Protected LED Array Driver

A6261

6

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Operation with Fewer LED Strings or Higher Currents

The A6261 may be configured to use fewer than four LED

strings, either by connecting outputs together for higher cur-

rents, or by connecting the output directly to VIN to disable the

regulator for that output. When a regulator is disabled it will

not indicate an open load and will not affect the fault flag or the

operation of the remaining regulator outputs.

Sleep Mode

When EN is held low the A6261 will be in shutdown mode and

all sections will be in a low power sleep mode. The input current

will be typically less than 10 μA. This means that the complete

circuit, including LEDs, may remain connected to the power sup-

ply under all conditions.

Safety Features

The circuit includes several features to ensure safe operation and

to protect the LEDs and the A6261:

• The current regulators between VIN and each LAx output pro-

vide a natural current limit due to the regulation.

• Each LAx output includes a short-to-ground detector that will

disable the output to limit the dissipation.

• An open circuit on any output will disable all outputs.

• The thermal monitor reduces the regulated current as the tem-

perature rises.

• Thermal shutdown completely disables the outputs under ex-

treme overtemperature conditions.

Short Circuit Detection

A short-to-ground on any LED

cathode (figure 1A) will not result in a short fault condition. The

current through the remaining LEDs will remain in regulation and

the LEDs will be protected. Due to the difference in the voltage

drop across the LEDs, as a result of the short the current match-

ing in the A6261 may exceed the specified limits.

Any LAx output that is pulled below the short detect voltage

(figure 1B) will disable the regulator on that output and allow

the fault flag, FF, to go high. A small current will be sourced

from the disabled output to monitor the short and detect when

it is removed. When the voltage at LAx rises above the short

detect voltage the fault flag will be removed and the regulator

re-enabled.

A shorted LED (figure 1C) will not result in a short fault condi-

tion. The current through the remaining LEDs will remain in

regulation and the LEDs will be protected. Due to the difference

in the voltage drop across the LEDs, as a result of the short, the

current matching in the A6261 may exceed the specified limits.

A short between LEDs in different strings (figure 1D) will not

result in a short fault condition. The current through the remain-

A6261

LA1

VIN

GND

LA2

LA3

LA4

A6261

LA1

VIN

GND

LA2

LA3

LA4

A6261

LA1

VIN

GND

LA2

LA3

LA4

A6261

LA1

VIN

GND

LA2

LA3

LA4

A. Any LED cathode short-to-ground.

Current remains regulated in

non-shorted LEDs. Matching may be

affected. FF is low.

B. Any LAx output short-to-ground.

Shorted output is disabled. Other

outputs remain active. FF is high.

C. Shorted LEDs.

Current remains regulated.

Matching may be affected.

Only the shorted LED is inactive.

FF is low.

D. Short between LEDs in different

strings. Current remains regulated.

Current is summed and shared by

affected strings. Intensity match

dependent on voltage binning.

FF is low.

Figure 1. Short circuit conditions.