MIC2176-1 MICREL [Micrel Semiconductor], MIC2176-1 Datasheet - Page 17

• Manufacturer Part Number: MIC2176-1
• Description: Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Application Information
MOSFET Selection
The MIC2176 controller works from power stage input
voltages of 4.5V to 73V and has an external 4.5V to 5.5V
V
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. 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
MIC2176
frequency, the gate charge can be a significant source of
power dissipation in the MIC2176. 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
G[high-side]
SW
November 2010
IN
DD
G
DS
= Total gate charge for the high-side MOSFET taken
= Switching Frequency
to provide power to turn the external N-Channel
> 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]
GS
(avg)
circuit.
DD
of 2.5V. For applications when
< 5V, it is necessary that the
=
Q
At
G
×
f
SW
300kHz
GS
= V
DD
switching
.
DS
= 0
(6)
17
accurately calculated using C
gate charge.
For the low-side MOSFET:
Since the current from the gate drive comes from the
V
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 MIC2176. Also, the R
MOSFET will determine the current-limit value. Please
refer to “Current Limit” subsection is 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
P
DD
DS(ON)
GATEDRIVE
G
. Lower numbers translate into higher efficiency. Low
, the power dissipated in the MIC2176 due to gate
Voltage rating
On-resistance
Total gate charge
DS
= On-resistance of the MOSFET switch
(max) of the MOSFETs to account for voltage
I
P
P
P
G[low
HSD
CONDUCTION
SW
AC
. A safety factor of 20% should be added to
=
-
=
=
side]
V
P
P
DD
CONDUCTION
AC(off
(avg)
OUT
×
(I
)
G[high-
=
+
/ V
=
I
P
SW(RMS)
C
HSD
AC(on)
ISS
+
side]
AC
P
×
).
ISS
AC
V
(avg)
GS
2
at V
×
DS(ON)
×
R
f
DS(ON)
+
SW
DS
I
M9999-111710-A
G[low
CONDUCTION
= 0 instead of
of the low-side
-
side]
MIC2176
DS(ON)
(avg))
) and
(10)
(11)
(7)
(9)
×
(8)