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

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TC115

300mA CMOS LDO

Microchip Technology 

TC115

Re-writing in terms of input and output currents and

voltages:

EQUATION 4-5:

(VINMIN) (IINMAX) =

(VOUTMAX) (IOUTMAX)

Efficiency

Solving for input curent:

EQUATION 4-6:

IINMAX =

(VOUTMAX) (IOUTMAX)

(Efficiency) (VINMAX)

The sawtooth current is centered on the DC current

level; swinging equally above and below the DC current

calculated in Equation 4-6. The peak inductor current is

the sum of the DC current plus half the AC current.

Note that minimum input voltage should be used when

calculating the AC inductor current (Equation 4-9).

EQUATION 4-7:

V

=

L(di)

dt

EQUATION 4-8:

di

=

V(dt)

dt

EQUATION 4-9:

di = [(VINMIN – VSW)tON]

L

where: VSW = The voltage drop across the internal

N-channel MOSFET.

Combining the DC current calculated in Equation 4-6,

with half the peak AC current calculated in Equation 4-

9, the peak inductor current is given by:

EQUATION 4-10:

IPK = IINMAX + 0.5(di)

4.3 Internal Transistor Switch

The LX pin has a typical ON resistance of 1.4Ω,

therefore peak switch current is given by (VIN/1.4). The

internal transistor switch has a maximum design rating

of 350mA. An oscillator frequency doubling circuit is an

included guard against high switching currents. Should

the voltage on the LX pin rise above 1.3V, max, while

the internal N-channel switch is ON, the oscillator

frequency automatically doubles to minimize ON time.

Although reduced, switch current still flows because

the PWM remains in operation. Therefore, the LX input

is not internally current limited and care must be taken

never to exceed the 350mA maximum limit. Failure to

observe this will result in damage to the regulator.

4.4 Output Capacitor

The effective series resistance of the output capacitor

directly affects the amplitude of the output voltage

ripple. (The product of the peak inductor current and

the ESR determines output ripple amplitude.) There-

fore, a capacitor with the lowest possible ESR should

be selected. Smaller capacitors are acceptable for light

loads or in applications where ripple is not a concern.

The Sprague 595D series of tantalum capacitors are

among the smallest of all low ESR surface mount

capacitors available. Table 4-1 lists suggested

components and suppliers.

4.5 Board Layout Guidelines

As with all inductive switching regulators, the TC115

generates fast switching waveforms which radiate

noise. Interconnecting lead lengths should be

minimized to keep stray capacitance, trace resistance

and radiated noise as low as possible. In addition, the

GND pin, input bypass capacitor and output filter

capacitor ground leads should be connected to a single

point. The input capacitor should be placed as close to

power and ground pins of the TC115 as possible.

TABLE 4-1: SUGGESTED COMPONENTS AND SUPPLIERS

Type

Surface Mount

Through-Hole

Inductors

Sumida

CD54 Series

CDR125 Series

Coiltronics

CTX Series

Sumida

RCH855 Series

RCH110 Series

Renco

RL1284-12

Capacitors

Matsuo

267 Series

Sprague

595D Series

Nichicon

F93 Series

Sanyo

OS-CON Series

Nichicon

PL Series

Diodes

Nihon

EC10 Series

Matsushita

MA735 Series

ON Semiconductor

1N5817 - 1N5822

DS21361B-page 6

© 2002 Microchip Technology Inc.

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