A8508GLWTR-T View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
A8508GLWTR-T
Allegro MicroSystems
A8508GLWTR-T Datasheet PDF : 28 Pages
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A8508
Wide Input Voltage Range, High Efficiency
8-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 A8508 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
Powering up: LED pin short-to-GND check
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 4.75 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 A8508 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 4.75 kΩ resistor, the voltage on that LEDx pin
during the LED detection period is about 200 mV. This is shown
in figure 2.
If an LEDx pin is shorted to ground such that LEDx pin voltage
is < 100 mV, the A8508 will not proceed with soft start until the
short is removed from the LEDx pin. This prevents the A8508
VLEDa(not used)
C3
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 4.75 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.
A8508
LED1
LED2
LED3
LED4
LED5
LED6
LED7
GND
LED8
4.75 kΩ
A8508
LED1
LED2
LED3
LED4
LED5
LED6
LED7
GND
LED8
Figure 4. Channel select setup: (left) channel LED8 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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