MIC2174-1YMM Micrel Inc, MIC2174-1YMM Datasheet - Page 14

• 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.
Application Information
MOSFET Selection
The MIC2174/MIC2174C controller works from input
voltages of 3V to 40V and has an external 3V to 5.5V V
to provide power to turn the external N-Channel power
MOSFETs for the high- and low-side switches. For
applications where V
power MOSFETs used are sub-logic level and are in full
conduction mode for V
V
specified at V
There are differing criteria for choosing the high-side and
low-side MOSFETs. These differences are more
significant at lower duty cycles, such as a 12V to 1.8V
conversion. In such an application, the high-side
MOSFET is required to switch as quickly as possible to
minimize transition losses, whereas the low-side
MOSFET can switch slower, but must handle larger
RMS currents. When the duty cycle approaches 50%,
the current carrying capability of the high-side MOSFET
starts to become critical.
It is important to note that the on-resistance of a
MOSFET increases with increasing temperature. A 75°C
rise in junction temperature will increase the channel
resistance of the MOSFET by 50% to 75% of the
resistance specified at 25°C. This change in resistance
must be accounted for when calculating MOSFET power
dissipation and in calculating the value of current limit.
Total gate charge is the charge required to turn the
MOSFET on and off under specified operating conditions
(V
MIC2174/MIC2174C
switching frequency and above, the gate charge can be
a significant source of power dissipation in the
MIC2174/MIC2174C. At low output load, this power
dissipation is noticeable as a reduction in efficiency. The
average current required to drive the high-side MOSFET
is:
where:
I
current
Q
from the manufacturer’s data sheet for V
f
G[high-side]
SW
September 2010
IN
G
DS
= Total gate charge for the high-side MOSFET taken
= Switching Frequency (300kHz)
> 5V; logic-level MOSFETs, whose operation is
and V
(avg) = Average high-side MOSFET gate
GS
GS
I
). The gate charge is supplied by the
G[high
= 4.5V must be used.
-
side]
IN
gate-drive
GS
(avg)
< 5V, it is necessary that the
of 2.5V. For applications when
=
Q
G
×
circuit.
f
SW
GS
= V
At
IN
.
300kHz
(5)
IN
14
The low-side MOSFET is turned on and off at V
because an internal body diode or external freewheeling
diode is conducting during this time. The switching loss
for the low-side MOSFET is usually negligible. Also, the
gate-drive current for the low-side MOSFET is more
accurately calculated using C
gate charge.
For the low-side MOSFET:
Since the current from the gate drive comes from the V
the power dissipated in the MIC2174/MIC2174C due to
gate drive is:
A convenient figure of merit for switching MOSFETs is
the on resistance times the total gate charge R
Q
gate-charge logic-level MOSFETs are a good choice for
use with the MIC2174/MIC2174C. Also, the R
the low-side MOSFET will determine the current limit
value. Please refer to “Current Limit” subsection in
“Functional Description” for more details.
Parameters that are important to MOSFET switch
selection are:
•
•
•
The voltage ratings for the high-side and low-side
MOSFETs are essentially equal to the power stage input
voltage V
the V
spikes due to circuit parasitic elements.
The power dissipated in the MOSFETs is the sum of the
conduction losses during the on-time (P
the switching losses during the period of time when the
MOSFETs turn on and off (P
where:
R
D = Duty Cycle = V
DS(ON)
G
P
. Lower numbers translate into higher efficiency. Low
GATEDRIVE
Voltage rating
On-resistance
Total gate charge
I
P
DS
G[low
CONDUCTION
= on-resistance of the MOSFET switch
(max) of the MOSFETs to account for voltage
HSD
-
side]
P
. A safety factor of 20% should be added to
P
=
AC
(avg)
SW
V
IN
=
=
=
×
P
OUT
I
=
P
(I
AC(off
SW(RMS)
CONDUCTION
G[high
C
/ V
ISS
)
HSD
-
×
+
side]
2
AC
P
V
×
AC(on)
GS
):
ISS
(avg)
R
×
+
DS(ON)
at V
f
P
MIC2174/MIC2174C
SW
AC
+
I
DS
G[low
M9999-091310-C
CONDUCTION
= 0 instead of
-
side]
(avg))
DS(ON)
DS(ON)
DS
) and
(10)
= 0
(6)
(7)
(8)
(9)
of
IN
×
,