MIC2166_1009 Datasheet, PDF (14/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Hyper Speed Control™ Family

Micrel, Inc.
Application Information
MOSFET Selection
The MIC2166 controller works from an input voltage of
4.5V to 28V and has an internal 5V V_DD supply to
provide power to turn the external N-Channel power
MOSFETs for the high-side and low-side switches. For
applications where V_IN < 5.5V, it is recommended to
connect VDD-to-VIN to bypass the internal linear
regulator. The external power MOSFETs should be
logic-level MOSFETs, whose operation is specified at
V_GS = 4.5V.
There are different criteria for choosing the high-side and
low-side MOSFETs. These differences are more
significant at lower duty cycles such as 24V to 1.2V
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 on-resistance 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. For a
MOSFET with a 0.4%/°C thermal coefficient a 75°C rise
in junction temperature will increase the channel
resistance of the MOSFET by 30% of the resistance
specified at 25°C. This change in resistance must be
accounted for when calculating MOSFET power
dissipation and 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 MIC2166 gate-
drive circuit. At 600kHz switching frequency, the gate
charge can be a significant source of power dissipation
in the MIC2166. At light 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[HS] (avg) = Q_G × f_SW
(4)
where:
I_G[HS](avg) = Average High-Side (HS) 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
MIC2166
The low-side MOSFET is turned on and off at V_DS = 0V
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 (LS) MOSFET:
I_G[LS] (avg) = C_ISS × V_GS × f_SW
(5)
Since the current from the gate drive comes from the
V_DD, which is the output of the internal linear regulator
power by V_IN, the power dissipated in the MIC2166 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 MIC2166. 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_IN. 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)
September 2010
14
M9999-092410-C