 2003 Microchip Technology Inc.

DS21361C-page 7

TC115

Rewriting in terms of input and output currents and volt-

ages:

EQUATION 5-5:

Solving for input current:

EQUATION 5-6:

The sawtooth current is centered on the DC current

level, swinging equally above and below the DC current

calculated in Equation 5-6. 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 5-9).

EQUATION 5-7:

EQUATION 5-8:

EQUATION 5-9:

Combining the DC current calculated in Equation 5-6,

with half the peak ac current calculated in Equation 5-9,

the peak inductor current is given by:

EQUATION 5-10:

5.3

Internal Transistor Switch

The LX pin has a typical ON resistance of 1.4

Ω.

The internal transistor switch has a maximum design

rating of 350 mA. An oscillator frequency doubling cir-

cuit is an included guard against high switching cur-

rents. 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 350 mA maximum limit.

Failure to observe this will result in damage to the

regulator.

5.4

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). Therefore, 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).

among the smallest of all low ESR surface mount

capacitors

available.

Table 5-1

lists

suggested

components and suppliers.

5.5

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.

TABLE 5-1:

SUGGESTED COMPONENTS AND SUPPLIERS

MIN

MAX

()

MAX

MA X

()

Efficiency

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

=

MA X

MAX

MAX

()

Efficiency

MIN

()

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

=

Ldi dt

⁄

()

=

di

⁄

()

=

di

MIN

–

L

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

=

Where:

N-channel MOSFET.

MA X

0.5 di

()

+

=

Type

Inductors

Capacitors

Diodes

Surface Mount

CD54 Series

CDR125 Series

Coiltronics™

CTX Series

267 Series

595D Series

Nichicon™

F93 Series

Nihon

EC10 Series

Matsushita™

MA735 Series

Through-Hole

RCH855 Series

RCH110 Series

RL1284-12

Sanyo™

OS-CON Series

Nichicon™

PL Series

1N5817 - 1N5822