AL8807AW5-7 データシートの表示（PDF） - Diodes Incorporated.

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AL8807AW5-7

HIGH EFFICIENCY LOW EMI WIDE ANALOG DIMMING RANGE 36V 1A/1.3A BUCK LED DRIVER

Diodes Incorporated. 

AL8807A

Application Information (cont.)

Analog Dimming

Further control of the LED current can be achieved by driving the CTRL pin with an external voltage lower than 2.5V; the average LED current

becomes:

ILED



VCTRL

VREF

VTH

R SET

Where VREF is nominally 2.5V

The LED current decreases linearly with the CTRL voltage when

VCTRL ≤ 2.5V, as in Figure 9 for 2 different current levels.

Note that 100% brightness setting corresponds to VCTRL = VREF,

nominally 2.5V.

If a voltage greater than 2.6V is applied to the CTRL pin an internal

clamp is activated which results in the internal reference voltage being

applied to the hysteresis control circuitry. This prevents the LED

current from being overdriven and will still set the LED current to

approximately.

ILED

 VTH

RSET

1.10

1.00

0.90

0.80

VIN = 12V

2 LEDs

L = 68µH

TA = 25°C

RS = 100m

0.70

0.60

RS = 150m

0.50

0.40

0.30

0.20

0.10

0.00

0

1

2

3

4

5

CTRL VOLTAGE (V)

Figure 33 LED Current vs. CTRL Pin Voltage

As the CTRL pin is reduced below 2.5V the sense voltage will proportionally decrease. This means that the time taken for the LED/Switch current

to ramp up to the upper threshold will decrease. The AL8807A, being a hysteretic converter, automatically compensates for the reduction in LED

current by reducing its lower threshold voltage and therefore its off-time. It therefore remains in continuous conduction mode maintaining a better

dimming accuracy than other peak-switch current control topologies.

A result of the reduced on- and off-times results in an increase of the switching frequency. This phenomenon can be seen in Figure 34.

Figure 34 Switching Frequency vs. VCTRL

Ultimately at very small CTRL pin voltages the AL8807A will switch much faster than its nominal switching frequency which due to propagation

delays leads to a non-linear degrading of accuracy.

The degradation in linear dimming accuracy at small CTRL pin voltages can be improved by using larger value inductors which cause the

AL8807A to oscillate at lower frequencies.

A further cause of loss of linearity as small CTRL pin voltages is the internal offsets of the control loop; at a CTRL pin voltage of 0.25V the

nominal LED current sense voltage has been reduced to 10mV.

AL8807A

Document number: DS35990 Rev. 2 - 2

11 of 20

www.diodes.com

March 2013

© Diodes Incorporated

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