LT3498 Просмотр технического описания (PDF) - Linear Technology

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LT3498

20mA LED Driver and OLED Driver with Integrated Schottky in 3mm x 2mm DFN

Linear Technology 

LT3498

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.

RSENSE1

=

200mV

ILED

Table 4: RSENSE1 Value Selection for 200mV Sense

ILED (mA)

5

RSENSE1 (Ω)

40

10

20

15

13.3

20

10

The LED current can be set by:

ILED

≈

200mV

RSENSE1

, when

VCTRL1 > 1.5V

ILED

≈

VCTRL1

6.25 • RSENSE1

, when

VCTRL1 < 1.25V

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).

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.

PWM

10kHz TYP

R1

100kΩ

LT3498

C1

0.1μF

CTRL1

3498 F04

Figure 4. Dimming Control Using a Filtered PWM Signal

3498fa

11

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