MIC2169B Datasheet, PDF (12/24 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

Micrel, Inc.
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, 500kHz
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:
( ) _IPP
=
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.
I_INDUCTOR =
(I_{OUT _ MAX} )²
+
I_PP ²
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Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance. The
high frequency operation of the MIC2169B 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
MIC2169B
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_INDUCTORCu = 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)} = room temperature winding resistance
(usually specified by the manufacturer)
Output Capacitor Selection
The output capacitor values are usually determined by
the 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. The maximum value of ESR is calculated:
R_ESR
≤
ΔV_OUT
I_PP
where:
V_OUT = peak-to-peak output voltage ripple
I_PP = peak-to-peak inductor ripple current
The total output ripple is a combination of the ripple due
to the output capacitors’ ESR and the ripple due to the
output capacitor. The total ripple is calculated below:
ΔV_OUT =
⎜⎜⎝⎛
I_PP × (1− D)
C_OUT × f_S
⎟⎟⎠⎞²
+
(I_PP
×
R_ESR
)²
where:
D = duty cycle
C_OUT = output capacitance value
f_S = switching frequency
The voltage rating of capacitor should be twice the
voltage for a tantalum and 20% greater for aluminum
electrolytic.
The output capacitor RMS current is calculated below:
June 2009
12
M9999- 060309-A
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