MIC2174 Datasheet, PDF (13/24 Pages) Micrel Semiconductor – 300kHz, Synchronous Buck Controller 300kHz, Synchronous Buck Controller

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_IN
to provide power to turn the external N-Channel power
MOSFETs for the high- and low-side switches. For
applications where V_IN < 5V, it is necessary that the
power MOSFETs used are sub-logic level and are in full
conduction mode for V_GS of 2.5V. For applications when
V_IN > 5V; logic-level MOSFETs, whose operation is
specified at V_GS = 4.5V must be used.
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 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_DS and V_GS). The gate charge is supplied by the
MIC2174 gate-drive circuit. At 300kHz switching
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:
I_G[high-side] (avg) = Q_G × f_SW
(5)
where:
I_G[high-side](avg) = Average high-side MOSFET gate
current
Q_G = Total gate charge for the high-side MOSFET taken
from the manufacturer’s data sheet for V_GS = V_IN.
f_SW = Switching Frequency (300kHz)
The low-side MOSFET is turned on and off at V_DS = 0
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_ISS at V_DS = 0 instead of
gate charge.
MIC2174
For the low-side MOSFET:
I_G[low-side] (avg) = C_ISS × V_GS × f_SW
(6)
Since the current from the gate drive comes from the V_IN,
the power dissipated in the MIC2174 due to gate drive is:
P_GATEDRIVE = V_IN × (I_G[high-side](avg) + I_{G[low -side]}(avg)) (7)
A convenient figure of merit for switching MOSFETs is
the on resistance times the total gate charge R_DS(ON) ×
Q_G. Lower numbers translate into higher efficiency. Low
gate-charge logic-level MOSFETs are a good choice for
use with the MIC2174. Also, the R_DS(ON) of 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:
• Voltage rating
• On-resistance
• Total gate charge
The voltage ratings for the high-side and low-side
MOSFETs are essentially equal to the power stage input
voltage V_HSD. A safety factor of 20% should be added to
the V_DS(max) of the MOSFETs to account for voltage
spikes due to circuit parasitic elements.
The power dissipated in the MOSFETs is the sum of the
conduction losses during the on-time (P_CONDUCTION) and
the switching losses during the period of time when the
MOSFETs turn on and off (P_AC).
P_SW = P_CONDUCTION + P_AC
(8)
P_CONDUCTION = I_SW(RMS) ² × R_DS(ON)
(9)
P_AC = P_{AC(off )} + P_AC(on)
(10)
where:
R_DS(ON) = on-resistance of the MOSFET switch
D = Duty Cycle = V_OUT / V_HSD
Making the assumption that the turn-on and turn-off
transition times are equal; the transition times can be
approximated by:
t_T
=
C_ISS
× V_IN
+ C_OSS
I_G
× V_HSD
(11)
where:
C_ISS and C_OSS are measured at V_DS = 0
I_G = gate-drive current
The total high-side MOSFET switching loss is:
P_AC = (V_HSD + V_D ) × I_PK × t _T × f_SW
(12)
September 2009
13
M9999-090409-B