MIC25400YML TR Micrel Inc, MIC25400YML TR Datasheet - Page 16

• Manufacturer Part Number: MIC25400YML TR
• Description: IC REG PWM SYNC BUCK 2A 24MLF
• Manufacturer: Micrel Inc
• Series: -r

Specifications of MIC25400YML TR

• Pwm Type: Voltage Mode
• Number Of Outputs: 2
• Frequency - Max: 1.2MHz
• Duty Cycle: 75%
• Voltage - Supply: 4.5 V ~ 13.2 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 24-VFQFN Exposed Pad, 24-MLF®
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: 576-3902-2

Proper snubber design requires the parasitic inductance
and capacitance be known. A method of determining
these values and calculating the damping resistor value
is outlined below.
1. Measure the ringing frequency at the switch node
which is determined by parasitic L
frequency as f
2. Add a capacitor C
the C
measure the new ringing frequency. Define this new
(lower) frequency as f
using the values of f
3. Add a resistor R
damping.
Step 1: First measure the ringing frequency on the
switch node voltage when the high-side MOSFET turns
on. This ringing is characterized by the equation:
W
C
Step 2: Add a capacitor, C
synchronous MOSFET, Q2. The capacitor value should
be approximately 3 times the C
frequency of the switch node ringing, f
Define f’ as:
Combining the equations for f
parasitic capacitance
L
Step 3: Calculate the damping resistor.
Critical damping occurs at Q=1
Micrel, Inc.
P
January 2011
P
here:
is solved by re-arranging the equation for f
and L
OSS
C
L
Q
f
f
f' =
P
2
of the FET) from the switch node-to-ground and
1
P
P
are the parasitic capacitance and inductance
=
=
=
=
f
f
=
R
2π
2
1
2π
1
S
( )
1
2
2π
.
⋅
L
(f
L
C
P
C
1
2
P
S
'
S
⋅
)
L
(C
S
1
1
⋅
S
2
+
⋅
, f
in series with C
P
C
1
C
S
C
(normally at least 3 times as big as
−
2
2
P
P
P
+
. L
1
and C
⋅
C
(f
=
P
P
1
1
)
)
and C
2
1
S
, f
.
S
2
OSS
, in parallel with the
and f’ to derive C
P
P
S
of Q2. Measure the
can now be solved
and C
2.
to generate critical
P
. Define this
1
.
P
, the
16
Solving for R
Figure 11 shows the snubber in the circuit and the
damped switch node waveform.
The snubber capacitor, Cs, is charged and discharged
each switching cycle. The energy stored in Cs is
dissipated by the snubber resistor, Rs, two times per
switching period.
equation below.
Where:
f
V
Low-side MOSFET Selection
An external N-channel logic level power MOSFET must
be used for the low-side switch. The MOSFET gate-to-
source drive voltage of the MIC25400 is regulated by an
internal 5V regulator. Logic level MOSFETs, whose
operation is specified at V
of MOSFETs with a lower specified V
2.5V) are not recommended since the low threshold can
cause them to turn on when the high-side FET is turning
on. When operating the regulator below a 6V input,
connect V
dropping out.
Total gate charge is the charge required to turn the
MOSFET on and off under specified operating conditions
(V
regulator’s gate drive circuit. Gate charge is a source of
power dissipation in the regulator due to the high
switching frequencies. At low output load this power
dissipation is noticeable as a reduction in efficiency. The
average current required to drive the MOSFETs is:
Where:
Q
This
manufacturer’s data sheet.
Since current from the gate drive is supplied by the input
voltage, power dissipated in the MIC25400 due to gate
drive is:
Parameters that are important to MOSFET selection are:
S
IN
G
is the switching frequency for each phase
DS
is the DC input voltage
is the gate charge for both of the external MOSFETs.
•
•
•
and V
information
Voltage rating
On resistance
Total Gate Charge
P
I
P
R
DD
snubber
GATE_DRIVE
S
DD
=
=
GS
S
to V
Q
). The gate charge is supplied by the
C
G
S
=
L
+
⋅
P
IN
f
f
S
C
S
should
P
=
to prevent the V
This power is calculated in the
⋅
C
Q
S
G
⋅
GS
⋅
V
f
IN
S
= 4.5V must be used. Use
be
2
⋅
V
IN
obtained
GS
DD
M9999-020111-C
(such as 3.3V or
regulator from
MIC25400
from
the