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MIC24420 Datasheet(PDF) 18 Page - Micrel Semiconductor

Part No. MIC24420
Description  2.5A Dual Output PWM Synchronous Buck Regulator IC
Download  34 Pages
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Maker  MICREL [Micrel Semiconductor]
Homepage  http://www.micrel.com
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MIC24420 Datasheet(HTML) 18 Page - Micrel Semiconductor

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Micrel, Inc.
MIC24420/MIC24421
June 2012
18
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
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 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:
P
P
1
C
L
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
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 re-arranging the equation for f1.
()
2
1
P
2
P
)
(f
C
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


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