Diodes AL8805 Manuel d'utilisateur télécharger pdf (Page 9)

AL8805

Document number: DS35030 Rev. 4 - 2

9 of 16

www.diodes.com

July 2012

L8805

Application Information

AL8805 Operation

In normal operation, when voltage is applied at +V

, the AL8805 internal switch is turned on. Current starts to flow through sense resistor R

inductor L1, and the LEDs. The current ramps up linearly, and the ramp rate is determined by the input voltage +Vin and the inductor L1.

This rising current produces a voltage ramp across R

. The internal circuit of the AL8805 senses the voltage across R

and applies a proportional

voltage to the input of the internal comparator.

When this voltage reaches an internally set upper threshold, the internal switch is turned off. The inductor current continues to flow through R

L1, the LEDs and the schottky diode D1, and back to the supply rail, but it decays, with the rate of decay determined by the forward voltage drop

of the LEDs and the schottky diode.

This decaying current produces a falling voltage at R

, which is sensed by the AL8805. A voltage proportional to the sense voltage across R

applied at the input of the internal comparator. When this voltage falls to the internally set lower threshold, the internal switch is turned on again.

This switch-on-and-off cycle continues to provide the average LED current set by the sense resistor R

LED Current Control

The LED current is controlled by the resistor R

in Figure 30.

Connected between V

and SET the nominal average output current in the LED(s) is defined as:

SET

THD

LED

I =

If the CTRL pin is driven by an external voltage (higher than 0.4V and lower than 2.5V), the average LED current is:

SET

THD

REF

CTRL

LED

I =

For example for a desired LED current of 660mA and a default voltage V

CTRL

=2.5V the resulting resistor is:

Ω≈== m150

5.2

66.0

1.0

REF

CTRL

LED

THD

SET

Figure 30 Typical Application Circuit

DC Dimming

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

CTRL

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

by RSET. The LED current decreases linearly with the CTRL voltage when 0.5V ≤ V

CTRL

≤ 2.5V, as in Figure 2 for 4 different current levels.

When the CTRL voltage falls below the threshold, 0.4V, the output switch is turned off which allows PWM dimming.

Note that 100% brightness setting corresponds to V

CTRL

= V

REF

, nominally 2.5V. For any voltage applied on the CTRL pin that is higher than V

REF

the device will not overdrive the LED current and will still set the current according to the equation V

CTRL

= V

REF

AL8805

SET

CTRL

GND

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Commentaires sur ces manuels

Pas de commentaire

Diodes AL8805 Manuel d'utilisateur Page 9

Commentaires sur ces manuels

Produits connexes et manuels pour Matériel Diodes AL8805