MIC2164 MICREL [Micrel Semiconductor], MIC2164 Datasheet - Page 16

• Manufacturer Part Number: MIC2164
• Description: Constant Frequency, Synchronous Buck Controllers Featuring Adaptive On-Time Control
• Manufacturer: MICREL [Micrel Semiconductor]

Micrel, Inc.
where:
t
V
f
The high-side MOSFET switching losses increase with
the switching frequency and the input voltage V
low-side MOSFET switching losses are negligible and
can be ignored for these calculations.
Inductor Selection
Values for inductance, peak, and RMS currents are
required to select the output inductor. The input and
output voltages and the inductance value determine the
peak-to-peak inductor ripple current. Generally, higher
inductance values are used with higher input voltages.
Larger peak-to-peak ripple currents will increase the
power dissipation in the inductor and MOSFETs. Larger
output ripple currents will also require more output
capacitance to smooth out the larger ripple current.
Smaller peak-to-peak ripple currents require a larger
inductance value and therefore a larger and more
expensive inductor. A good compromise between size,
loss and cost is to set the inductor ripple current to be
equal to 20% of the maximum output current. The
inductance value is calculated by the equation below.
where:
f
20% = ratio of AC ripple current to DC output current
V
The peak-to-peak inductor current ripple is:
The peak inductor current is equal to the average output
current plus one half of the peak-to-peak inductor current
ripple.
The RMS inductor current is used to calculate the I
losses in the inductor.
Maximizing efficiency requires the proper selection of
core material and minimizing the winding resistance. The
high frequency operation of the MIC2164/-2/-3 requires
the use of ferrite materials for all but the most cost
sensitive applications.
T
SW
SW
September 2009
D
HSD(max)
= Switching transition time
= Body diode drop (0.5v)
= Switching Frequency
= switching frequency
L
Δ
I
=
I
( L
L(PK)
PP
I
V
L(RMS)
= maximum power stage input voltage
HSD
)
V
=
=
OUT
(max)
I
V
OUT(max)
=
OUT
×
V
I
×
OUT(max)
HSD
(
×
V
f
SW
HSD
(
V
(max)
+
HSD
×
0.5
(max)
20
2
(max)
×
×
%
+
f
Δ
−
SW
ΔI
×
I
L(PP)
V
I
−
L(PP)
OUT
12
OUT
×
V
L
OUT
(max)
2
)
)
(14)
HSD
(15)
. The
(12)
(13)
2
R
16
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:
The resistance of the copper wire, R
with the temperature. The value of the winding
resistance used should be at the operating temperature.
R
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 capacitors
are tantalum, low-ESR aluminum electrolytic, OS-CON
and POSCAPS. 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 below:
H
20°C
WINDING
WINDING(20°C)
L(PP)
OUT(pp)
= temperature of wire under full load
ΔV
= ambient temperature
= peak-to-peak inductor current ripple
OUT(pp)
P
= R
= peak-to-peak output voltage ripple
INDUCTORCu
WINDING(20°c)
ESR
=
= room temperature winding resistance
⎛
⎜
⎜
⎝
C
C
OUT
=I
OUT
L(RMS)
ΔI
≤
× (1 + 0.0042 × (T
L(PP)
⋅
ΔV
f
ΔI
SW
2
OUT(pp)
× R
L(PP)
⋅
8
WINDING
⎞
⎟
⎟
⎠
2
+
(
ΔI
WINDING
L(PP)
M9999-090409-B
(16)
MIC2164/-2/-3
H
– T
⋅
ESR
, increases
20°C
)) (17)
C
OUT
(18)
(19)
)
2