MIC2199_10 Datasheet, PDF (9/14 Pages) Micrel Semiconductor – 300kHz 4mm × 4mm Synchronous Buck Converter

Micrel, Inc.
Applications Information
Following applications information includes component selec-
tion 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 × (VIN(max) - VOUT )}
V_IN(max) × f_S × 0.2 × I_OUT(max)
where:
f_S = switching frequency
0.2 = ratio of AC ripple current to DC output current
V_IN(max) = maximum input voltage
The peak-to-peak inductor current (AC ripple current) is:
I_PP
=
V_{OUT ×} (V_IN(max)
V_IN(max) × f_S
− ^V_OUT⁾
×L
The peak inductor current is equal to the average output current
plus one half of the peak-to-peak inductor ripple current.
I_PK = I_OUT(max) + 0.5 × I_PP
The RMS inductor current is used to calculate the I²Ã—R losses
in the inductor.
I_{INDUCTOR(rms)} = I_OUT(max) ×
1+
1
3



I_P
I_OUT(max)



2
Maximizing efficiency requires the proper selection of core
material and minimizing the winding resistance. The high
frequency operation of the MIC2199 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 efficiency of the power supply.
This is especially noticeable at low output power. The winding
resistance decreases efficiency 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 insignificant and can be ignored. At
lower output currents, the core losses can be a significant
contributor. Core loss information is usually available from
the magnetics vendor.
MIC2199.
Copper loss in the inductor is calculated by the equation
below:
P_{INDUCTORCu =} I_{INDUCTOR(rms)2 ×RWINDING}
The resistance of the copper wire, R_WINDING, increases with
temperature. The value of the winding resistance used should
be at the operating temperature.
( ) ^R_WINDING(hot) ^{= R}_{WINDING(20°C)} ^{× 1+ 0.0042 ×(T}_HOT ^{− T}_20°C ⁾
where:
T_HOT = temperature of the wire under operating load
T_20°C = ambient temperature
R_{WINDING(20°C)} is room temperature winding
resistance
(usually specified by the manufacturer)
Current-Sense Resistor Selection
Low inductance power resistors, such as metal film resistors
should be used. Most resistor manufacturers make low induc-
tance resistors with low temperature coefficients, designed
specifically for current-sense applications. Both resistance
and power dissipation must be calculated before the resis-
tor is selected. The value of R_SENSE 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.
R_SENSE
=
55mV
I_OUT(max)
The maximum overcurrent threshold is:
I_{OVERCURRENT(max)}
=
95mV
R_CS
The maximum power dissipated in the sense resistor is:
P_{D(RSENSE )} = I_{OVERCURRENT(max)}² ×R_CS
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 MIC2199 is regulated by
an internal 5V V_DD regulator. Logic-level MOSFETs, whose
operation is specified at V_GS = 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 specified 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 specified operating conditions (V_DS and
V_GS). The gate charge is supplied by the MIC2199 gate drive
circuit. At 500kHz switching frequency, the gate charge can
be a significant source of power dissipation in the MIC2199.
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_S
January 2010
9
M9999-011310