LT1937ES5 Linear Technology, LT1937ES5 Datasheet - Page 7

• Manufacturer Part Number: LT1937ES5
• Manufacturer: Linear Technology

Specifications of LT1937ES5

• Operating Supply Voltage (typ): 3.3/5/9V
• Number Of Segments: 6
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Package Type: TSOT-23
• Pin Count: 5
• Mounting: Surface Mount
• Operating Supply Voltage (min): 2.5V
• Operating Supply Voltage (max): 10V
• Lead Free Status / Rohs Status: Not Compliant

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
pin bias current. For V
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 8. Dimming Control Using a Filtered PWM Signal
Figure 7. Dimming Control Using a DC Voltage
PWM
10k
V
DC
U
DC
0.1 F
90k
90k
R3
R3
range from 0V to 2V, the selection
U
LT1937
LT1937
FB
FB
R2
R2
5k
5k
W
R1
6.3
R1
6.3
1937 F07
1937 F08
U
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
increases when the NMOS is turned on. The selection of
R1 and R
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.
Figure 10. Start-Up Waveforms Without Soft-Startup Circuit
100mA/DIV
100mV/DIV
2V/DIV
SHDN
Figure 9. Dimming Control Using a Logic Signal
I
FB
IN
INC
SIGNAL
V
THREE LEDs
15mA
LOGIC
IN
= 3.6V
follows formula (1) and Table 4.
INC
2N7002
sets how much the LED current
R
INC
50 s/DIV
LT1937
FB
R1
1937 F09
1937 F09
LT1937
7
1937f