4

LTC1574

LTC1574-3.3/LTC1574-5

to the main voltage comparator input. On the LTC1574-5

and LTC1574-3.3, this pin goes to an internal resistive

divider which sets the output voltage.

1.25V.

Comparator. The (+) input is connected to a reference

voltage of 1.25V.

NC (Pins 1, 8, 9, 16): No Connection.

GND (Pins 2, 4, 13, 15): Ground.

SW (Pins 3, 14): Drain of P-Channel MOSFET Switch and

Cathode of Schottky Diode.

close to ground (Pin 4).

reduced to 340mA.

SHDN (Pin 7): Pulling this pin to ground keeps the internal

switch off and puts the LTC1574 in micropower shutdown.

Operating Frequency and Inductor

Since the LTC1574 utilizes a constant off-time architecture,

frequency of operation can be expressed as:

f

t

VV

VV

Hz

OFF

IN

OUT

IN

D

=

−

+







1

internal Schottky diode. Note that the operating frequency

is a function of the input and output voltage.

Although the size of the inductor does not affect the fre-

quency or inductor peak current, it does affect the ripple

current. The peak-to-peak ripple current is given by:

I

VV

L

RIPPLE

OUT

D

=×

+







−

410

6

By choosing a smaller inductor, a low ESR (Effective Series

Resistance) output filter capacitor has to be used. Core loss

will increase due to higher ripple current.

Short-Circuit Protection

The LTC1574 is protected from output short circuits by its

internal current limit. Depending on the condition of 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 12) which is

compared with a 1.25V reference voltage. With the current

addition, the off-time of the switch is increased to allow the

inductor current to decay far enough to prevent any current

build-up (see Figure 1).

GND

L = 100

µH

20

µs/DIV

1574 • F01

Figure 1. Inductor Current with Output Shorted