LT3497 datasheet(13/20 Pages) LINER

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LT3497

3497f

time to settle to its ﬁnal value. Figure 9 shows the avail-

able dimming range for different PWM frequencies with

a settling time of 40μs.

Figure 9. Dimming Ratio vs Frequency

lower battery voltage. This technique allows the LEDs to

be powered off two alkaline cells. Most portable devices

have a 3.3V supply voltage which can be used to power

the LT3497. The LEDs can be driven straight from the

battery, resulting in higher efﬁciency.

Figure 11 shows 3/3 LEDs powered by two AA cells.

The battery is connected to the inductors and the chip is

powered off a 3.3V logic supply voltage.

The dimming range can be further extended by changing

the amplitude of the PWM signal. The height of the PWM

signal sets the commanded sense voltage across the sense

resistor through the CTRL pin. In this manner both analog

dimming and direct PWM dimming extend the dimming

range for a given application. The color of the LEDs no

longer remains constant because the forward current of

the LED changes with the height of the CTRL signal. For

the 4-LED application described above, the LEDs can be

dimmed ﬁrst, modulating the duty cycle of the PWM signal.

Once the minimum duty cycle is reached, the height of the

PWM signal can be decreased below 1.5V down to 100mV.

The use of both techniques together allows the average LED

current for the 4-LED application to be varied from 20mA

down to less than 20µA. Figure 10 shows the application

for dimming using both analog dimming and PWM dim-

ming. A potentiometer must be added to ensure that the

gate of the NMOS receives a logic-level signal, while the

CTRL signal can be adjusted to lower amplitudes.

LOW INPUT VOLTAGE APPLICATIONS

The LT3497 can be used in low input voltage applica-

tions. The input supply voltage to the LT3497 must be

2.5V or higher. However, the inductors can be run off a

Figure 11. 2 AA Cells to 3/3 White LEDs

APPLICATIONS INFORMATION

PWM FREQUENCY (Hz)

100

1000

10000

100

1000

10000

3497 F09

PULSING MAY BE VISIBLE

SW1

VIN

LT3497

GND

SW2

µH

RSENSE1

Ω

Si2318DS

RSENSE2

Ω

µF

3497 F10

3V TO 5V

µH

CAP1

CAP2

LED1

LED2

CTRL1

CTRL2

µF

100k

PWM

FREQ

PWM

FREQ

Figure 10. Li-Ion to 4/4 White LEDs with Both PWM Dimming

and Analog Dimming

SW1

VIN

LT3497

GND

SW2

µH

RSENSE2

Ω

RSENSE1

Ω

µF

C1, C2: TAIYO YUDEN LMK212BJ105MG

C3, C4: TAIYO YUDEN GMK212BJ105KG

L1, L2: MURATA LQH32CN150K53

µF

3497 F11

3.3V

µH

2 AA CELLS

2V TO 3.2V

CAP1

CAP2

LED1

LED2

CTRL1

SHUTDOWN

AND DIMMING

CONTROL 1

SHUTDOWN

AND DIMMING

CONTROL 2

CTRL2

OFF ON

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