MIC2165_1011 Datasheet, PDF (15/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
Application Information
MOSFET Selection
The MIC2165 controller works from an input voltage of
4.5V to 28V and has an internal 5V V_DD 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 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. 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% 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 MIC2165 gate-drive circuit. At 600kHz switching
frequency, the gate charge can be a significant source of
power dissipation in the MIC2165. 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
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 MIC2165 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 MIC2165. 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).
September 2010
15
M9999-092410-E