7

LT1303/LT1303-5

Inductor Section

Inductors suitable for use with the LT1303 usually fall in

the 5

µH to 50µH range. The inductor must: (1) handle

current of 1.25A without saturating, (2) have enough

inductance to provide a di/dt lower than 400mA/

µs, and

(3) have low enough DC resistance to avoid excessive

heating or efficiency losses. Higher value inductors will

deliver more power but tend to be physically larger. Most

ferrite core drum or rod inductors such as those specified

in Table 1 are suitable for use. It is acceptable to bias open-

flux inductors (e.g. Sumida CD54) into saturation by 10 to

20% without adverse effects.

hysteresis built in, but hysteresis can be added by

connecting a high-value resistor from LBI to LBO as

shown in Figure 5. The internal reference can be accessed

via the comparator as shown in Figure 6.

Figure 7 shows inductor current of a suitable inductor,

di/dt is controlled at all times. The rapid rise in current

shown in Figure 8 results from this inductor saturating at

approximately 1A. Saturation occurs when the inductor

cannot hold any more magnetic energy in the core. Current

then increases rapidly, limited only by the resistance of the

winding. Figure 9’s inductor has high DC resistance which

results in the exponential time constant shape of the

inductor current.

Figure 7. Properly Chosen Inductor Does Not Saturate

500mA/DIV

5

µs/DIV

LT1303 F07

Figure 6. Accessing Internal Reference

to about 2

µs. Response time of C2, which determines

minimum on-time, is approximately 300ns.

Low Battery Detector

The low battery detector is enabled when SHDN is low and

disabled when SHDN is high. The comparator has no

Figure 5. R3 Adds Hysteresis to Low-Battery Detector

–

+

1.24V

LT1303

R4

47k

R1

R3

2M

R2

49.9k

1%

5V

HYSTERESIS

≈ 30mV

LT1303 F05

PHONE

VENDOR

SERIES

APPROPRIATE VALUES NUMBERS

Coilcraft

D03316

10

µH to 47µH

(708) 639-6400

D01608

10

µH

Coiltronics

OCTAPAK

(407) 241-7876

CTX20-1

20

µH

CTX20-2

20

µH

CTX33-4

33

µH

Sumida

CD54

10

µH to 33µH

(708) 956-0666

Gowanda

GA10

10

µH to 33µH

(716) 532-2234

Table 1. Recommended Inductors

GND

LBO

LBI

LT1303

+

100k

R2

R1

2.2

µF

2N3906

OUTPUT

R1 + R2

≈ 33k

R2

R1

()

LT1303 F06