LT3591 datasheet(9/16 Pages) LINER

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LT3591

3591f

APPLICATIONS INFORMATION

Table 4. RSENSE Value Selection for 200mV Sense

ILED (mA)

RSENSE (Ω)

540

13.3

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.

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:

when V

LED

SENSE

CTRL

LED

CTRL

≈>

≈

200

125

•

when V

SENSE

CTRL <

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

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 of the CTRL

pin which is 10MΩ (typ).

Direct PWM Dimming

Changing the forward current ﬂowing in the LEDs not only

changes the intensity of the LEDs, it also changes the color.

The chromaticity of the LEDs changes with the change in

forward current. Many applications cannot tolerate any

shift in the color of the LEDs. Controlling the intensity of

the LEDs with a direct PWM signal allows dimming of the

LEDs without changing the color. In addition, direct PWM

dimming offers a wider dimming range to the user.

Dimming the LEDs via a PWM signal essentially involves

turning the LEDs on and off at the PWM frequency. The

typical human eye has a limit of ~60 frames per second.

By increasing the PWM frequency to ~80Hz or higher,

the eye will interpret that the pulsed light source is con-

tinuously on. Additionally, by modulating the duty cycle

(amount of “on-time”), the intensity of the LEDs can be

controlled. The color of the LEDs remains unchanged in

this scheme since the LED current value is either zero or

a constant value.

Figure 6 shows a Li-Ion powered driver for ten white LEDs.

Direct PWM dimming method requires an external NMOS

tied between the cathode of the lowest LED in the string

LT3591

CTRL

0.1

µF

PWM

10kHz TYP

3591 F05

100k

Figure 5. Dimming Control Using a Filtered PWM Signal

CTRL

PWM

FREQ

VIN

µH

VIN

3V TO

RSENSE

Ω

LT3591

CAP

LED

100k

GND

2.2

µF

Si2308

µF

Figure 6. Li-Ion to Ten White LEDs with Direct PWM Dimming

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