Protected LED Array Driver

A6261

7

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

ing LEDs will remain in regulation and the LEDs will be pro-

tected. The current will be summed and shared by the affected

strings. Current matching in the strings will then depend on the

LED forward voltage differences.

Open Load Detection

An open load condition is detected

will be disabled and the fault flag allowed to go high.

The regulators will remain disabled until either the power is

cycled off and on, the EN input is taken low then high, or the

fault flag, FF, is pulled low. If the power is cycled or EN is pulsed

low, the regulators will start in the enabled state, unless disabled

by tying the output to VIN, and the open load detection timer will

be reset. If the open load is still present the regulators will again

be disabled after the open load detect time.

Pulling the fault flag low will override the open load fault action

and all enabled regulators will be switched on. This state will

be maintained while the fault flag is held low. If the fault flag is

allowed to go high the A6261 will return to the open load fault

condition and will disable all regulators.

Each of the four regulators includes a limiter to ensure that

the output voltage will not rise higher than the output disable

means that the voltage across the regulator will not be less than

the output disable voltage, unless it is forced by connecting the

LAx pin to VIN. However if a load becomes disconnected, the

regulator will pull the LAx pin up to the limit, which will ensure

Note that an open load may also be detected if the sum of the for-

ward voltages of the LEDs in a string is close to or greater than

the supply voltage on VIN.

Temperature Monitor

A temperature monitor function,

included in the A6261, reduces the LED current as the silicon

junction temperature of the A6261 increases (see figure 2). By

mounting the A6261 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 A6261 increases, the regulated

current level is reduced, reducing the dissipated power in the

A6261 and in the LEDs. The current is reduced from the 100%

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

this temperature the current will continue to reduce at a lower

rate until the temperature reaches the overtemperature shutdown

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 specified, in the characteristics table, at the 90% current level.

reduced and is defined as approximately:

0.0039

=

(°C)

(3)

100

80

60

40

20

0

90

25

70

90

110

130

150

170

Figure 2. Temperature monitor current reduction.