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TC1313-AM1EMF View Datasheet(PDF) - Microchip Technology

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TC1313-AM1EMF
Microchip
Microchip Technology Microchip
TC1313-AM1EMF Datasheet PDF : 28 Pages
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5.5 Inductor Selection
For most applications, a 4.7 µH inductor is recom-
mended to minimize noise. There are many different
magnetic core materials and package options to select
from. That decision is based on size, cost and
acceptable radiated energy levels. Toroid and shielded
ferrite pot cores will have low radiated energy but tend
to be larger and more expensive. With a typical
2.0 MHz switching frequency, the inductor ripple
current can be calculated based on the following
formulas.
EQUATION 5-2:
DutyCycle = -V---O----U----T-
VIN
Duty cycle represents the percentage of switch-on
time.
EQUATION 5-3:
Where:
TON
=
DutyCycle × ----1-----
FSW
FSW = Switching Frequency.
The inductor ac ripple current can be calculated using
the following relationship:
EQUATION 5-4:
VL
=
L
×
Δ-----I--L-
Δt
Where:
VL = voltage across the inductor (VIN – VOUT)
Δt = on-time of P-channel MOSFET
Solving for ΔIL = yields:
EQUATION 5-5:
ΔIL
=
-V---L- × Δt
L
When considering inductor ratings, the maximum DC
current rating of the inductor should be at least equal to
the maximum buck regulator load current (IOUT1), plus
one half of the peak-to-peak inductor ripple current
(1/2 * ΔIL). The inductor DC resistance can add to the
buck converter I2R losses. A rating of less than 200 mΩ
is recommended. Overall efficiency will be improved by
using lower DC resistance inductors.
TC1313
TABLE 5-2: TC1313 RECOMMENDED
INDUCTOR VALUES
Part
Number
Value
(µH)
DCR
Ω
(max)
MAX
IDC (A)
Size
WxLxH (mm)
Coiltronics®
SD10 2.2 0.091 1.35 5.2, 5.2, 1.0 max.
SD10 3.3 0.108 1.24 5.2, 5.2, 1.0 max.
SD10 4.7 0.154 1.04 5.2, 5.2, 1.0 max.
Coiltronics
SD12 2.2 0.075 1.80 5.2, 5.2, 1.2 max.
SD12 3.3 0.104 1.42 5.2, 5.2, 1.2 max.
SD12 4.7 0.118 1.29 5.2, 5.2, 1.2 max.
Sumida Corporation®
CMD411 2.2 0.116 0.950 4.4, 5.8, 1.2 max.
CMD411 3.3 0.174 0.770 4.4, 5.8, 1.2 max.
CMD411 4.7 0.216 0.750 4.4, 5.8, 1.2 max.
Coilcraft®
1008PS 4.7 0.35 1.0 3.8, 3.8, 2.74 max.
1812PS 4.7 0.11 1.15 5.9, 5.0, 3.81 max.
5.6 Thermal Calculations
5.6.1
BUCK REGULATOR OUTPUT
(VOUT1)
The TC1313 is available in two different 10-pin
packages (MSOP and 3X3 DFN). By calculating the
power dissipation and applying the package thermal
resistance, (θJA), the junction temperature is estimated.
The maximum continuous junction temperature rating
for the TC1313 is +125°C.
To quickly estimate the internal power dissipation for
the switching buck regulator, an empirical calculation
using measured efficiency can be used. Given the
measured efficiency (Section 2.0 “Typical Perfor-
mance Curves”), the internal power dissipation is
estimated below.
EQUATION 5-6:
V----O--E--U--f--Tf--i-1-c---×i--e---In--O-c---Uy---T---1-⎠⎞
(VOUT1 × IOUT1)
=
PDissipation
The first term is equal to the input power (definition of
efficiency, POUT/PIN = Efficiency). The second term is
equal to the delivered power. The difference is internal
power dissipation. This estimate assumes that most of
the power lost is internal to the TC1313. There is some
percentage of power lost in the buck inductor, with very
little loss in the input and output capacitors.
© 2005 Microchip Technology Inc.
DS21974A-page 17
 

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