MIC2165_1011 Datasheet, PDF (18/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
The power dissipated in the input capacitor is:
P_{DISS(CIN )} = I_CIN(RMS)2 × ESR _CIN
(24)
External Schottky Diode
An external freewheeling diode, which is recommended to
improve the efficiency in discontinuous mode, can be used
to keep the inductor current flow continuous while both
MOSFETs are turned off.
In continuous mode, 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)
In the discontinuous mode, the average current through
the diode is large.
I_D(avg)DM
≈
(1 −
D) × ( ^VZC
Rds(on)
−
ΔI_L(PP)
2
)
(26)
where V_ZC is the zero cross comparator offset.
The reverse voltage requirement of the diode is:
V_DIODE(rrm) = V_IN
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 6 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.
The power dissipated by the Schottky diode is:
P_DIODE = I_D(avg) × V_F
(27)
where, VF = forward voltage at the peak diode current.
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 6. Output Parasitics
September 2010
18
M9999-092410-E