6

LTC1574

LTC1574-3.3/LTC1574-5

I

I

A

RMS

PEAK

RMS

≈

()

=

2

300mA

Absolute Maximum Ratings and Latchup Prevention

The absolute maximum ratings specify that SW

0.3V. Normally this situation should never occur. It could,

however, if the output is held up while the supply is pulled

down. A condition where this could potentially occur is

when a battery is supplying power to an LTC1574 regula-

tor and also to one or more loads in parallel with the the

LTC1574 regulator is supplying a light load and one of the

parallel circuits is a heavy load, the input capacitor of the

LTC1574 regulator could be pulled down faster than the

output capacitor, causing the absolute maximum ratings

to be exceeded. The result is often a latchup which can be

sible as a result of mechanical stress, bad battery contacts

or use of a lithium-ion battery with a built-in internal

disconnect. The user needs to assess his/her application

to determine whether this situation could occur. If so,

additional protection is necessary.

Prevention against latchup can be accomplished by

simply connecting a Schottky diode across the SW and

than the 0.6V required for latchup. Note that a low

leakage Schottky should be used to minimize the effect

inductor L1 is chosen for its excellent self-shielding prop-

erties. Open magnetic structures such as drum and rod

cores are to be avoided since they inject high flux levels

into their surroundings. This can become a major source

of noise in any converter circuit.

Design Example

the LTC1574-5 is operating in continuous mode.

Figure 6. Continuous Inductor Current

and is also equal to:

I

VV

L

RIPPLE

OUT

D

=

+







−

•

L = 44

µH. The next higher standard value of L is 50µH

(example: Coiltronics CTX50-4). The operating frequency,

f

V

V

kHz

OUT

IN

≈−









=

2 5 10 1

111

5

.•

should be at least:

I

IV

V

V

V

A

mA

RMS

OUT

OUT

IN

OUT

IN

RMS

=

−

()

=

12

174

/

TIME

=

= 350mA

2

1574 • F06

APPLICATIO S I FOR ATIO

1574 F07

LATCHUP

PROTECTION

SCHOTTKY

SW

LTC1574

+

Figure 7. Preventing Absolute Maximum

Ratings from Being Exceeded