MIC2169A Datasheet, PDF (9/17 Pages) Micrel Semiconductor – 500kHz PWM Synchronous Buck Control IC

MIC2169A
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
I_{INDUCTOR(rms)}  I_OUT max  
1
1
3


I_P 
I_OUT max
Maximizing efficiency requires the proper selection of core
material and minimizing the winding resistance. The high
frequency operation of the MIC2169A requires the use of
ferrite materials for all but the most cost sensitive applica-
tions. 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 induc-
tor. 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
feedback loop from stability point of view. See “Feedback Loop
Compensation” section for more information. The maximum
value of ESR is calculated:
Micrel
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:
2
  ^V_OUT ^



I_PP  (1 D)
C_OUT  f_S




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
inductor 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 )}
Voltage Setting Components
The MIC2169A requires two resistors to set the output volt-
age as shown in Figure 2.
June 2005
9
M9999-111803