LT3497 データシートの表示（PDF） - Linear Technology

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LT3497

Dual Full Function White LED Driver with Integrated Schottky Diodes

Linear Technology 

LT3497

APPLICATIONS INFORMATION

Table 4: RSENSE Value Selection for 200mV Sense

ILED (mA)

RSENSE (Ω)

5

40

10

20

15

13.3

20

10

R1

PWM 100k

10kHz TYP

LT3497

CTRL1,2

C1

0.1µF

3497 F05

Figure 5. Dimming Control Using a Filtered PWM Signal

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:

ILED

≈

200mV

RSENSE

when VCTRL > 1.5V

ILED

≈

VCTRL

6.25 •RSENSE

when VCTRL < 1.25V

Feedback voltage variation versus control voltage is given

in the Typical Performance Characteristics.

Using a Filtered PWM Signal

A filtered 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 CTRL1, CTRL2 pins.

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

pins 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 4/4 white LED driver. Direct

PWM dimming method requires an external NMOS tied

between the cathode of the lowest LED in the string and

ground as shown in Figure 6. Si2318DS MOSFETs can be

used since its sources are connected to ground. The PWM

signal is applied to the (CTRL1 and CTRL2) control pins of

the LT3497 and the gate of the MOSFET. The PWM signal

should traverse between 0V to 5V to ensure proper turn

on and off of the converters and the NMOS transistors (Q1

and Q2). When the PWM signal goes high, LEDs are con-

nected to ground and a current of ILED = (200mV/RSENSE)

flows through the LEDs. When the PWM signal goes low,

the LEDs are disconnected and turn off. The low PWM

input applied to the LT3497 ensures that the respective

3497f

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