AN26

© 2005 by NIDEC COPAL ELECTRONICS CORP.

Characteristics subject to change without notice

4

Document No. 6043, Rev. A

b)

Set the R2 potentiometer to the middle position.

c)

Apply the external voltage supply, Vext (3.0V < VIN < 5V) between VIN (T1) and GND (T2).

d)

The W-6137 is in the shutdown mode (SHDN pin is connected to GND) if the J1 connector is not jumpered (or S1

switch is not pressed).

-

Connect a current meter between Vext and VIN pad to measure the shutdown current: ISD << 1µA

-

In this mode of operation the LEDs are disconnected from the output: LEDs are OFF.

e)

Connect SHDN pin of the W-6137 to VIN using a jumper shunt between Pin #1 and Pin #2 of J1 connector (or press

the S1 switch).

-

Observe that LEDs are ON.

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 VFB = 300mV +/- 15 mV.

The current into the LED pin can be set according to the following equation:

ILED (mA) = VFB (mV) / RSET(ohm) = VFB / (R1 + R2).

-

Set the input voltage: ex. VIN = 3.6V.

-

Disconnect the jumper from the J3 connector and insert a current meter between these pins to monitor the LED

current, ILED.

-

Rotate the potentiometer R2 and observe the ILED value on the current meter. The current can be adjusted

between 3mA and 40mA, approximately. Various LED current values and the associated RSET values are listed

below:

ILED (mA)

RSET (ohm)

559.1

10

29.8

15

20

20

15

25

12

30

10

-

Monitor the voltage on FB (T5), VOUT (T7), and SW (T9).

-

Verify the internal switch frequency (fSW = 1.0MHz – typically) using a scope probe connected on SW (T9) test

point (GND = T8).

Figure 3 shows the internal switch output, VSW (DC coupled, 5V/div) and the regulated voltage on FB pin, VFB (DC,

500mV/div) for VIN = 3.6V, ILED = 20 mA, 3 LEDs in series connected to the W-6137 output.

Figure 4 shows the VOUT (CH2, AC coupled, 100mV/div) against VSW (CH1).