A8521 datasheet(18/35 Pages) ALLEGRO

18 / 35 page

Wide Input Voltage Range, High Efficiency

Fault Tolerant LED Driver

A8521

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Although the APWM dimming function has a wide frequency

range, if this function is used strictly as an analog dimming

function it is recommended to use frequency ranges between

50 and 500 kHz for best accuracy. The frequency range must be

considered only if the user is not using this function as a closed

loop trim function. Another limitation is that the propagation

delay between this APWM signal and IOUT takes several milli-

seconds to change the actual LED current. This effect is shown in

figures 16, 17, and 19.

Analog dimming

The A8521 can also be dimmed by using an external DAC or

another voltage source applied either directly to the ground side

of the RISET resistor or through an external resistor to the ISET

pin (see figure 19). The limit of this type of dimming depends on

the range of the ISET pin. In the case of the A8521 the limit is

20 to 125 μA.

• For a single resistor (panel A of figure 20), the ISET current is

controlled by the following formula:

ISET =

VISET – VDAC

RISET

(3)

where VISET is the ISET pin voltage and VDAC is the DAC output

voltage.

When the DAC voltage is 0 V the LED current will be at its

maximum. To keep the internal gain amplifier stable, the user

should not decrease the current through the RISET resistor to less

than 20 μA

• For a dual-resistor configuration (panel B of figure 20), the ISET

current is controlled by the following formula:

ISET =

–

VISET

RISET

VDAC – VISET

(4)

The advantage of this circuit is that the DAC voltage can be

higher or lower, thus adjusting the LED current to a higher or

lower value of the preset LED current set by the RISET resistor:

▫ VDAC = 1.003 V; the output is strictly controlled by RISET

▫ VDAC > 1.003 V; the LED current is reduced

▫ VDAC < 1.003 V; the LED current is increased

Figure 20. Simplified diagrams of voltage control of ILED: typical

applications using a DAC to control ILED using a single resistor (upper),

and dual resistors (lower).

GND

DAC

VDAC

GND

A8521

ISET

GND

DAC

VDAC

GND

A8521

ISET

RISET

(A)

(B)

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