MIC2176-1 MICREL [Micrel Semiconductor], MIC2176-1 Datasheet - Page 19

• Manufacturer Part Number: MIC2176-1
• Description: Wide Input Voltage, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
Copper loss in the inductor is calculated by Equation 18:
The resistance of the copper wire, R
with the temperature. The value of the winding
resistance used should be at the operating temperature.
P
where:
T
T
R
(usually specified by the manufacturer)
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 capacitor
types are tantalum, low-ESR aluminum electrolytic, OS-
CON and POSCAP. 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:
where:
Δ V
ΔI
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 21:
November 2010
H
20°C
ΔV
WINDING(Ht)
WINDING(20°C)
L(PP)
OUT(pp)
= temperature of wire under full load
OUT(pp)
= ambient temperature
= peak-to-peak inductor current ripple
P
(1 + 0.0042 × (T
= peak-to-peak output voltage ripple
INDUCTOR(Cu)
= R
=
ESR
= room temperature winding resistance
WINDING(20°C)
⎛
⎜
⎜
⎝
C
C
OUT
OUT
ΔI
= I
≤
×
L(PP)
L(RMS)
f
H
ΔV
SW
– T
ΔI
×
OUT(pp)
L(PP)
×
2
20°C
8
× R
⎞
⎟
⎟
⎠
))
2
WINDING
+
(
ΔI
L(PP)
WINDING
×
ESR
, increases
C
OUT
(18)
(19)
(20)
(21)
)
2
19
where:
D = duty cycle
C
f
As described in the “Theory of Operation” subsection in
Functional Description , the MIC2176 requires at least
20mV peak-to-peak ripple at the FB pin to make the g
amplifier and the error comparator behave properly. Also,
the output voltage ripple should be in phase with the
inductor current. Therefore, the output voltage ripple
caused by the output capacitors value 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, a ripple injection
method should be applied to provide the enough
feedback voltage ripple. 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 22:
The power dissipated in the output capacitor is:
Input Capacitor Selection
The input capacitor for the power stage input V
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. 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 on the input
capacitor’s ESR. The peak input current is equal to the
peak inductor current, so:
sw
OUT
= switching frequency
= output capacitance value
ΔV
I
P
C
DISS(C
OUT
IN
(RMS)
= I
OUT
L(pk)
)
=
=
× ESR
ΔI
I
C
L(PP)
12
OUT
(RMS)
CIN
2
×
ESR
M9999-111710-A
C
OUT
MIC2176
IN
should
(22)
(23)
(24)
m