A8521 datasheet(19/35 Pages) ALLEGRO

19 / 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

VOUT

PWM

SW node

Output disconnect

event detected

IOUT

Overvoltage protection

The A8521 has overvoltage protection (OVP) and open Schottky

diode (D1 in figure 1) protection. The OVP protection has a

default level of 8.1 V and can be increased up to 53 V by con-

necting resistor ROVP between the OVP pin and VOUT. When

the current into the OVP pin exceeds 199 μA (typical), the OVP

comparator goes low and the boost stops switching.

The following equation can be used to determine the resistance

for setting the OVP level:

ROVP = ( VOUTovp – VOVP(th)) / IOVPH

(5)

where:

VOUTovp is the target overvoltage level,

ROVP is the value of the external resistor, in Ω,

VOVP(th) is the pin OVP trip point found in the Electrical Charac-

teristics table, and

IOVPH is the current into the OVP pin.

There are several possibilities for why an OVP condition would

be encountered during operation, the two most common being: a

disconnected output, and an open LED string. Examples of these

are provided in figures 21 and 22.

Figure 21 illustrates when the output of the A8521 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. If the output is

drained off, eventually the boost might start switching for a short

duration until the OVP threshold is hit again.

Figure 22 displays a typical OVP event caused by an open LED

string. After the OVP condition is detected, the boost stops

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

Afterwards VOUT is allowed to fall, and eventually the boost will

resume switching and the A8521 will resume normal operation.

Figure 21. OVP protection in an output disconnect event; shows VOUT

(ch1, 10 V/div.), SW node (ch2, 50 V/div.), PWM (ch3, 5 V/div.), and

IOUT (ch4, 200 mA/div.), time = 1 ms/div.

Figure 22. OVP protection in an open LED string event; shows VOUT

(ch1, 10 V/div.), SW node (ch2, 50 V/div.), PWM (ch3, 5 V/div.), and

IOUT (ch4, 200 mA/div.), time = 500 μs/div.

VOUT

PWM

SW node

IOUT

LED string open

condition detected

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