7

LT1937

1937f

APPLICATIO S I FOR ATIO

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

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

50

µs/DIV

1937 F09

THREE LEDs

15mA

FB

100mV/DIV

SHDN

2V/DIV

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

increases when the NMOS is turned on. The selection of

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.

R2

5k

R1

6.3

Ω

1937 F07

R3

90k

LT1937

FB

PWM

R2

5k

R1

6.3

Ω

0.1

µF

1937 F08

R3

90k

10k

LT1937

FB

LOGIC

SIGNAL

2N7002

R1

1937 F09

LT1937

FB