8

LTC1174

LTC1174-3.3/LTC1174-5

S

APPLICATI

I FOR ATIO

the diode conducts only a small fraction of the time. The

most stressful condition for the diode is when the output is

short-circuited. Under this condition the diode must safely

diode must also be used to optimize efficiency. Schottky

diodes are a good choice for low forward drop and fast

switching times. Most LTC1174 circuits will be well served

by either a 1N5818, a MBRS140T3 or a MBR0520L Schot-

tky diode.

Short-Circuit Protection

The LTC1174 is protected from output short by its internal

limit is either set to 340mA or 600mA. In addition, the off-

time of the switch is increased to allow the inductor’s

current to decay far enough to prevent any current build-up

(see Figure 2).

current is given by:

I

IV

V

V

V

A

RMS

OUT

OUT

IN

OUT

IN

RMS

≈

−

()

12

/

because even significant deviations do not offer much relief.

Note that ripple current directly affects capacitor’s lifetime.

DO NOT UNDERSPECIFY THIS COMPONENT. An additional

0.1

frequency decoupling.

To avoid overheating, the output capacitor must be sized to

handle the ripple current generated by the inductor. The

worst case RMS ripple current in the output capacitor is

given by:

I

I

A

mA

RMS

PEAK

RMS

≈

()

=

2

170

or 300mA

Although the output voltage ripple is determined by the

hysteresis of the voltage comparator, ESR of the output

capacitor is also a concern. Too high of an ESR will create

a higher ripple output voltage and at the same time cause the

LTC1174 to sleep less often. This will affect the efficiency of

the LTC1174. For a given technology, ESR is a direct

function of the volume of the capacitor. Several small-sized

capacitors can also be paralleled to obtain the same ESR as

one large can. Manufacturers such as Nichicon, Chemicon

and Sprague should be considered for high performance

capacitors. The OS-CON semiconductor dielectric capaci-

tor available from Sanyo has the lowest ESR for its size, at

a higher price.

Catch Diode Selection

The catch diode carries load current during the off-time. The

average diode current is therefore dependent on the

P-channel switch duty cycle. At high input voltages the

Low-Battery Detector

The low-battery indicator senses the input voltage through

an external resistive divider. This divided voltage connects

to the “–” input of a voltage comparator (pin 3) which is

compared with a 1.25V reference voltage. With the current

going into pin 3 being negligible, the following expression

is used for setting the trip limit:

V

R

R









125 1

4

3

.

GND

L = 100

µH

20

µs/DIV

1174 F02

Figure 2. Inductor's Current with Output Shorted