MIC2155 Datasheet, PDF (25/33 Pages) Micrel Semiconductor – 2-Phase, Single Output, PWM Synchronous Buck Control IC

Micrel, Inc.
For the high-side switch, the maximum DC power
dissipation is:
( ) P_SWITCH1(DC) = R_DSON1 × I_SW1(rms) ²
For the low-side switch, the DC power dissipation is:
( ) P_SWITCH2(DC) = R_DSON2 × I_SW2(rms) ²
The switching loss for each of the high-side MOSFETs
is:
P_AC = V_IN ×I_SW(peak) × T_t × f_S
The total power dissipation for each MOSFET is:
P_{FET _ total} = P_SWITCH1(DC) + P_AC
External Schottky Diode
A freewheeling diode in parallel with the low-side FET is
needed to keep the inductor current flow continuous
while both MOSFETs are turned off (dead time). Dead
time is necessary to prevent current from flowing
unimpeded through both MOSFETs. An external
Schottky diode is not necessary for circuit operation
since the low-side MOSFET contains a parasitic body
diode. An external diode will improve efficiency due to its
lower forward voltage drop as compared to the internal
parasitic diode in the FET. It may also decrease high
frequency noise because the schottky diode junction
does not suffer from reverse recovery.
If the MOSFET body diode is used, it must be rated to
handle the peak and average current. The body diode
may have 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. If the internal FET
diode is used, power dissipated during the dead time
should be added to the P_DISS of the low-side MOSFET.
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 is less
ringing and power loss. Depending on the circuit
components and operating conditions, an external
Schottky diode may give a ½% to 1% improvement in
efficiency.
This power dissipation is calculated below
I_D(ave) = I_OUT × 2 × t_d × f_S
Where t_d is the dead time when both MOSFETs are off.
The reverse voltage requirement of the diode is:
MIC2155/2156
V_{DIODE _ RRM} = V_IN
The power dissipated by the diode is:
P_DIODE = I_{D _ AVE} × V_F
where V_F is the forward voltage at the peak diode
current.
Snubber Design
A snubber is used to damp out high frequency ringing
caused by parasitic inductance and capacitance in the
buck converter circuit. A snubber is needed for each of
the two phases in the converter. Figure 22 shows a
simplified schematic of one of the buck converter
phases. Stray capacitance consists mostly of the two
MOSFET’s output capacitance (C_OSS). 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.
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 22. Output Parasitics
One method of reducing the ringing is to use a resistor
and capacitor to lower the Q of the resonant circuit. The
circuit in Figure 23 shows the resistor in 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 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.
The snubber components should be placed as close as
possible to the low-side MOSFET and/or external
schottky diode since in contributes to most of the stray
capacitance. Placing the snubber too far from the FET or
using etch that is too long or thin will add inductance to
May 2009
25
M9999-052709-A
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