A6263 datasheet(6/11 Pages) ALLEGRO

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Protected LED Array Driver

A6263

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Safety Features

The A6263 includes several features to ensure safe operation and

to protect the LEDs and the IC:

• 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 the affected string

only.

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

perature rises above a programmable thermal threshold.

• Thermal shutdown completely disables the outputs under ex-

treme overtemperature conditions.

Temperature Monitor

A temperature monitor function reduces the LED current as the

silicon junction temperature of the IC increases (see figure 2). By

mounting the A6263 on the same thermal substrate as the LEDs,

this feature can also be used to limit the dissipation of the LEDs.

As the junction temperature of the A6263 increases, the regulated

current level is reduced, reducing the dissipated power in the

A6263 and in the LEDs. The current is reduced from the 100%

level at typically 4% per degree Celsius until the point at which

the current drops to 25% of the full value, defined at TJL. Above

this temperature the current will continue to reduce at a lower

rate until the temperature reaches the overtemperature shutdown

threshold temperature, TJF.

The temperature at which the current reduction begins can be

adjusted by changing the voltage on the THTH pin. When THTH

is left open the temperature at which the current reduction begins

is defined as the thermal monitor activation temperature, TJM,

and is specified, in the Electrical Characteristics table, at the 90%

current level.

TJM will increase as the voltage at the THTH pin, VTHTH, is

reduced and is defined as approximately:

0.0039

TJM

(°C)

1.46 –VTHTH

(3)

A resistor connected between THTH and GND will reduce VTHTH

and increase TJM. A resistor connected between THTH and a refer-

ence supply greater than 1 V will increase VTHTH and reduce TJM.

Figure 3 shows how the nominal value of the thermal monitor

activation temperature varies with the voltage at THTH and with

either a pull-down resistor, RTH, to GND or with a pull-up resis-

tor, RTH, to 3 V and to 5 V.

In extreme cases, if the chip temperature exceeds the overtem-

perature limit, TJF, all regulators will be disabled. The tempera-

ture will continue to be monitored and the regulators re-activated

when the temperature drops below the threshold provided by the

specified hysteresis.

100

TJM

TJL

TJF

110

Junction Temperature, TJ (°C)

130

150

170

Figure 2. Temperature monitor current reduction

250

200

150

100

1.3

1.2

1.1

1.0

0.9

0.8

VTHTH

110

100

Thermal Monitor Activation Temperature, TJM (°C)

130

120

150

140

RTH pull-up

to 5 V

RTH pull-up

to 3 V

RTH pull-down

to GND

Figure 3. TJM versus a pull-up or pull-down resistor, RTH, and VTHTH

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