Application Note 32

Doc. No. 6050 Rev. A

4

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2)

LED Current Evaluation

a) Programming LED current

LED

current

is

programmed

using

the

external

resistors,

RSET

=

R1

+

R2,

connected to the FB pin. The voltage at the

FB pin is internally regulated to the value

The current into the LED pin can be set

according to the following equation:

–

–

Disconnect the jumper from the J4,

connector and insert a current meter

between these pins to monitor the LED

–

Rotate the potentiometer R2 and observe

current can be adjusted between 3mA

and 30mA, approximately. Various LED

current values and the associated RSET

values are listed below:

RSET ( )

559

10

29.8

15

20

20

15

25

12

30

10

–

and SW (T9).

–

Verify

the

internal

switch

frequency

probe connected on SW (T9) test point

(GND = T8).

coupled, 5V/div) and the regulated voltage on FB pin,

LEDs in series connected to the W-6238 output.

Figure 3. Internal Switch Output and Regulated

b) Evaluating LED current regulation versus the

input voltage, VIN.

–

the R2 potentiometer (i.e. 10mA, 20mA,

–

between 3.0V and 5.5V

–

W-6238

FB

voltage

versus

input

voltage.

W-6238 LED Current versus Input Voltage

(10 LEDs in series)

0

5

10

15

20

25

30

35

3.00

3.50

4.00

4.50

5.00

5.50

VIN (V)

RSET = 29.8ohm

RSET = 15 ohm

RSET = 10 ohm

W-6238 FB Voltage versus Input Voltage

200

210

220

230

240

250

260

270

280

290

300

310

3.00

3.50

4.00

4.50

5.00

5.50

VIN (V)

ILED = 10 mA

ILED = 20 mA

ILED = 30 mA

Note: RSET = R1 + R2

Figure 4. LED current Line Regulation

()

2

R

+

1

R

v

=

)

(

RSET

)

mV

(

V

=

mA

I

FB

FB

LED