A8519KETTR-R datasheet(22/35 Pages) ALLEGRO

22 / 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

Input Overcurrent Protection and Disconnect Switch

The primary function of the input disconnect switch is to protect

the system and the device from catastrophic input currents during

a fault condition.

If the input current level goes above the preset current limit

threshold, the part will be shut down in less than 3 µs—this is a

latched condition. The fault flag is also set low to indicate a fault.

This protection feature prevents catastrophic failure in the system

due to a short of the inductor, inductor short to GND, or short at

the output GND. Figure 33 illustrates the typical input overcur-

rent fault condition. As soon as input OCP limit is reached, the

part disables the gate of the disconnect switch Q1.

During startup when Q1 first turns on, an inrush current flows

through Q1 into the output capacitance. If Q1 turns on too fast

(due to its low gate capacitance), the inrush current may trip

input OCP limit. In this case, an external gate capacitance CG is

added to slow down the turn-on transition. Typical value for CG

is around 4.7 to 22 nF. Do not make CG too large, since it also

slows down the turn-off transient during a real input OCP fault.

VIN

GND

VSENSE

adj

To L1

A8519

GATE

Figure 32: Typical Circuit Showing Implementation of Input

Disconnect Feature

Figure 33: Startup into Output Shorted to GND fault. Input OCP

tripped at 4 A (RSC = 0.024 W, Radj =383 Ω)

PWM

VGATE

Input Current

Figure 31: Secondary Boost Switch OCP

There is also a secondary current limit (ISW(LIM2)) that is sensed

through the boost switch. This current limit, once detected,

immediately shuts down the A8519. The level of this current

limit is set above the cycle-by-cycle current limit to protect the

switch from destructive currents when boost inductor is shorted.

Figure 31 shows the secondary boost switch OCP. Once this limit

is reached, the A8519 will immediately shut down.

VOUT

VSW

VPWM

Inductor Current

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