MIC2168A Datasheet, PDF (11/18 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

Micrel
MIC2168A
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:
L = ^{VOUT × (VIN(max) − VOUT )}
V_IN(max) × f_S × 0.2 × I_OUT (max)
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance.
The high frequency operation of the MIC2168A 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 output currents, the core losses can be a
significant contributor. Core loss information is usually
available from the magnetics vendor. Copper loss in the
inductor is calculated by the equation below:
P_INDUCTOR Cu = I_INDUCTOR(rms)² × R_WINDING
where:
f_S = switching frequency, 1MHz
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 I2 × R
losses in the inductor.
2
IINDUCTOR (rms) = I_OUT (max) ×
1+
1
3
⎜⎜⎝⎛
IP
I_OUT (max)
⎟⎟⎠⎞
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)
Output Capacitor Selection
The output capacitor values are usually determined
capacitors ESR (equivalent series resistance). Voltage
and RMS current capability are two other important
factors selecting the output capacitor. Recommended
capacitors are tantalum, low-ESR aluminum
electrolytics, and POSCAPS. The output capacitor’s
ESR is usually the main cause of output ripple. The
output capacitor ESR also affects the overall voltage
feedback loop from stability point of view. See
“Feedback Loop Compensation” section for more
information.
January 2010
11
M9999-011510