LT3591 查看數據表（PDF） - Linear Technology

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LT3591

White LED Driver with Integrated Schottky in 3mm × 2mm DFN

Linear Technology 

LT3591

APPLICATIONS INFORMATION

and ground as shown in Figure 6. A Si2308 MOSFET can

be used since its source is connected to ground. The PWM

signal is applied to the CTRL pin of the LT3591 and the gate

of the MOSFET. The PWM signal should traverse between

0V to 5V, to ensure proper turn on and off of the driver

and the NMOS transistor Q1. When the PWM signal goes

high, the LEDs are connected 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 MOSFET ensures that the LEDs quickly turn

off without discharging the output capacitor which in turn

allows the LEDs to turn on faster. Figure 7 shows the PWM

dimming waveforms for the circuit in Figure 6.

The calculations show that for a 100Hz signal the dimming

range is 83 to 1. In addition, the minimum PWM duty cycle

of 1.2% ensures that the LED current has enough time

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

range achievable for different frequencies with a settling

time of 120µs.

10000

1000

100

PULSING MAY BE VISIBLE

PWM

10

5V/DIV

IL

500mA/DIV

ILED

20mA/DIV

VIN = 3.6V

10 LEDs

2ms/DIV

3591 F07

Figure 7. Direct PWM Dimming Waveforms

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 120µs for a

3.6V input voltage. The achievable dimming range for this

application and 100Hz PWM frequency can be determined

using the following method.

Example:

ƒ = 100Hz, tSETTLE = 120µs

tPERIOD

=

1

ƒ

=

1

100

=

0.01s

Dim Range = tPERIOD = 0.01s = 83:1

tSETTLE 120µs

Min Duty Cycle = tSETTLE • 100 = 120µs • 100 = 1.2%

tPERIOD

0.01s

Duty Cycle Range = 100%→ 1.2% at 100Hz

10

1

10

100

1000

10000

PWM DIMMING FREQUENCY (Hz)

3591 F08

Figure 8. Dimming Range vs Frequency

In addition to extending the dimming range, PWM dimming

improves the efficiency of the converter for LED currents

below 20mA. Figure 9 shows the efficiency for traditional

analog dimming of the front page application and PWM

dimming of the application in Figure 6.

80

PWM DIMMING

75

70

65

ANALOG DIMMING

60

VIN = 3.6V

10 LEDs

55

0

5

10

15

20

LED CURRENT (mA)

3591 F09

Figure 9. PWM vs Analog Dimming Efficiency

3591f

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