Diodes AL8808 Manuel d'utilisateur télécharger pdf (Page 10)

AL8808

Document number: DS35648 Rev. 2 - 2

10 of 20

www.diodes.com

January 2013

NEW PRODUCT

L8808

Application Information (cont.)

Analog Dimming

The CTRL pin can be driven by an external analog voltage (V

CTRL

), to adjust the output current to a value below the nominal average value

defined by R1. The LED current decreases linearly with the CTRL voltage when 0.5V  V

CTRL

 2.5V.

Note that 100% brightness is achieved when either the CTRL pin is left floating or pulled above 2.5V by an external voltage source.

For 2.6V  V

CTRL

 5.5V the LED current will not get overdriven and will be set the current according to the equation V

CTRL

= 2.5V (the internal

reference voltage). See Figure 33 below.

PWM Dimming

LED current can be adjusted digitally, by applying a low frequency Pulse Width Modulated (PWM) logic signal to the CTRL pin to turn the device

on and off. This will produce an average output current proportional to the duty cycle of the control signal. In particular, a PWM signal with a

max resolution of 10bit (~0.1% duty cycle) can be applied to the CTRL pin to change the output current to a value below the nominal average

value set by resistor R

SET

. To achieve this resolution the PWM frequency has to be lower than 500Hz, however higher dimming frequencies can

be used, at the expense of dimming dynamic range and accuracy.

Typically, for a PWM frequency of 500Hz the accuracy is better than 2% for PWM ranging from 5% to 100%.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

V (V)

Figure 33. LED Current Dimming Ration and

Typical Error vs. Control Voltage

CTRL

110

100

DIMMED : 100% LED

(%)

LED Current Error

Dimmed LED Current Ratio

2.0

2.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

TYPI

AL E

(%)

T = 25°C

V = 12V

L = 68µH

50 60010203040 9010070 80

PWM DUTY CYCLE (%)

Figure 34. PWM Dimming at 500Hz

0.8

0.7

0.1

0.3

0.2

0.6

0.5

0.4

LED

(A)

V = 12V

T = +25°C

L = 68µH

R = 150m

1 LED

SET

V = 12V

T = +25°C

L = 68µH

R = 150m

1 LED

SET

7.50.0 2.5 5.0 15.010.0 12.5

PWM DUTY CYCLE (%)

Figure 35. Low Duty Cycle PWM Dimming at 500Hz

(%)

1 2 ... 5 6 7 8 9 10 11 12 13 14 15 ... 19 20

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Diodes AL8808 Manuel d'utilisateur Page 10

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