LT3517(RevA) 데이터 시트보기 (PDF) - Linear Technology

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LT3517
(Rev.:RevA)

Full-Featured LED Driver with 1.5A Switch Current

Linear Technology 

LT3517

APPLICATIONS INFORMATION

than 1V, the LED current is:

ILED

=

10

VCTRL

• RSENSE

(2)

When VCTRL is higher than 1V, the LED current is clamped

to be:

ILED

=

100mV

RSENSE

(3)

The LED current programming feature possibly increases

total dimming range by a factor of ten.

The CTRL pin should not be left open (tie to VREF if not

used). The CTRL pin can also be used in conjunction with

a PTC thermistor to provide overtemperature protection

for the LED load.

VREF

2V

45.3k

5k

PTC

49.9k

CTRL

3517 F02

Figure 2

Setting Output Voltage

For a boost application, the output voltage can be set by

selecting the values of R1 and R2 (see Figure 3) according

to the following equation:

VOUT

=

⎛

⎝⎜

R1

R2

+

1⎞⎠⎟

•

1.01V

(4)

VOUT

LT3517

R1

FB

R2

3517 F03

Figure 3

For a buck or a buck-boost configuration, the output

voltage is typically level-shifted to a signal with respect

to GND as illustrated in the Figure 4. The output can be

expressed as:

VOUT

=

R1

R2

•

1.01V

+

VBE(Q2)

(5)

+

R1

VOUT

LT3517 Q2

–

FB

R2

3517 F04

RSENSE

LED

ARRAY

Figure 4

Inductor Selection

The inductor used with the LT3517 should have a satura-

tion current rating of 2A or greater. For buck mode LED

drivers, the inductor value should be chosen to give a

ripple current “ΔI” of ~30% to 40% of the LED current.

In the buck mode, the inductor value can be estimated

using the formula:

( ) L(µH) = DBUCK • tSW(µs) • VIN – VLED

ΔI

(6)

DBUCK

=

VLED

VIN

VLED is the voltage across the LED string, VIN is the input

voltage to the converter, and tSW is the switching period.

In the boost configuration, the inductor can be estimated

using the formula:

L(µH) = DBOOST • tSW(µs) • VIN

ΔI

(7)

DBOOST

=

VLED – VIN

VLED

3517fa

9

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