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LT3491ESC8

White LED Driver with Integrated Schottky in SC70 and 2mm × 2mm DFN

Linear Technology 

LT3491

APPLICATIO S I FOR ATIO

The time it takes for the LED current to reach its pro-

grammed value sets the achievable dimming range for a

given PWM frequency. For example, the settling time of

the LED current in Figure 7 is approximately 30µs for a 3V

input voltage. The achievable dimming range for this

application and 100Hz PWM frequency can be determined

using the following method.

Example:

down to 100mV. The use of both techniques together

allows the average LED current for the four LED applica-

tion to be varied from 20mA down to less than 20µA.

Figure 9 shows the application for dimming using both

analog dimming and PWM dimming. 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.

ƒ = 100Hz, tSETTLE = 30µs

tPERIOD

=

1

ƒ

=

1

100

=

0.01s

Dim Range = tPERIOD = 0.01s = 300:1

tSETTLE 30µs

Min Duty Cycle = tSETTLE • 100 = 30µs • 100 = 0.3%

tPERIOD

0.01s

Duty Cycle Range = 100%→ 0.3% at 100Hz

The calculations show that for a 100Hz signal the dimming

range is 300 to 1. In addition, the minimum PWM duty

cycle of 0.3% ensures that the LED current has enough

time to settle to its final value. Figure 8 shows the dimming

range achievable for three different frequencies with a

settling time of 30µs.

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 four 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

1kHz

10kHz

100Hz

1

10

100

1000

PWM DIMMING RANGE

3491 F08

Figure 8. Dimming Range Comparison

of Three PWM Frequencies

VIN

3V TO 5V

L1

10µH

C1

1µF

VIN

SW

CAP

LT3491

GND

LED

CTRL

PWM

FREQ

2.5V

RSENSE

10Ω

C2

1µF

0V

Q1

Si2302

100k

3491 F09

Figure 9. Li-Ion to Four White LEDs with Both

PWM Dimming and Analog Dimming

3491fa

10

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