Protected LED Array Driver

A6263

5

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Functional Description

The A6263 is a linear current regulator that is designed to pro-

vide drive current and protection for parallel strings of series-

connected high brightness LEDs. It provides up to 4 matched pro-

grammable current outputs at up to 100 mA, with low minimum

dropout voltages below the main supply voltage. For 12 V power

net applications, optimum performance is achieved when driving

4 strings of 1 to 3 LEDs, at current up to 100 mA per string.

Current regulation is maintained and the LEDs protected during a

short-to-ground at any point in the LED string. A short-to-ground

on any regulator output terminal disables that offending string

only. Similarly, in the case of an open output pin or an open-LED

fault, all other LED strings remain in regulation. Individual out-

puts can be disabled by connecting the output to VIN. Multiple

outputs can be connected in parallel to drive higher current LED

strings.

Integrated thermal management reduces the regulated current

level at high internal junction temperatures to limit power dis-

sipation. This thermal threshold is programmable and can be

disabled if necessary.

Pin Functions

VIN

Supply to the control circuit and current regulators. A small

value ceramic bypass capacitor, typically 100 nF, should be con-

nected from close to this pin to the GND pin.

GND

Ground reference connection. Should be connected directly

to the ground plane of the circuit board.

IREF

to GND to set reference current and thereby LED current.

THTH

current starts to reduce with increasing temperature. Connecting

THTH directly to GND will disable the thermal monitor function.

LA[1:4]

Current source connected to the anode of the first LED

in each string. Connect directly to VIN to disable the respective

output. In this document “LAx” indicates any one of the outputs.

LED Current Level

The LED current is controlled by 4 matching linear current regu-

lators, between the VIN pin and each of the LAx outputs. The

basic equation that determines the nominal output current at each

LAx pin is:

15

(1)

The output current may be reduced from the set level by the ther-

mal monitor circuit.

Conversely the reference resistors may be calculated from:

=

15

(2)

For example, where the required current is 90 mA for both chan-

nels the resistor value will be:

90

==

0.167 kΩ

15

These equations completely define the output currents with

respect to the setting resistors. However, for further reference, a

more detailed description of the internal reference current calcu-

lations is included below.

It is important to note that because the A6263 is a linear regu-

lator, the maximum regulated current is limited by the power

dissipation and the thermal management in the application. All

current calculations assume adequate heatsinking for the dissi-

pated power. Thermal management is at least as important as the

electrical design in all applications. In high current high ambient

temperature applications the thermal management is the most

important aspect of the systems design. The application section

below provides further detail on thermal management and the

associated limitations.

Operation with Fewer LED Strings or Higher Currents

The A6263 may be configured to use fewer than all four LED

strings, either by connecting outputs together for higher currents,

or by connecting the output directly to VIN to disable the regula-

tor for that output. It is also acceptable, though not recommended,

to leave an unused LAx pin floating.