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LT3590

3590f

Programming LED Current

The feedback resistor (R1 in Figure 1) and the sense volt-

age (VIN -VLED) control the LED current.

LED

−

The CTRL pin controls the sense reference voltage as

shown in the Typical Performance Characteristics. For

CTRL higher than 1.5V, the sense reference is 200mV,

which results in full LED current. In order to have accu-

rate LED current, precision resistors are preferred (1%

is recommended). The formula and table for R1 selection

are shown below.

ILED

200

Table 3. R1 Theoretical Value for 200mV Sense

ILED (mA)

R1 (Ω)

6.8

5.0

4.0

Dimming Control

There are three different types of dimming control circuits.

The LED current can be set by modulating the CTRL pin

with a DC voltage, a ﬁltered PWM signal or directly with

a PWM signal.

APPLICATIONS INFORMATION

Using a DC Voltage

For some applications, the preferred method of brightness

control is a variable DC voltage to adjust the LED current.

The CTRL pin voltage can be modulated to set the dim-

ming of the LED string. As the voltage on the CTRL pin

increases from 0V to 1.5V, the LED current increases from

0 to ILED. As the CTRL pin voltage increases beyond 1.5V,

it has no effect on the LED current.

The LED current can be set by:

ILED =

200mV

,when VCTRL > 1.5V

ILED =

VCTRL

6.25 • R1

,when VCTRL < 1.25V

Feedback voltage variation versus control voltage is

shown in Figure 4.

Using a Filtered PWM Signal

A variable duty cycle PWM can be used to control the

brightness of the LED string. The PWM signal is ﬁltered

(Figure 5) by a RC network and fed to the CTRL 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 in the CTRL

pin which is 100kΩ.

Figure 4. Dimming and Shutdown Using CTRL Pin

VCTRL (V)

0.10

0.15

2.0

3590 F04

0.05

0.5

1.0

1.5

0.25

0.20

Figure 5. Dimming Control Using a Filtered PWM Signal

PWM

kHz TYP

CTRL

1μF

10k

3590 F05

LT3590

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