LT3590 Просмотр технического описания (PDF) - Linear Technology

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LT3590

48V Buck Mode LED Driver in SC70 and 2mm x 2mm DFN

Linear Technology 

LT3590

APPLICATIONS INFORMATION

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.

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 6 is approximately 50μs for a

48V input voltage. The achievable dimming range for this

application and 100Hz PWM frequency can be determined

using the following method.

Example:

ƒ = 100Hz, tSETTLE = 50μs

tPERIOD

=

1

ƒ

=

1

100

=

0.01s

Dim Range = tPERIOD = 0.01s = 200:1

tSETTLE 50μs

Min Duty Cycle = tSETTLE • 100 = 50μs • 100 = 0.5%

tPERIOD

0.01s

Duty Cycle Range = 100%→ 0.5% at 100Hz

The calculations show that for a 100Hz signal the dimming

range is 200 to 1. In addition, the minimum PWM duty

cycle of 0.5% ensures that the LED current has enough

time to settle to its final value. Figure 7 shows the dim-

ming range achievable for three different frequencies with

a settling time of 50μs.

PWM

5V/DIV

VSW

20V/DIV

ILED

20mA/DIV

VIN = 48V

4 LEDs

2ms/DIV

3590 F06

Figure 6. Direct PWM Dimming Waveforms

1kHz

10kHz

100Hz

1

10

100

1000

PWM DIMMING RANGE

3590 F07

Figure 7. Dimming Range Comparison

of Three PWM Frequencies

3590f

10

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