MIC2182 Datasheet, PDF (16/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller Final Information

MIC2182
Applications Information
The following applications information includes component
selection and design guidelines. See Figure 14 and Tables 1
through 5 for predesigned circuits.
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 inductor
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
current. 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_OUT )
V_IN(max) × f_S × 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 I²Â·R losses
in the inductor.
I_inductor(rms) = I_OUT(max) ×
1+
1
3



I_PP
I_OUT(max)



2
Maximizing efficiency requires the proper selection of core
material and minimizing the winding resistance. The high
frequency operation of the MIC2182 requires the use of ferrite
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
Micrel
output currents, the core losses can be a significant contribu-
tor. Core loss information is usually available from the mag-
netics vendor.
Copper loss in the inductor is calculated by the equation
below:
P_{inductor Cu} = I_inductor(rms)² × R_winding
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
inductance resistors with low temperature coefficients, de-
signed specifically for current-sense applications. Both resis-
tance and power dissipation must be calculated before the
resistor 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
=
75mV
I_OUT(max)
The maximum overcurrent threshold is:
I_{overcurrent(max)}
=
135mV
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 MIC2182 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
increases with increasing temperature. A 75°C rise in junc-
tion 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.
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 MIC2182 gate drive
circuit. At 300kHz switching frequency and above, the gate
MIC2182
16
June 2000