TS1937

Boost (Step-up) WLED Driver

6/8

Version: C07

Application Information (Continue)

2. Using a Filtered PWM Signal

The filtered PWM signal can be considered as an adjustable DC voltage. It can be used to replace the variable DC

voltage source in dimming control. The circuit is shown as follow:

Dimming Control using a Filtered PWM Signal

LED Current Control

The LED current is controlled by the feedback resistor (R1). The feedback reference is 95mV. The LED current is

95mV/R1. In order to have accurate LED current, precision resistors are preferred (1% is recommended). The formula

and table 3 for R1 selection are shown below.

R1 = 95mV/ILED

ILED (mA)

R1(Ω)

5

19.1

10

9.53

12

7.87

15

6.34

20

4.75

Duty

Iout ( mA)

0%

20

20%

17.4

30%

15

40%

12.4

50%

10

60%

8

70%

6.4

80%

4.07

100%

0

PWM : 2V ; 1KHz ; Vin=3.6V