3

LT1268B/LT1268

Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However,

no responsibility is assumed for its use. Linear Technology Corporation makes no representation that

the interconnection of its circuits as described herein will not infringe on existing patent rights.

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Supply Current Increase

25

45

mA/A

During Switch-ON Time

f

Switching Frequency

120

150

180

kHz

q

120

180

kHz

Maximum Switch Duty Cycle

65

85

92

%

Shutdown Mode Supply Current

3V

100

500

µA

Shutdown Mode Threshold Voltage

3V

100

150

250

mV

q

50

300

mV

ELECTRICAL C

C

HARA TERISTICS

temperature range.

Note 2: For duty cycles (DC) between 50% and 65%, minimum guaranteed

Note 3: Minimum current limit is reduced by 0.5A at 125

°C. 100°C test

limits are guaranteed by correlation to 125

°C tests.

Note 4: LT1268B reference voltage is specified at

±9mV to guarantee ±1%

output voltage accuracy when 0.1% external resistors are used to set

output voltage. To maintain output accuracy under load, load current

should be taken from the case and the ground pin should be connected

separately to output ground. See AN19 for details.

Note 5: The Q package is intended for surface mount without a separate

heat sink. See graph for thermal resistance as a function of the mounting

area. This curve assumes no other heat dissipators adjacent to package.

Note 6: Maximum switch current may be limited by package power

dissipation, especially for the surface mount (Q) package. This package

has a thermal resistance of 20

°C/W to 50°C/W (see graph). The following

formula will allow an estimate of maximum continuous switch current as a

function of power loss and duty cycle. See AN19 for more details.

P

P = Power dissipation due to switch current

DC = Switch duty cycle

In a typical application where thermal resistance is 30

°C/W, maximum

power might be limited to 2W and power allocated to switch loss is 1.5W.

For a duty cycle of 40%, this yields

= 5A

1.5

0.15

× 0.4

Obviously, a combination of high thermal resistance and high duty cycle

may restrict switch current to a value well below the 7.5A electrical limit.

TYPICAL PERFOR A CE CHARACTERISTICS

DUTY CYCLE (%)

0

18

24

80

LT1268 • TPC01

12

6

0

20

40

60

100

70

10

30

50

90

–55°C

125°C

25°C

Switch Current Limit vs Duty Cycle

0

0

10

20

30

40

60

5

10

15

20

LT1268 • 603

25

30

50

SINGLE-SIDED BOARD, 1oz COPPER

DOUBLE-SIDED BOARD, 1oz COPPER

SWITCH CURRENT (A)

0

1.2

1.6

8

LT1268 • TPC02

0.8

0.4

1.0

1.4

0.6

0.2

0

2

4

6

10

7

1

3

5

9

Switch Saturation Voltage

Q Package Thermal Resistance