TC1303B Datasheet, PDF (19/30 Pages) Microchip Technology – 500 mA Synchronous Buck Regulator, + 300 mA LDO with Power-Good Output

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 accept-
able radiated energy levels. Toroid and shielded ferrite
pot cores will have low radiated energy but tend to be
larger and higher is cost. With a typical 2.0 MHz switch-
ing frequency, the inductor ripple current can be
calculated based on the following formulas.
EQUATION 5-2:
DutyCycle = -^V---^O----^U----^T-
V_IN
Duty cycle represents the percentage of switch-on
time.
EQUATION 5-3:
Where:
T_ON
=
D
ut
y
C
y
cle
×
----¹-----
F_SW
F_SW = Switching Frequency.
The inductor ac ripple current can be calculated using
the following relationship:
EQUATION 5-4:
V_L = L × ^Δ--_Δ---^I-_t-^L-
Where:
V_L = voltage across the inductor (V_IN – V_OUT)
Δt = on-time of P-channel MOSFET
Solving for ΔI_L = yields:
EQUATION 5-5:
ΔI_L
=
-^V---^L-
L
×
Δ
t
When considering inductor ratings, the maximum DC
current rating of the inductor should be at least equal to
the maximum buck regulator load current (I_OUT1), plus
one half of the peak-to-peak inductor ripple current
(1/2 * ΔI_L). 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.
TC1303B
TABLE 5-2: TC1303B RECOMMENDED
INDUCTOR VALUES
Part
Number
Value
(µH)
DCR
Ω
(MAX)
MAX
I_DC (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
(V_OUT1)
The TC1303B is available in two different 10-pin
packages (MSOP and 3X3 DFN). By calculating the
power dissipation and applying the package thermal
resistance, (θ_JA), the junction temperature is estimated.
The maximum continuous junction temperature rating
for the TC1303B 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-⎠⎞}
– (V_OUT1 × I_OUT1)
=
P_Dissipation
The first term is equal to the input power (definition of
efficiency, P_OUT/P_IN = Efficiency). The second term is
equal to the delivered power. The difference is internal
power dissipation. This is an estimate assuming that
most of the power lost is internal to the TC1303B.
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.
DS21949A-page 19