TS19371 Datasheet, PDF (6/8 Pages) Taiwan Semiconductor Company, Ltd – Boost (Step-up) WLED Driver with OVP

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
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Version: F08