A8519KETTR-R datasheet(15/35 Pages) ALLEGRO

15 / 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

Table 3: Sync Pulse Duty Cycle Range for Selected Switch-

ing Frequencies.

Sync Pulse Frequency

Duty Cycle Range

2.2 MHz

33% to 66%

2 MHz

30% to 70%

1 MHz

15% to 85%

600 kHz

9% to 91%

300 kHz

4.5% to 95.5%

Figure 10: Synchronized FSET Pin and Switch Node SW Voltage.

Figure 11: Transition of the Switch Wave Form when the Sync

Pulse is Detected. The A8519 is switching at 2 MHz, and the

applied sync pulse is 1 MHz. The LED current does not show any

variation while the frequency changeover occurs.

VOUT

VFSET

ILED(TOTAL)

VSW

Suppose the A8519 is started up with a valid external SYNC sig-

nal, but the SYNC signal is lost during normal operation. In that

case, one of the following happens:

•

If the external SYNC signal is high impedance (open), the

A8519 continues normal operation after approximately 5 µs,

at the switching frequency set by RFSET. No FAULT flag is

generated.

•

If the external SYNC signal is stuck low (shorted to ground),

the A8519 will detect an FSET-shorted-to-GND fault. The

FAULT pin is pulled low after approximately 10 µs, and

switching is disabled. Once the FSET pin is released or

SYNC signal is detected again, the A8519 will proceed to

soft-start.

To prevent generating a fault when the external SYNC signal is

stuck at low, the circuit shown in Figure 12 can be used. When

the external SYNC signal goes low, the A8519 will continue to

operate normally at the switching frequency set by the RFSET. No

FAULT flag is generated.

A8519

Schottky

Barrier

Diode

FSET

10.2 k

220 pF

External

Synchronization

Signal

Figure 12: Countermeasure to Prevent External Sync Signal

Stuck-at-Low Fault.

LED Current Setting and LED Dimming

The maximum LED current can be up to 100 mA per chan-

nel, and is set through the ISET pin. Connect a resistor, RISET,

between this pin and GND. To set ILED calculate RISET as fol-

lows:

I = I × A

LED

SET

ISET

I =

SET

ISET

R =

ISET

(V × A )

ISET

LED

where ILED current is in A and RISET is in Ω.

2MHz operation

1MHz operation

VOUT

VFSET

ILED(TOTAL)

VSW

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