A8521 datasheet(13/35 Pages) ALLEGRO

13 / 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

All unused pins should be connected with a 2.37 kΩ resistor to

ground, as shown in figure 5. The unused pin, with the pull-down

resistor, will be taken out of regulation at this point and will not

contribute to the boost regulation loop.

If a LEDx pin is shorted to ground the A8521 will not proceed

with soft start until the short is removed from the LEDx pin. This

prevents the A8521 from powering-up and putting an uncon-

trolled amount of current through the LEDs.

Soft start function

During soft start the LEDx pins are set to sink (ILEDSS) and the

boost switch current is reduced to the ISWSS(LIM) level to limit

the inrush current generated by charging the output capacitors.

When the converter senses that there is enough voltage on the

LEDx pins the converter proceeds to increase the LED current to

the preset regulation current and the boost switch current limit is

switched to the ISW(LIM) level to allow the A8521 to deliver the

necessary output power to the LEDs. This is shown in figure 6.

Frequency selection

The switching frequency on the boost regulator is set by the resis-

tor connected to the FSET/SYNC pin. The switching frequency

can be can be anywhere from 580 kHz to 2.3 MHz. Figure 7

shows the typical switching frequencies, in MHz, for given resis-

tor values, in kΩ.

In case during operation a fault occurs that will increase the

switching frequency, the FSET/SYNC pin is clamped to a

maximum switching frequency of no more than 3.5 MHz. If the

FSET/SYNC pin is shorted to GND the part will shut down. For

more details see the Fault Mode table later in this datasheet.

Figure 5. Channel select setup: (left) using only LED1, LED2, and LED3,

and (right) using all four channels.

2.37 kΩ

LED1

LED2

LED3

LED4

GND

A8521

LED1

LED2

LED3

LED4

GND

A8521

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

Resistance for RSET (kΩ)

10.0

30.0

20.0

12.5

32.5

22.5

17.5

15.0

25.0

35.0

Figure 6. Startup diagram showing the input current, output voltage, and

output current; shows IOUT (ch1, 200 mA/div.), IIN (ch2, 1 A/div.), VOUT

(ch3, 20 V/div.), and PWM/EN (ch4, 5 V/div.), time = 1 ms/div.

Figure 7. Typical Switching Frequency versus value of RFSET resistor.

Inrush current caused by

enabling the disconnect

switch (when used)

Operation during

ISWSS(lim)

Normal operation

ISW(lim)

PWM/EN

IIN

IOUT

VOUT

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