LT3517

9

3517ff

APPLICATIONS INFORMATION

than 1V, the LED current is:

(2)

to be:

100mV

(3)

The LED current programming feature possibly increases

total dimming range by a factor of ten.

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

a PTC thermistor to provide overtemperature protection

for the LED load.

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:

R1

R2

• 1.01V

(5)

49.9k

3517 F02

45.3k

2V

5k

PTC

CTRL

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:

R1

R2

+1

⎛

⎝

⎜

⎞

⎠

⎟ •1.01V

(4)

LT3517

FB

R1

R2

3517 F03

Figure 3

LT3517

FB

R1

Q2

+

–

LED

ARRAY

R2

3517 F04

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:

LμH

()

ΔI

(6)

In the boost configuration, the inductor can be estimated

using the formula:

LμH

ΔI

(7)