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A8509GLBTR-T

Wide Input Voltage Range, High Efficiency 4-Channel Fault Tolerant LED Driver

Allegro MicroSystems 

A8509

Wide Input Voltage Range, High Efficiency

4-Channel Fault Tolerant LED Driver

Enabling the IC

Functional Description

The IC turns on when a logic high signal is applied on the EN

VOUT

pin, and the input voltage present on the VIN pin is greater than

the 8.5 V necessary to clear the UVLO (VUVLOrise ) threshold.

Before the LEDs are enabled, the A8509 driver goes through a

C1

system check to determine if there are any possible fault condi-

VEN

tions that might prevent the system from functioning correctly.

C2

VLEDa(not used)

Powering up: LED pin short-to-GND check C3

After the VIN pin goes above the UVLO threshold, and a high sig-

nal is present on the EN pin, the IC proceeds to check if any LEDx

pins are shorted to GND and/or are not used. Each unused pin

should be connected to GND with a 2.4 kΩ pull-down resistor.

After the voltage threshold on the LEDx pins exceeds 120 mV, a

timer of 1536 clock cycles (2 ms at 800 kHz switching frequency,

see figure 2) is applied during which the A8509 determines the

status of the pins. Any unused pin connected to GND with the

pull-down resistor will be taken out of regulation at this point and

will not contribute to the boost regulation loop (see figure 3). A

typical example is shown in figure 4. When a pin is connected

to GND through a 2.4 kΩ resistor, the voltage on that LEDx pin

during the LED detection period is about 200 mV. This is shown

in figure 2.

VLEDb(used)

C4

t

Figure 3. LED detect circuit operation for an LED pin that is not being

used; shows VOUT (ch1, 10 V/div.), VEN (ch2, 5 V/div.), an unused LEDx

with a 2.4 kΩ resistor from this pin to GND, VLEDa (ch3, 500 mV/div.), and

a used LEDx, VLEDb (ch4, 500 mV/div.), t = 2 ms/div.

A8509

LED1

LED2

LED3

GND

LED4

2.4 kΩ

A8509

LED1

LED2

LED3

GND

LED4

If an LEDx pin is shorted to ground such that LEDx pin voltage

is < 100 mV, the A8509 will not proceed with soft start until the

short is removed from the LEDx pin. This prevents the A8509

Figure 4. Channel select setup: (left) channel LED4 not used,

(right) using all channels.

VOUT

VOUT

C1

VEN

C2

VLEDa

C3

LED detection period

C1

Short removed

C2

C3

VEN

LED detection period

VLEDa (with temporary short)

VLEDb

C4

t

Figure 2. LED detect circuit operation for two connected LEDs at fSW =

800 kHz; shows VOUT (ch1, 10 V/div.), VEN (ch2, 5 V/div.), an LEDx, VLEDa

(ch3, 500 mV/div.), another LEDx, VLEDb (ch4, 500 mV/div.), t = 2 ms/div.

C4

VLEDb (no short)

t

Figure 5. LED detect circuit operation: device powers-up after the short

is removed from the LED pin; shows VOUT (ch1, 10 V/div.), VEN (ch2,

5 V/ div.), an LEDx with short, VLEDa (ch3, 500 mV/div.), and an LEDx

without short, VLEDb (ch4, 500 mV/div.), t = 2 ms/div.

Allegro MicroSystems, Inc.

8

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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