TC115 データシートの表示（PDF） - Microchip Technology

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TC115

300mA CMOS LDO

Microchip Technology 

4.0 APPLICATIONS

4.1 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 4-1:

TC115 TYPICAL

APPLICATION

VIN

+

C1

L1

D1

+

C2

5

4

GND

LX

TC115

NC

PS

SHDN

1

2

3

VOUT

OFF ON

(Tie to VIN or VOUT

if not used)

4.2 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 50µH

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

are recommended. For highest efficiency, use an

inductor with a series resistance less than 20 mΩ).

TC115

The inductor value directly affects the output ripple

voltage. Equation 4-3 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 4-1:

VRIPPLE ≈ ESR(di)

where ESR is the equivalent series resistance of the

output filter capacitor, and VRIPPLE is in volts.

Expressing di in terms of switch ON resistance and

time:

EQUATION 4-2:

VRIPPLE ≈

ESR [(VIN – VSW)tON]

L

Solving for L:

EQUATION 4-3:

L ≈ ESR [(VIN – VSW)tON]

VRIPPLE

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 4-10) without saturating.

Calculating the peak inductor current is straightforward.

Inductor current consists of an AC (sawtooth) current

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

Equation 4-6 calculates the average DC current. Note

that minimum input voltage and maximum load current

values should be used:

EQUATION 4-4:

Output Power

Input Power = Efficiency

© 2002 Microchip Technology Inc.

DS21361B-page 5

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