TS19370

Boost (Step-up) WLED Driver with OVP

5/8

Version: D08

Application Information (Continue)

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

LED Dimming Control

There are some different types of dimming control circuits:

1. Using a PWM Signal to SHDN Pin

With the PWM signal applied to the SHDN pin, the TS19370 is turned on or off by the PWM signal. The LEDs operate

at either zero or full current. The average LED current increases proportionally with the duty cycle of the PWM signal. A

0% duty cycle will turn off the TS19370 and corresponds to zero LED current. A 100% duty cycle corresponds to full

current. The typical frequency range of the PWM signal is 1kHz to 10kHz.

The magnitude of the PWM signal should be higher than the minimum SHDN voltage high.

For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED current.

The dimming control using a DC voltage is shown in Figure 4. As the DC voltage increases, the voltage drop on R2

increases and the voltage drop on R1 decreases. Thus, the LED current decreases. The selection of R2 and R3 will

make the current from the variable DC source much smaller than the LED current and much larger than the FB pin bias

current. For VDC range from 0V to 2V, the selection of resistors in Figure 4 gives dimming control of LED current from

0mA to 20mA.

Dimming Control using a DC Voltage

VDC(V)

VFB

Iout(mA)

2

0

0

1.8

2.2

0.43

1.6

9.2

1.8

1.4

19.6

3.6

1.2

31.1

6

1

43.4

8.5

0.8

63

12.3

0.6

74

14.5

0.4

86.4

16.9

0.2

96.9

19

0

102

20