PT6937 Datasheet, PDF (6/15 Pages) Princeton Technology Corp – White LED Step-Up Converter

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White LED Step-Up Converter
PT6937
USING A DC VOLTAGE
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 the figure below. 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
5V, the selection of resistors in Figure 3 gives dimming control of LED current from 0mA to 15mA.
V IN
>2V
0V
L1
22µ H
D4
1 N58 19
C1
1µF ⁵
1
VIN
SW
4
P T 6937
3
/SHDN
FB
GND
2
D1
WLED
D2
WLED
D3
WLED
R1
4.7
VIN
C2
0.22 µF
L1
22µH
D4
1N5819
C1
1µF
5
1
VIN
SW
R2
4
PT6937
3
1K
/SHDN FB
GND
R3
51K
2
DC 0~5V
D1
WLED
D2
WLED
D3
WLED
R1
4.7
C2
0.22µF
Figure 2. PWM Dimming control Using the /SHDN Figure 3. Dimming Control Using DC Voltage
Pin
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 in Figure 4.
L1
22µH
D4
1N5819
VIN
C1
1µF
5
1
VIN
SW
R2
4
PT6937
3
5K
/SHDN FB
D1
WLED
D2
WLED
D3
WLED
C2
0.22µF
GND
2
R3
R1
90K
4.7
PWM 0~5V
C3
0.1µF
R4
10K
Figure 4. Dimming Control Using Filtered PWM Signal.
PT6937 V1.1
-6-
March, 2006