MIC2169 데이터 시트보기 (PDF) - Micrel
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MIC2169 Datasheet PDF : 15 Pages
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MIC2169
IPK = IOUT (max) + 0.5 × IPP
The RMS inductor current is used to calculate the I2 × R
losses in the inductor.
2
IINDUCTOR(rms) = IOUT (max ) ×
1+
1
3
⎛
⎝⎜
IOUT
IP
(max)
⎞
⎠⎟
Maximizing efficiency requires the proper selection of core
material and minimizing the winding resistance. The high
frequency operation of the MIC2169 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 con-
tributor. Core loss information is usually available from the
magnetics vendor. Copper loss in the inductor is calculated
by the equation below:
PINDUCTORCu = IINDUCTOR(rms)2 × 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) × 1+ 0.0042 × (THOT − T20°C )
where:
THOT = temperature of the wire under operating load
T20°C = ambient temperature
RWINDING(20°C) is room temperature winding resistance (usu-
ally 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 to consider when
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:
RESR
≤
ΔVOUT
IPP
where:
VOUT = peak-to-peak output voltage ripple
Micrel
IPP = peak-to-peak inductor ripple current
The total output ripple is a combination of the ESR output
capacitance. The total ripple is calculated below:
2
( ) ΔVOUT =
⎛
⎜
⎝
IPP × (1− D)
COUT × fS
⎞
⎟
⎠
+
IPP × RESR
2
where:
D = duty cycle
COUT = output capacitance value
fS = switching frequency
The voltage rating of capacitor should be twice the voltage for
a tantalum and 20% greater for an aluminum electrolytic.
The output capacitor RMS current is calculated below:
ICOUT(rms)
=
IPP
12
The power dissipated in the output capacitor is:
PDISS(COUT ) = ICOUT(rms)2 × RESR(COUT )
Input Capacitor Selection
The input capacitor should be selected for ripple current rating
and voltage rating. Tantalum input capacitors may fail when
subjected to high inrush currents, caused by turning the input
supply on. To maximize reliability, tantalum input capacitor
voltage rating should be at least two times the maximum in-
put voltage. Aluminum electrolytic, OS-CON, and multilayer
polymer film capacitors can handle the higher inrush currents
without voltage derating. The input voltage ripple will primar-
ily depends upon the input capacitor’s ESR. The peak input
current is equal to the peak inductor current, so:
ΔVIN = IINDUCTOR(peak) × RESR(CIN )
The input capacitor must be rated for the input current ripple.
The RMS value of input capacitor current is determined at
the maximum output current. Assuming the peak-to-peak
inductor ripple current is low:
ICIN ≈ (rms) IOUT (max ) × D × (1− D)
The power dissipated in the input capacitor is:
PDISS(CIN ) = ICIN(rms)2 × RESR(CIN )
Voltage Setting Components
The MIC2169 requires two resistors to set the output voltage
as shown in Figure 2.
March 2009
9
M9999-032409
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