LT3474 Просмотр технического описания (PDF) - Linear Technology

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LT3474

Step-Down 1A LED Driver

Linear Technology 

LT3474/LT3474-1

APPLICATIONS INFORMATION

Inductor Selection and Maximum Output Current

A good first choice for the inductor value is

L

=

(VOUT

+

VF ) •

900kHz

f

where VF is the voltage drop of the catch diode (~0.4V), f

is the switching frequency and L is in μH. With this value

the maximum load current will be 1.1A, independent of

input voltage. The inductor’s RMS current rating must be

greater than the maximum load current and its saturation

current should be at least 30% higher. For highest efficiency,

the series resistance (DCR) should be less than 0.2Ω.

Table 2 lists several vendors and types that are suitable.

For robust operation at full load and high input voltages

(VIN > 30V), use an inductor with a saturation current

higher than 2.5A.

Table 2. Inductors

VALUE

IRMS

PART NUMBER

(μH)

(A)

Sumida

CR43-3R3

3.3

1.44

CR43-4R7

4.7

1.15

CDRH4D16-3R3

3.3

1.1

CDRH4D28-3R3

3.3

1.57

CDRH4D28-4R7

4.7

1.32

CDRH5D28-100

10

1.3

CDRH5D28-150

15

1.1

CDRH73-100

10

1.68

CDRH73-150

15

1.33

Coilcraft

DO1606T-332

3.3

1.3

DO1606T-472

4.7

1.1

DO1608C-332

3.3

2

DO1608C-472

4.7

1.5

MOS6020-332

3.3

1.8

MOS6020-472

10

1.5

DCR

HEIGHT

(Ω)

(mm)

0.086

3.5

0.109

3.5

0.063

1.8

0.049

3

0.072

3

0.048

3

0.076

3

0.072

3.4

0.13

3.4

0.1

2

0.12

2

0.08

2.9

0.09

2.9

0.046

2

0.05

2

The optimum inductor for a given application may differ

from the one indicated by this simple design guide. A

larger value inductor provides a higher maximum load

current, and reduces the output voltage ripple. If your

load is lower than the maximum load current, then you

can relax the value of the inductor and operate with higher

ripple current. This allows you to use a physically smaller

inductor, or one with a lower DCR resulting in higher

efficiency. Be aware that if the inductance differs from

the simple rule above, then the maximum load current

will depend on input voltage. In addition, low inductance

may result in discontinuous mode operation, which further

reduces maximum load current. For details of maximum

output current and discontinuous mode operation, see

Linear Technology’s Application Note 44. Finally, for duty

cycles greater than 50% (VOUT/VIN > 0.5), a minimum

inductance is required to avoid sub-harmonic oscillations.

See Application Note 19.

The current in the inductor is a triangle wave with an average

value equal to the load current. The peak switch current

is equal to the output current plus half the peak-to-peak

inductor ripple current. The LT3474 limits its switch cur-

rent in order to protect itself and the system from overload

faults. Therefore, the maximum output current that the

LT3474 will deliver depends on the switch current limit,

the inductor value, and the input and output voltages.

When the switch is off, the potential across the inductor

is the output voltage plus the catch diode drop. This gives

the peak-to-peak ripple current in the inductor

ΔIL

=

(1–

DC) ( VOUT

(L • f)

+

VF

)

where f is the switching frequency of the LT3474 and L

is the value of the inductor. The peak inductor and switch

current is

ISW (PK )

=IL(PK)

=IOUT

+

ΔIL

2

3474fd

11

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