LT3498
APPLICATIONS INFORMATION—LED DRIVER
Programming LED Current
The LED current can be set by:
The feedback resistor (R
) and the sense voltage
200mV
SENSE1
ILED
≈
≈
, when VCTRL1 >1.5V
(V
– V
) control the LED current. The CTRL1 pin
CAP1
LED1
RSENSE1
VCTRL1
6.25•RSENSE1
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
resistorsarepreferred(1%isrecommended).Theformula
ILED
, when VCTRL1 <1.25V
Feedback voltage variation versus control voltage is
given in the Typical Performance Characteristics.
and table for R
selection are shown below.
SENSE
200mV
ILED
Using a Filtered PWM Signal
RSENSE1
=
A filtered PWM signal can be used to control the bright-
ness of the LED string. The PWM signal is filtered (Figure 4)
by a RC network and fed to the CTRL1 pin.
Table 4: R
Value Selection for 200mV Sense
SENSE1
(mA)
I
R
(Ω)
LED
SENSE1
5
40
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).
10
15
20
20
13.3
10
Dimming Control
Therearethreedifferenttypesofdimmingcontrolcircuits.
The LED current can be set by modulating the CTRL1 pin
with a DC voltage, a filtered PWM signal or directly with
a PWM signal.
LT3498
R1
100kΩ
PWM
10kHz TYP
CTRL1
3498 F04
C1
0.1µF
Using a DC Voltage
Forsomeapplications,thepreferredmethodofbrightness
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
Figure 4. Dimming Control Using a Filtered PWM Signal
0toI . AstheCTRL1pinvoltageincreasesbeyond1.5V,
LED
it has no effect on the LED current.
3498f
11
Linear [ Linear ]
