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TS19371

Boost (Step-up) WLED Driver with OVP

TSC Corporation 

TS19371

Boost (Step-up) WLED Driver with OVP

Application Information (Continue)

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 TS19371 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 TS19371 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 below figure. 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)

2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

0

VFB

0

2.2

9.2

19.6

31.1

43.4

63

74

86.4

96.9

102

Iout(mA)

0

0.43

1.8

3.6

6

8.5

12.3

14.5

16.9

19

20

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

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

3. Using Control pin for dimming

The PWM signal can be considered as TTL signal to control pin to change the power on/off time.

7/10

Version: G08

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