A8519KETTR-R datasheet(18/35 Pages) ALLEGRO

18 / 35 page

Wide Input Voltage Range, High-Efficiency,

Fault-Tolerant LED Driver

A8519 and

A8519-1

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Figure 22: Transition of output current level when a 50% duty

cycle APWM signal is applied to the APWM pin, in conjunction

with 50% duty cycle applied to the PWM pin.

Extending LED Dimming Ratio

The dynamic range of LED brightness can be further extended by

using a combination of PWM duty cycle, APWM duty cycle, and

analog dimming method.

For example, the following approach can be used to achieve a

50,000:1 dimming ratio at 200 Hz PWM frequency:

•

Vary PWM duty cycle from 100% down to 0.02% to give

5,000:1 dimming.

•

With PWM duty cycle at 0.02%, vary APWM duty from 0%

to 90% to reduce LED current down to 10%. This gives a net

effect of 50,000:1 dimming.

Analog Dimming

Besides using APWM signal, the LED current can also be

reduced by using an external DAC or another voltage source.

Connect RISET between the DAC output and the ISET pin. The

limit of this type of dimming is dependant of the range of the

ISET pin. In the case of the A8519, the limit is 20 to 144 µA.

VAPWM

ILED(TOTAL)

VPWM

Figure 20: Transition of Total LED Current from 240 mA to

180 mA, when a 25% APWM signal is applied to the APWM pin.

(Dimming PWM = 100%)

Figure 21: Transition of Total LED Current from 180 mA to

240 mA, when a 25% APWM stops being applied to the APWM

pin. (Dimming PWM = 100%)

VAPWM

ILED(TOTAL)

VPWM

VAPWM

ILED(TOTAL)

VPWM

Applied

Although the APWM dimming function has a wide frequency

range, if used strictly as an analog dimming function, it is recom-

mended to use frequency ranges between 50 and 500 kHz for

best accuracy. The frequency range needs to be considered only

if the user is not using APWM as a closed-loop trim function.

It takes about 1 millisecond to change the actual LED current

due to propagation delay between the APWM signal and the

ILED(TOTAL).

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