MIC2164_10 Datasheet, PDF (16/39 Pages) Micrel Semiconductor – Synchronous Buck Controllers Featuring Adaptive On-Time Control 28V Input, Constant Frequency

Micrel, Inc.
Application Information
MOSFET Selection
The MIC2164/-2/-3 controller works from power stage
input voltages of 3V to 28V 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 VGS of 2.5V. For applications when
V_IN > 5V; logic-level MOSFETs, whose operation is
specified at VGS = 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 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
(VDS and VGS). The gate charge is supplied by the
MIC2164/-2/-3 gate-drive circuit. At 300kHz switching
frequency and above, the gate charge can be a
significant source of power dissipation in the MIC2164/-
2/-3. 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
(4)
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
MIC2164/-2/-3/C
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.
For the low-side MOSFET:
I_G[low-side] (avg) = C_ISS × V_GS × f_SW
(5)
Since the current from the gate drive comes from the
V_IN, the power dissipated in the MIC2164/-2/-3 due to
gate drive is:
P_GATEDRIVE = V_IN.(I_G[high-side] (avg) + I_G[low-side] (avg)) (6)
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 MIC2164/-2/-3. 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
(7)
P_CONDUCTION = I_SW(RMS) ² * R_DS(ON)
(8)
P_AC = P_{AC(off )} + P_AC(on)
(9)
where:
R_DS(ON) = on-resistance of the MOSFET switch
D = Duty Cycle = V_OUT / V_HSD
September 2010
16
M9999-091310-D