MIC2166_1009 Datasheet, PDF (17/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Hyper Speed Control™ Family

Micrel, Inc.
MIC2166
External Schottky Diode (Optional)
An external freewheeling diode can be used to keep the
inductor current flow continuous while both MOSFETs
are turned off.
The diode conducts current during the dead-time. The
dead-time prevents current from flowing unimpeded
through both MOSFETs and is typically 30ns. The diode
conducts twice during each switching cycle. Although the
average current through this diode is small, the diode
must be able to handle the peak current.
I_D(avg)CM = I_OUT × 2 × 30ns × f_SW
(25)
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
The reverse voltage requirement of the diode is:
V_DC
V_DIODE(rrm) = V_IN
The power dissipated by the Schottky diode is:
–
P_DIODE = I_D(avg) × V_F
(26)
Sync_buck
Controller
Q2
C_OSS2
L_STRAY4
C_OUT
Figure 5. Output Parasitics
where, VF = forward voltage at the peak diode current.
An external Schottky diode is recommended, even
though the low-side MOSFET contains a parasitic body
diode since the Schottky diode has much less forward
voltage than the body diode. The external diode will
improve efficiency and reduce the high frequency noise.
If the MOSFET body diode is used, 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.
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
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 2 and 10 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
September 2010
17
M9999-092410-C