LT1268BCQ データシートの表示（PDF） - Linear Technology

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LT1268BCQ

7.5A, 150kHz Switching Regulators

Linear Technology 

LT1268B/LT1268

ELECTRICAL CHARACTERISTICS VIN = 15V, VC = 0.5V, VFB = VREF, switch pin open, unless otherwise noted.

SYMBOL

∆IIN

∆ISW

f

DCMAX

PARAMETER

Supply Current Increase

During Switch-ON Time

Switching Frequency

Maximum Switch Duty Cycle

Shutdown Mode Supply Current

Shutdown Mode Threshold Voltage

CONDITIONS

3V ≤ VIN ≤ VMAX, VC = 0.05V

3V ≤ VIN ≤ VMAX

MIN TYP MAX

25

45

120

150

180

q

120

180

65

85

92

100 500

100 150 250

q

50

300

UNITS

mA/A

kHz

kHz

%

µA

mV

mV

The q denotes specifications which apply over the full operating

temperature range.

Note 1: Measured with VC in hi clamp, VFB = 0.8V.

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

switch current is given by ILIM = 6.25 (1.7 – DC).

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.

IMAX =

P

RSW × DC

P = Power dissipation due to switch current

RSW = Switch-ON resistance ≈ 0.15Ω

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

IMAX =

1.5 = 5A

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.

WU

TYPICAL PERFOR A CE CHARACTERISTICS

Switch Current Limit vs Duty Cycle

24

18

12

–55°C

25°C

125°C

6

0

0 10 20 30 40 50 60 70 80 90 100

DUTY CYCLE (%)

LT1268 • TPC01

Switch Saturation Voltage

1.6

TJ = 25°C

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

0 1 2 3 4 5 6 7 8 9 10

SWITCH CURRENT (A)

LT1268 • TPC02

Q Package Thermal Resistance

60

50

SINGLE-SIDED BOARD, 1oz COPPER

40

30

20

DOUBLE-SIDED BOARD, 1oz COPPER

BOTTOM COPPER AREA >100cm2

10

0

0 5 10 15 20 25 30

TOP SURFACE COPPER AREA (cm2)

LT1268 • 603

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.

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