Protected LED Array Driver

A6261

9

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Application Information

Power Dissipation

The most critical design considerations when using a linear regu-

lator such as the A6261 are the power produced internally as heat

and the rate at which that heat can be dissipated.

There are three sources of power dissipation in the A6261:

• The quiescent power to run the control circuits

• The power in the reference circuit

• The power due to the regulator voltage drop

The elements relating to these dissipation sources are illustrated

in figure 4.

Quiescent Power

The quiescent power is the product of the

fore defined as:

(4)

Reference Power

The reference circuit draws the reference

current from the supply and passes it through the reference resis-

tor to ground. The reference current is 8% of the output current

on any one active output. The reference circuit power is the prod-

uct of the reference current and the difference between the supply

voltage and the reference voltage, typically 1.2 V. The reference

(5)

Regulator Power

In most application circuits the largest dis-

sipation will be produced by the output current regulators. The

power dissipated in each current regulator is simply the product

of the output current and the voltage drop across the regulator.

The total current regulator dissipation is the sum of the dissipa-

tion in each output regulator. The regulator power for each output

is defined as:

(6)

where x is 1, 2, 3, or 4.

output current is regulated by making this voltage large enough

to provide the voltage drop from the supply voltage to the total

The total power dissipated in the A6261 is the sum of the qui-

escent power, the reference power, and the power in each of the

four regulators:

(7)

The power that is dissipated in each string of LEDs is:

(8)

LEDs in the string.

A6261

LAx

VIN

GND

IREF

Figure 4. Internal power dissipation sources.