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A8509 Datasheet(PDF) 8 Page - Allegro MicroSystems

Part # A8509
Description  Wide Input Voltage Range, High Efficiency
Download  28 Pages
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Manufacturer  ALLEGRO [Allegro MicroSystems]
Direct Link  http://www.allegromicro.com
Logo ALLEGRO - Allegro MicroSystems

A8509 Datasheet(HTML) 8 Page - Allegro MicroSystems

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Wide Input Voltage Range, High Efficiency
4-Channel Fault Tolerant LED Driver
A8509
8
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Functional Description
Enabling the IC
The IC turns on when a logic high signal is applied on the EN
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
system check to determine if there are any possible fault condi-
tions that might prevent the system from functioning correctly.
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 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.
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.
GND
2.4 kΩ
A8509
LED1
LED2
LED3
LED4
A8509
LED1
LED2
LED3
LED4
GND
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.
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.
t
VLEDa(not used)
VLEDb(used)
VOUT
VEN
C1
C3
C4
C2
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.
t
LED detection period
Short removed
VEN
VOUT
VLEDa (with temporary short)
VLEDb (no short)
C1
C3
C4
C2
t
LED detection period
VEN
VLEDa
VLEDb
VOUT
C1
C2
C3
C4


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