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MIC2198_05 Datasheet, PDF (9/14 Pages) Micrel Semiconductor – 500kHz 4mm × 4mm Synchronous Buck Controller | |||
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MIC2198
Applications Information
The following applications information includes component
selection and design guidelines.
Inductor Selection
Values for inductance, peak, and RMS currents are required
to select the output inductor. The input and output voltages
and the inductance value determine the peak to peak induc-
tor ripple current. Generally, higher inductance values are
used with higher input voltages. Larger peak to peak ripple
currents will increase the power dissipation in the inductor
and MOSFETs. Larger output ripple currents will also require
more output capacitance to smooth out the larger ripple cur-
rent. Smaller peak to peak ripple currents require a larger
inductance value and therefore a larger and more expensive
inductor. A good compromise between size, loss and cost is
to set the inductor ripple current to be equal to 20% of the
maximum output current.
The inductance value is calculated by the equation below.
L = VOUT Ã (VIN(max) â VOUT)
VIN(max) Ã fS Ã 0.2 Ã IOUT(max)
where:
fS = switching frequency
0.2 = ratio of AC ripple current to DC output current
VIN(max) = maximum input voltage
The peak-to-peak inductor current (AC ripple current) is:
IPP
=
VOUT Ã (VIN(max)
VIN(max) Ã fS
âVOUT)
ÃL
The peak inductor current is equal to the average output current
plus one half of the peak to peak inductor ripple current.
IPK = IOUT(max) + 0.5 Ã IPP
The RMS inductor current is used to calculate the I2ÃR losses
in the inductor.
IINDUCTOR(rms) = IOUT(max) Ã
1+
1
3


ï£
IP
IOUT(max)



2
Maximizing efï¬ciency requires the proper selection of core
material and minimizing the winding resistance. The high
frequency operation of the MIC2198 requires the use of fer-
rite materials for all but the most cost sensitive applications.
Lower cost iron powder cores may be used but the increase
in core loss will reduce the efï¬ciency of the power supply.
This is especially noticeable at low output power. The winding
resistance decreases efï¬ciency at the higher output current
levels. The winding resistance must be minimized although
this usually comes at the expense of a larger inductor.
The power dissipated in the inductor is equal to the sum
of the core and copper losses. At higher output loads, the
core losses are usually insigniï¬cant and can be ignored. At
lower output currents, the core losses can be a signiï¬cant
contributor. Core loss information is usually available from
the magnetics vendor.
Micrel, Inc.
Copper loss in the inductor is calculated by the equation
below:
PINDUCTORCu = IINDUCTOR(rms)2 Ã RWINDING
The resistance of the copper wire, RWINDING, increases with
temperature. The value of the winding resistance used should
be at the operating temperature.


RWINDING(hot)
=
RWINDING(20°C)
Ã
1
ï£
+
0.0042
Ã
(T HOT â T20°C)
where:
THOT = temperature of the wire under operating load
T20°C = ambient temperature
RWINDING(20°C) is room temperature winding
resistance (usually speciï¬ed by the manufacturer)
Current-Sense Resistor Selection
Low inductance power resistors, such as metal ï¬lm resistors
should be used. Most resistor manufacturers make low induc-
tance resistors with low temperature coefï¬cients, designed
speciï¬cally for current-sense applications. Both resistance
and power dissipation must be calculated before the resis-
tor is selected. The value of RSENSE is chosen based on the
maximum output current and the maximum threshold level.
The power dissipated is based on the maximum peak output
current at the minimum overcurrent threshold limit.
RSENSE
=
55mV
IOUT(max)
The maximum overcurrent threshold is:
IOVERCURRENT(max)
=
95mV
RCS
The maximum power dissipated in the sense resistor is:
PD(RSENCE) = IOVERCURRENT(max)2 Ã RCS
MOSFET Selection
External N-Channel logic-level power MOSFETs must be
used for the high- and low-side switches. The MOSFET
gate-to-source drive voltage of the MIC2198 is regulated by
an internal 5V VDD regulator. Logic-level MOSFETs, whose
operation is speciï¬ed at VGS = 4.5V must be used.
It is important to note the on-resistance of a MOSFET in-
creases with increasing temperature. A 75°C rise in junction
temperature will increase the channel resistance of the MOS-
FET by 50% to 75% of the resistance speciï¬ed at 25°C. This
change in resistance must be accounted for when calculating
MOSFET power dissipation.
Total gate charge is the charge required to turn the MOSFET
on and off under speciï¬ed operating conditions (VDS and
VGS). The gate charge is supplied by the MIC2198 gate drive
circuit. At 500kHz switching frequency, the gate charge can
be a signiï¬cant source of power dissipation in the MIC2198.
At low output load this power dissipation is noticeable as a
reduction in efï¬ciency. The average current required to drive
the high-side MOSFET is:
IG[high-side](avg) = QG Ã fS
October 2005
9
MIC2198
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