LT1937 LINER [Linear Technology], LT1937 Datasheet - Page 11

• Manufacturer Part Number: LT1937
• Description: 20mA LED Driver and OLED Driver with Integrated Schottky in 3mm x 2mm DFN
• Manufacturer: LINER [Linear Technology]

Programming LED Current
The feedback resistor (R
(V
controls the sense reference voltage as shown in the
Typical Performance Characteristics. For CTRL1 higher
than 1.5V, the sense reference is 200mV, which results in
full LED current. To have accurate LED current, precision
resistors are preferred (1% is recommended). The formula
and table for R
Table 4: R
Dimming Control
There are three different types of dimming control circuits.
The LED current can be set by modulating the CTRL1 pin
with a DC voltage, a fi ltered PWM signal or directly with
a PWM signal.
Using a DC Voltage
For some applications, the preferred method of brightness
control is a variable DC voltage to adjust the LED current.
The CTRL1 pin voltage can be modulated to set the dim-
ming of the LED string. As the voltage on the CTRL1 pin
increases from 0V to 1.5V, the LED current increases from
0 to I
it has no effect on the LED current.
APPLICATIONS INFORMATION— LED DRIVER
CAP1
R
SENSE
LED
– V
. As the CTRL1 pin voltage increases beyond 1.5V,
SENSE1
1
I
LED1
LED
10
15
20
5
200
(mA)
Value Selection for 200mV Sense
SENSE
) control the LED current. The CTRL1 pin
I
LED
mV
selection are shown below.
SENSE1
) and the sense voltage
R
SENSE1
13.3
40
20
10
(Ω)
The LED current can be set by:
Feedback voltage variation versus control voltage is
given in the Typical Performance Characteristics.
Using a Filtered PWM Signal
A fi ltered PWM signal can be used to control the bright-
ness of the LED string. The PWM signal is fi ltered (Figure 4)
by a RC network and fed to the CTRL1 pin.
The corner frequency of R1, C1 should be much lower
than the frequency of the PWM signal. R1 needs to be
much smaller than the internal impedance of the CTRL1
pin which is 10MΩ (typ).
I
I
LED
LED
Figure 4. Dimming Control Using a Filtered PWM Signal
R
6 25
200
10kHz TYP
.
SENSE
PWM
V
mV
•
CT
R
1
R R L
SENSE
,
1
when V
100k
R1
1
,
when V
CTRL1
C1
0.1 F
CTRL1
1 5
.
V
CTRL1
LT3498
1 25
3498 F04
LT3498
.
V
11
3498f