7
LT1937
1937f
APPLICATIO S I FOR ATIO
WUUU
2. 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
Figure␣ 7. 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 V
DC
range from 0V to 2V, the selection
of resistors in Figure 7 gives dimming control of LED
current from 0mA to 15mA.
3. Using a Filtered PWM Signal
The filtered PWM signal can be considered as an adjust-
able DC voltage. It can be used to replace the variable DC
voltage source in dimming control. The circuit is shown in
Figure 8.
Figure 7. Dimming Control Using a DC Voltage
Figure 8. Dimming Control Using a Filtered PWM Signal
Figure 9. Dimming Control Using a Logic Signal
V
IN
= 3.6V 50µs/DIV 1937 F09
THREE LEDs
15mA
FB
100mV/DIV
SHDN
2V/DIV
I
IN
100mA/DIV
Figure 10. Start-Up Waveforms Without Soft-Startup Circuit
4. Using a Logic Signal
For applications that need to adjust the LED current in
discrete steps, a logic signal can be used as shown in
Figure 9. R1 sets the minimum LED current (when the
NMOS is off). R
INC
sets how much the LED current
increases when the NMOS is turned on. The selection of
R1 and R
INC
follows formula (1) and Table 4.
Start-up and Inrush Current
To achieve minimum start-up delay, no internal soft-start
circuit is included in LT1937. When first turned on without
an external soft-start circuit, inrush current is about 200mA
as shown in Figure␣ 10. If soft-start is desired, the recom-
mended circuit and the waveforms are shown in Figure 11.
If both soft-start and dimming are used, a 10kHz PWM
signal on SHDN is not recommended. Use a lower fre-
quency or implement dimming through the FB pin as
shown in Figures 7, 8 or 9.
V
DC
R2
5k
R1
6.3Ω
1937 F07
R3
90k
LT1937
FB
PWM
R2
5k
R1
6.3Ω
0.1µF
1937 F08
R3
90k
10k
LT1937
FB
LOGIC
SIGNAL
2N7002
R1
1937 F09
R
INC
LT1937
FB
