LED Backlight Driver for LCD Monitors and Televisions

A8516

9

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Typical PWM Operation Waveforms

t

t

C1

C1

C2

C2

C4

Symbol

Parameter Units/Division

C1

10 V

C2

10 V

–*

1 V

C4

100 mA

t

time

2 ms

*Offset = 46 V

Symbol

Parameter Units/Division

C1

10 V

C2

10 V

–*

1 V

C4

100 mA

t

time

50 μs

*Offset = 46 V

PWM dimming at 200 Hz 10% duty cycle;

Output voltage ripple approximately 0.8 V (out of 50 V)

PWM dimming at 5 kHz 10% duty cycle

1

100

90

80

70

60

50

40

30

20

10

0

10

0.1

100

10

1

0.1

PWM Frequency (kHz)

PWM Duty Cycle (%)

Ratio of LED Current versus PWM Frequency

PWM duty cycle = 10%

Ratio of LED Current versus PWM Duty Cycle

PWM frequency = 200 Hz

0.1

1

10

100

Compensated

pulse width

Uncompensated

pulse width

To improve the accuracy of PWM dimming at very high frequency and/or very low duty cycle, it is

necessary to compensate the PWM pulse width, as described in Application Information section.