Automotive Stop/Tail LED Array Driver

A6264

7

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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

current matching in the A6264 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-

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

is cycled, 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 A6264 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

threshold voltage below VIN when driven by the regulator. This

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 A6264, reduces the LED current as the silicon

junction temperature of the A6264 increases (see figure 2). By

mounting the A6264 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 A6264 increases, the regulated

current level is reduced, reducing the dissipated power in the

A6264 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

begins is defined as the thermal monitor activation temperature,

rent level.

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

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

when the temperature drops below the threshold provided by the

specified hysteresis.

Note that it is possible for the A6264 to transition rapidly

between thermal shutdown and normal operation. This can hap-

pen if the thermal mass attached to the exposed thermal pad is

power, the thermal mass of any heatsink present, and the ambient

temperature.

100

80

60

40

20

0

90

25

70

90

110

130

150

170

Figure 2. Temperature monitor current reduction.