MIC2174-1YMM Micrel Inc, MIC2174-1YMM Datasheet - Page 17

• Manufacturer Part Number: MIC2174-1YMM
• Description: IC BUCK SYNC ADJ 25A 10MSOP
• Manufacturer: Micrel Inc
• Series: Hyper Speed Control™r
• Type: Step-Down (Buck)r

Specifications of MIC2174-1YMM

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.8 ~ 5.5 V
• Frequency - Switching: 300kHz
• Voltage - Input: 3 ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Power - Output: 421mW
• Primary Input Voltage: 40V
• No. Of Outputs: 2
• Output Current: 25A
• Voltage Regulator Case Style: MSOP
• No. Of Pins: 10
• Operating Temperature Range: -40°C To +125°C
• Svhc: No SVHC (15-Dec-2010)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: 576-3546-5

Micrel, Inc.
The reverse voltage requirement of the diode is:
The power dissipated by the Schottky diode is:
where, V
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
capacitance (C
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.
September 2010
F
= forward voltage at the peak diode current.
mostly
I
P
D(avg)
DIODE
OSS
=
). The stray inductance consists
V
=
I
of
OUT
DIODE(rrm)
I
D(avg)
×
the
2
×
×
30ns
V
=
two
F
V
HSD
×
f
MOSFETs’
SW
output
(27)
(26)
17
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
minimize the power dissipation in the resistor. This
capacitor value should be between two and ten times the
parasitic capacitance of the MOSFET C
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 FET or
using trace that is too long or thin will add inductance to
the snubber and diminishes its effectiveness.
+
–
V
DC
L
STRAY1
C
IN
L
STRAY1
R
Figure 5. Output Parasitics
DS
Figure 6. Snubber Circuit
Sync_buck
Controller
C
Q1
OSS1
L
STRAY2
L
STRAY2
Q2
S
is used to block DC and
L
L
STRAY3
STRAY4
MIC2174/MIC2174C
C
L
L
OSS2
STRAY3
STRAY4
M9999-091310-C
C
OSS
L
OSS2
. A capacitor
R
C
S
S
C
OUT