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LT3498

3498f

The LED current can be set by:

LED

SENSE

LED

≈>

≈

200

when VCTRL1

SENSE

625

125

.•

when VCTRL1<

Feedback voltage variation versus control voltage is

given in the Typical Performance Characteristics.

Using a Filtered PWM Signal

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.

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).

APPLICATIONS INFORMATION— LED DRIVER

Programming LED Current

The feedback resistor (RSENSE1) and the sense voltage

(VCAP1 – VLED1) control the LED current. The CTRL1 pin

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 RSENSE selection are shown below.

SENSE

LED

200

Table 4: RSENSE1 Value Selection for 200mV Sense

ILED (mA)

RSENSE1 (Ω)

540

13.3

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 ﬁltered 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 ILED. As the CTRL1 pin voltage increases beyond 1.5V,

it has no effect on the LED current.

Figure 4. Dimming Control Using a Filtered PWM Signal

LT3498

CTRL1

0.1

µF

PWM

10kHz TYP

3498 F04

100k

Ω

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