MIC2169 Datasheet, PDF (9/14 Pages) Micrel Semiconductor – 500 KHZ PWM SYNCHRONOUS BUCK CONTROL IC

MIC2169
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 I² × R
losses in the inductor.
I_{INDUCTOR(rms)} = I_OUT (max) ×
1+
1
3


I_OUT
I_P ^
(max)
2
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 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_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)} is room temperature winding resistance (usu-
ally specified by the manufacturer)
Output Capacitor Selection
The output capacitor values are usually determined capaci-
tors ESR (equivalent series resistance). Voltage and RMS
current capability are two other important factors selecting
the output capacitor. Recommended capacitors 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
Micrel
feedback loop from stability point of view. See “Feedback
Loop Compensation” section for more information. The
maximum value of ESR is calculated:
R_ESR
≤
∆V_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 ESR output
capacitance. The total ripple is calculated below:
( ) ^∆V_OUT ⁼



I_PP × (1−
C_OUT ×
D)^
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 an aluminum electrolytic.
The output capacitor RMS current is calculated below:
I_COUT(rms)
=
I_PP
12
The power dissipated in the output capacitor is:
P_{DISS(COUT )} = I_COUT(rms)2 × R_{ESR(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. Tantalum input capacitor voltage rating
should be at least 2 times the maximum input voltage to
maximize reliability. Aluminum electrolytic, OS-CON, and
multilayer polymer film capacitors can handle the higher
inrush currents without voltage derating. The input voltage
ripple will primarily depend on the input capacitor’s ESR. The
peak input current is equal to the peak inductor current, so:
∆V_IN = I_{INDUCTOR(peak)} × R_{ESR(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 induc-
tor ripple current is low:
I_CIN ≈ _(rms) I_OUT (max) × D × (1− D)
The power dissipated in the input capacitor is:
P_{DISS(CIN )} = I_CIN(rms)2 × R_{ESR(CIN )}
November 2003
9
M9999-111803