A8519KETTR-R datasheet(14/35 Pages) ALLEGRO

14 / 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

Soft-Start Function

During startup, the A8519 ramps up its boost output voltage

following a fixed ramp function. This technique limits the input

inrush current and ensures the same startup time regardless of the

PWM duty cycle.

The soft-start process is completed when any one of the follow-

ing conditions is met:

1. All LED currents have reached their regulation targets,

2. Output voltage has reached 93% of its OVP threshold, or

3. Soft-start ramp time (tSS) has expired.

Frequency Selection

Figure 7: Startup Diagram Showing the Input Current, Output

Voltage, Total LED Current, and Switch Node Voltage

Synchronization

The A8519 can also be synchronized using an external clock. At

power-up, if the FSET pin is held low, the IC will not power-up.

Only when the FSET pin is tri-stated to allow for the pin to rise

to about 1 V, or when a sync clock is detected, the A8519 will try

to power up. The basic requirement of the sync signal is 150 ns

minimum on-time and 150 ns minimum off-time as dictated by

the requirements of pulse-width on- and off-times.

Figure 9 shows timing for a synchronization clock into the A8519

at 2.2 MHz.

Any pulse with a duty cycle of 33% to 66% at 2.2 MHz can be

used to synchronize the IC. Table 3 summarizes the duty cycle

range at various synchronization frequencies.

Figure 8: Switching Frequency versus RFSET Resistor

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

010

100

110

Resistance in k

Figure 9: Sync Pulse On- and Off-Time Requirements

Pulse Width

Sync On Time

Pulse Width

Sync OffTime

150 ns

154 ns

T= 454 ns

The switching frequency on the boost regulator is set by a single

resistor connected to the FSET pin. The switching frequency can

be can be anywhere from 200 kHz to 2.15 MHz. Figure 8 shows

typical switching frequency in MHz for a given resistor value (in

kΩ). The following equation can also be used to determine typi-

cal switching frequency from FSET resistance:

fSW = 21.4/RFSET + 0.008

where fSW is in MHz, RFSET is in kΩ.

If a fault occurs during operation that will increase the switch-

ing frequency, the FSET pin is clamped to a maximum switching

frequency of no more than 3.5 MHz. If the FSET pin is shorted

to GND, the part will shut down. For more details, see the Fault

Mode table on page 25.

VOUT

VSW

ILED(TOTAL)

IIN

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