TC115331EMTTR datasheet(6/14 Pages) MICROCHIP

6 / 14 page

TC115

DS21361C-page 6

 2003 Microchip Technology Inc.

5.0

APPLICATIONS

5.1

Input Bypass Capacitors

Using an input bypass capacitor reduces peak current

transients drawn from the input supply and reduces the

switching noise generated by the regulator. The source

impedance of the input supply determines the size of

the capacitor that should be used.

FIGURE 5-1:

TC115 Typical Application.

5.2

Inductor Selection

Selecting the proper inductor value is a trade-off

between physical size and power conversion require-

ments. Lower value inductors cost less, but result in

higher ripple current and core losses. They are also

more prone to saturate since the coil current ramps to

a higher value. Larger inductor values reduce both

ripple current and core losses, but are larger in physical

size and tend to increase the start-up time slightly.

Practical inductor values, therefore, range from 50 µH

to 300 µH. Inductors with a ferrite core (or equivalent)

are recommended. For highest efficiency, use an

inductor with a series resistance less than 0.1

Ω).

The inductor value directly affects the output ripple

voltage. Equation 5-3 is derived as shown below, and

can be used to calculate an inductor value, given the

required output ripple voltage (VRIPPLE) and output

capacitor series resistance:

EQUATION 5-1:

Expressing di in terms of switch ON resistance and

time:

EQUATION 5-2:

Solving for L:

EQUATION 5-3:

Care must be taken to ensure the inductor can handle

peak switching currents, which can be several times

load currents. Exceeding rated peak current will result

in core saturation and loss of inductance. The inductor

should be selected to withstand currents greater than

IPK (Equation 5-10) without saturating.

Calculating the peak inductor current is straightfor-

ward. Inductor current consists of an AC (sawtooth)

current centered on an average DC current (i.e., input

current). Equation 5-6 calculates the average DC cur-

rent. Note that minimum input voltage and maximum

load current values should be used:

EQUATION 5-4:

TC115

SHDN

GND

VOUT

VIN

OFF ON

(Tie to VIN or VOUT

if not used)

Where:

ESR: the equivalent series resistance of the

output filter capacitor; VRIPPLE is in

volts.

di: represents the peak to peak ripple

current in the inductor.

VRIPPLE ESR di

()

≈

VRIPPLE

VIN VSW

–

()tON

[]

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

≈

Where:

VSW = voltage drop across the switch.

TON = the amount of time the switch is ON.

VIN VSW

–

()tON

[]

VRIPPLE

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

≈

Input Power

Output Power

Efficiency

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

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