MIC2174 Micrel Semiconductor, MIC2174 Datasheet - Page 13

• Manufacturer Part Number: MIC2174
• Description: Synchronous Buck Controller
• Manufacturer: Micrel Semiconductor

Micrel, Inc.
Application Information
MOSFET Selection
The MIC2174 controller works from power stage 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 different 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
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
frequency and above, the gate charge can be a
significant source of power dissipation in the MIC2174.
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
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.
G[high-side]
SW
September 2009
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
gate-drive
GS
GS
I
). The gate charge is supplied by the
G[high
= 4.5V must be used.
-
side]
IN
losses,
GS
(avg)
circuit.
< 5V, it is necessary that the
of 2.5V. For applications when
=
ISS
Q
whereas
At
G
at V
×
f
300kHz
SW
DS
GS
= 0 instead of
the
= V
IN
switching
.
low-side
DS
= 0
(5)
IN
13
For the low-side MOSFET:
Since the current from the gate drive comes from the V
the power dissipated in the MIC2174 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. Also, the R
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
Making the assumption that the turn-on and turn-off
transition times are equal; the transition times can be
approximated by:
where:
C
I
The total high-side MOSFET switching loss is:
G
DS(ON)
ISS
G
= gate-drive current
P
. Lower numbers translate into higher efficiency. Low
GATEDRIVE
•
•
•
P
and C
I
P
DS
AC
G[low
CONDUCTION
= on-resistance of the MOSFET switch
(max) of the MOSFETs to account for voltage
Voltage rating
On-resistance
Total gate charge
t
HSD
T
=
-
OSS
side]
(V
=
. A safety factor of 20% should be added to
P
HSD
P
C
=
AC
are measured at V
(avg)
SW
ISS
V
IN
+
=
=
=
×
×
P
OUT
V
I
=
P
V
D
(I
AC(off
SW(RMS)
CONDUCTION
G[high
IN
C
)
/ V
×
ISS
+
I
I
)
HSD
G
PK
C
-
×
+
side]
OSS
2
AC
P
×
V
×
AC(on)
GS
t
).
(avg)
www.DataSheet4U.com
T
R
×
DS
×
×
+
DS(ON)
V
f
P
f
HSD
SW
= 0
SW
DS(ON)
AC
+
I
G[low
M9999-090409-B
CONDUCTION
of the low-side
-
side]
MIC2174
(avg))
DS(ON)
) and
(10)
(11)
(12)
(6)
(7)
(8)
(9)
IN
×
,