TC1313 datasheet(17/28 Pages) MICROCHIP

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DS21974A-page 17

TC1313

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:

Duty cycle represents the percentage of switch-on

time.

EQUATION 5-3:

The inductor ac ripple current can be calculated using

the following relationship:

EQUATION 5-4:

Solving for

ΔIL = yields:

EQUATION 5-5:

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 m

is recommended. Overall efficiency will be improved by

using lower DC resistance inductors.

TABLE 5-2:

TC1313 RECOMMENDED

INDUCTOR VALUES

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:

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.

DutyCycle

OUT

-------------

DutyCycle

----------

Where:

FSW = Switching Frequency.

ΔI

Δt

--------

Where:

VL = voltage across the inductor (VIN – VOUT)

Δt = on-time of P-channel MOSFET

ΔI

------

Δt

Part

Number

Value

(µH)

DCR

(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.

OUT1

Efficiency

-------------------------------------

⎝⎠

⎛⎞

OUT1

()

–

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