping resistor.

Critical damping occurs at Q=1

Solving for RS

Figure 11 shows the snubber in the circuit and the

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