TS19370_10 Datasheet, PDF (5/8 Pages) Taiwan Semiconductor Company, Ltd – Boost (Step-up) WLED Driver with OVP

TS19370
Boost (Step-up) WLED Driver with OVP
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)
5
R1(Ω)
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)
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
5/8
Version: D08