MIC2176-1 Datasheet, PDF (17/31 Pages) Micrel Semiconductor – Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control

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_IN to provide power to turn the external N-Channel
power MOSFETs for the high- and low-side switches.
For applications where V_DD < 5V, it is necessary that the
power MOSFETs used are sub-logic level and are in full
conduction mode for VGS of 2.5V. For applications when
V_DD > 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. 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
(VDS and VGS). The gate charge is supplied by the
MIC2176 gate-drive circuit. At 300kHz switching
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:
I_G[high-side] (avg) = Q_G × f_SW
(6)
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_DD.
f_SW = Switching Frequency
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
MIC2176
accurately calculated using C_ISS at V_DS = 0 instead of
gate charge.
For the low-side MOSFET:
I_G[low-side] (avg) = C_ISS × V_GS × f_SW
(7)
Since the current from the gate drive comes from the
V_DD, the power dissipated in the MIC2176 due to gate
drive is:
P_GATEDRIVE = V_DD × (I_G[high-side] (avg) + I_G[low-side] (avg)) (8)
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 MIC2176. Also, the R_DS(ON) of the low-side
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:
• 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
P_CONDUCTION = I_SW(RMS)2 × R_DS(ON)
P_AC = P_{AC(off )} + P_AC(on)
(9)
(10)
(11)
where:
R_DS(ON) = On-resistance of the MOSFET switch
D = Duty Cycle = V_OUT / V_HSD
November 2010
17
M9999-111710-A