LT1937
W U U
APPLICATIO S I FOR ATIO
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2. Using a DC Voltage
4. Using a Logic Signal
Forsomeapplications,thepreferredmethodofbrightness
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
pinbiascurrent.ForVDC rangefrom0Vto2V,theselection
of resistors in Figure 7 gives dimming control of LED
current from 0mA to 15mA.
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). RINC sets how much the LED current
increases when the NMOS is turned on. The selection of
R1 and RINC 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
anexternalsoft-startcircuit,inrushcurrentisabout200mA
as shown in Figure 10. If soft-start is desired, the recom-
mendedcircuitandthewaveformsareshowninFigure11.
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.
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.
LT1937
FB
LT1937
FB
R3
90k
R2
5k
R
INC
V
DC
LOGIC
SIGNAL
R1
6.3Ω
2N7002
R1
1937 F09
1937 F07
Figure 7. Dimming Control Using a DC Voltage
Figure 9. Dimming Control Using a Logic Signal
IIN
100mA/DIV
LT1937
FB
FB
100mV/DIV
R3
R2
5k
90k
10k
SHDN
2V/DIV
PWM
R1
6.3Ω
0.1µF
VIN = 3.6V
THREE LEDs
15mA
50µs/DIV
1937 F09
1937 F08
Figure 8. Dimming Control Using a Filtered PWM Signal
Figure 10. Start-Up Waveforms Without Soft-Startup Circuit
1937f
7
Linear [ Linear ]
