MIC2174_10 Datasheet, PDF (17/27 Pages) Micrel Semiconductor – Synchronous Buck Controller Featuring Adaptive On-Time Control 40V Input, 300kHz

Micrel, Inc.
MIC2174/MIC2174C
I_D(avg) = I_OUT × 2 × 30ns × f_SW
(26)
The reverse voltage requirement of the diode is:
V_DIODE(rrm) = V_HSD
The power dissipated by the Schottky diode is:
P_DIODE = I_D(avg) × V_F
(27)
where, V_F = forward voltage at the peak diode current.
The external Schottky diode is not necessary for the
circuit operation since the low-side MOSFET contains a
parasitic body diode. The external diode will improve
efficiency and decrease the high frequency noise. If the
MOSFET body diode is used, then it must be rated to
handle the peak and average current. The body diode
has a relatively slow reverse recovery time and a
relatively high forward voltage drop. The power lost in
the diode is proportional to the forward voltage drop of
the diode. As the high-side MOSFET starts to turn on,
the body diode becomes a short circuit for the reverse
recovery period, dissipating additional power. The diode
recovery and the circuit inductance will cause ringing
during the high-side MOSFET turn-on.
An external Schottky diode conducts at a lower forward
voltage preventing the body diode in the MOSFET from
turning on. The lower forward voltage drop dissipates
less power than the body diode. The lack of a reverse
recovery mechanism in a Schottky diode causes less
ringing and less power loss. Depending upon the circuit
components and operating conditions, an external
Schottky diode will give a 1/2% to 1% improvement in
efficiency.
Snubber Design
A snubber is used to damp out high frequency ringing
caused by parasitic inductance and capacitance in the
buck converter circuit. Figure 5 shows a simplified
schematic of the buck converter. Stray capacitance
consists mostly of the two MOSFETs’ output
capacitance (C_OSS). The stray inductance consists
mostly package inductance and trace 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.
C_OSS1
L_STRAY1
+
L_STRAY2
L
Q1
C_IN
L_STRAY3
V_DC
Sync_buck
Q2
C_OSS2
C_OUT
Controller
L_STRAY4
–
Figure 5. Output Parasitics
One method of reducing the ringing is to use a resistor
and capacitor to lower the Q of the resonant circuit, as
shown in Figure 6. Capacitor C_S is used to block DC and
minimize the power dissipation in the resistor. This
capacitor value should be between two and ten times the
parasitic capacitance of the MOSFET C_OSS. 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.
L_STRAY1
R_DS
L_STRAY2
L_STRAY3
R_S
C_OSS2
L_STRAY4
C_S
Figure 6. Snubber Circuit
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 FET or
using trace that is too long or thin will add inductance to
the snubber and diminishes its effectiveness.
September 2010
17
M9999-091310-C