Diodes AL8807 Manuel d'utilisateur télécharger pdf (Page 10)

AL8807

Document number: DS35281 Rev. 5 - 2

10 of 20

www.diodes.com

March 2013

L8807

Application Information (cont.)

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 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 1% for PWM ranging from 1% to 100%.

100

200

300

400

500

600

700

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

PWM dimming [%]

LED current [mA

]

Figure 31 PWM Dimming at 500Hz

Zooming in at duty cycles below 10% shows:

Figure 32 Low Duty Cycle PWM Dimming at 300Hz

The accuracy of the low duty cycle dimming is affected by both the PWM frequency and also the switching frequency of the AL8807. For best

accuracy/resolution the switching frequency should be increased while the PWM frequency should be reduced.

The CTRL pin is designed to be driven by both 3.3V and 5V logic levels directly from a logic output with either an open drain output or push pull

output stage.

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Diodes AL8807 Manuel d'utilisateur Page 10