MIC2168_05 Datasheet, PDF (9/14 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

MIC2168
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.
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 MIC2168 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:
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 (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.
Micrel, Inc.
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
≤ ^∆VOUT
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_{C OUT(rms)}
= ^IPP
12
The power dissipated in the output capacitor is:
P _{DISS(C OUT )}=
I_C
OUT(rms)
×
2
R _{ESR(C OUT )}
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(C IN )}
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:
April 2005
9
M9999-040805