LT1303 データシートの表示（PDF） - Linear Technology

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LT1303

Micropower High Efficiency DC/DC Converters with Low-Battery Detector Adjustable and Fixed 5V

Linear Technology 

U

OPERATION

to about 2µs. 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

VBAT

R1

LT1303

1.24V –

5V

R4

47k

R2

49.9k

1%

+

R3

2M

R1 = (VTRIP –1.24V) (43.5k)

HYSTERESIS ≈ 30mV

LT1303 F05

Figure 5. R3 Adds Hysteresis to Low-Battery Detector

LT1303/LT1303-5

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.

VIN

2N3906

VREF

OUTPUT +

100k

VIN

LBO

R2

2.2µF

LT1303

LBI

R1 GND

( ) VREF = 1.24V

1 + R2

R1

VIN ≥ VREF + 200mV

R1 + R2 ≈ 33k

LT1303 F06

Figure 6. Accessing Internal Reference

APPLICATIONS INFORMATION

Inductor Section

Inductors suitable for use with the LT1303 usually fall in

the 5µH 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.

Table 1. Recommended Inductors

VENDOR

SERIES

PHONE

APPROPRIATE VALUES NUMBERS

Coilcraft

Coiltronics

D03316

D01608

OCTAPAK

CTX20-1

CTX20-2

CTX33-4

10µH to 47µH

10µH

20µH

20µH

33µH

(708) 639-6400

(407) 241-7876

Sumida

CD54

10µH to 33µH

(708) 956-0666

Gowanda GA10

10µH to 33µH

(716) 532-2234

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.

IL

500mA/DIV

5µs/DIV

LT1303 F07

Figure 7. Properly Chosen Inductor Does Not Saturate

7

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