MIC2165_1011 Datasheet, PDF (17/28 Pages) Micrel Semiconductor – Adaptive On-Time DC-DC Controller Featuring HyperLight Load®

Micrel, Inc.
MIC2165
P_INDUCTORCu=I_L(RMS)2 × R_WINDING
(16)
The resistance of the copper wire, R_WINDING, increases with
the temperature. The value of the winding resistance used
should be at the operating temperature.
R_WINDING = R_{WINDING(20°C)} × (1+ 0.0042 × (T_H – T_20°C)) (17)
where:
T_H = temperature of wire under full load
T_20°C = ambient temperature
R_{WINDING(20°C)} = room temperature winding
(usually specified by the manufacturer)
resistance
Output Capacitor Selection
The type of the output capacitor is usually determined by
its ESR (equivalent series resistance). Voltage and RMS
current capability are two other important factors for
selecting the output capacitor. Recommended capacitors
are tantalum, low-ESR aluminum electrolytic, OS-CON,
POSCAPS, and ceramic. The output capacitor’s ESR is
usually the main cause of the output ripple. The output
capacitor ESR also affects the control loop from a stability
point of view. The maximum value of ESR is calculated:
ESR _COUT
≤
ΔV_OUT(pp)
ΔI_L(PP)
(18)
where:
ΔVOUT(PP) = peak-to-peak output voltage ripple
ΔI_L(PP) = peak-to-peak inductor current ripple
and the error comparator to behavior properly. Also, the
output voltage ripple should be in phase with the inductor
current. Therefore, the output voltage ripple caused by the
output capacitor C_OUT should be much smaller than the
ripple caused by the output capacitor ESR. If low ESR
capacitors, such as ceramic capacitors, are selected as
the output capacitors, then a ripple injection method
should be applied to provide the enough FB voltage ripples.
Please refer to the “Ripple Injection” subsection for more
details.
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for
aluminum electrolytic or OS-CON. The output capacitor
RMS current is calculated in Equation 20:
I_{COUT (RMS)}
=
ΔI_L(PP)
12
(20)
The power dissipated in the output capacitor is:
P_{DISS(COUT )}
=
I_COUT
2
(RMS)
× ESR _COUT
(21)
Input Capacitor Selection
The input capacitor for the power stage input V_IN should be
selected for ripple current rating and voltage rating.
Tantalum input capacitors may fail when subjected to high
inrush currents, caused by turning on a “hot-plugging”. A
tantalum input capacitor’s voltage rating should be at least
two 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 de-rating. The input voltage ripple
will primarily depend upon the input capacitor’s ESR. The
peak input current is equal to the peak inductor current, so:
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 19:
( ) ΔV_OUT(PP) =
⎜⎜⎝⎛
ΔI_L(PP)
C_OUT × f_SW
×
8
⎟⎟⎠⎞²
+
ΔI_L(PP) × ESR_COUT
2
(19)
ΔV_IN = I_L(PK) × ESR_CIN
(22)
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 current ripple is low:
where:
C_OUT = output capacitance value
f_SW = switching frequency
As described in the “Theory of Operation” subsection in
“Functional Description”, MIC2165 requires at least 20mV
peak-to-peak ripple at the FB pin to make the g_m amplifier
( ) I_CIN(RMS) ≈ I_OUT(max) × D × 1 − D
(23)
September 2010
17
M9999-092410-E