A8519KETTR-R datasheet(20/35 Pages) ALLEGRO

20 / 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 26: Output of A8519 when Disconnected from Load During

Normal Operation

Figure 27: Typical OVP Condition Caused by an Open LED String

Figure 27 illustrates a typical OVP condition caused by an open

LED string. Once OVP is detected, the boost stops switching,

and the open LED string is removed from operation. Afterwards,

VOUT is allowed to fall, the boost will resume switching, and the

A8519 will resume normal operation.

VOUT

VSW

VPWM

ILED(TOTAL)

VOUT

VSW

VPWM

ILED(TOTAL)

While the IC is being PWM dimmed, the IC will recheck the dis-

abled LED every time the PWM signal goes high to prevent false

tripping of LED short. This also allows for some self-correction if

an intermittent LED pin short-to-VOUT is present.

At least one LED must be in regulation for the LED string short-

detect protection to activate. In case all of the LED pins are above

regulation voltage (this could happen when the input voltage

rises too high for the LED strings), they will continue to operate

normally.

Overvoltage Protection

The A8519 has output overvoltage protection (OVP) and open

Schottky diode protection (secondary OVP). The OVP pin has

a threshold level of 8.3 V typical. A resistor can be used to set

the output overvoltage protection threshold up to 40 V approxi-

mately. This is sufficient for driving 11 white LED in series.

The formula for calculating the OVP resistor is shown below:

where VOVP(th) = 8.3 V typical and IOVP(th) = 200 µA typical.

The OVP function is not a latched fault. If the OVP condition

occurs during a load dump, the IC will stop switching but not

shut down.

There are several possibilities why an OVP condition is encoun-

tered during operation, the two most common being an open LED

string and a disconnected output condition.

Figure 26 illustrates when the output of the A8519 is discon-

nected from load during normal operation. The output voltage

instantly increases up to OVP voltage level, and then the boost

stops switching to prevent damage to the IC. When the output

voltage decreases to a low value, the boost converter will begin

switching. If the condition that caused the OV event still exists,

OVP will be triggered again.

OVP

– V

)

OVP

OVP(th)

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