A8502 View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
'A8502' PDF : 35 Pages View PDF
A8502
Wide Input Voltage Range, High Efficiency
Fault Tolerant LED Driver
If a LEDx pin is shorted to ground the A8502 will not proceed
with soft start until the short is removed from the LEDx pin. This
prevents the A8502 from powering-up and putting an uncon-
trolled amount of current through the LEDs.
Soft start function
During soft start the LEDx pins are set to sink (ILEDSS) and the
boost switch current is reduced to the ISWSS(LIM) level to limit
the inrush current generated by charging the output capacitors.
When the converter senses that there is enough voltage on the
LEDx pins the converter proceeds to increase the LED current to
the preset regulation current and the boost switch current limit is
switched to the ISW(LIM) level to allow the A8502 to deliver the
necessary output power to the LEDs. This is shown in figure 6.
Frequency selection
The switching frequency on the boost regulator is set by the resis-
tor connected to the FSET/SYNC pin. The switching frequency
can be can be anywhere from 580 kHz to 2.3 MHz. Figure 7
shows the typical switching frequencies, in MHz, for given resis-
tor values, in kΩ.
In case during operation a fault occurs that will increase the
switching frequency, the FSET/SYNC pin is clamped to a
maximum switching frequency of no more than 3.5 MHz. If the
FSET/SYNC pin is shorted to GND the part will shut down. For
more details see the Fault Mode table later in this datasheet.
Inrush current caused by
enabling the disconnect
switch (when used)
C1
IOUT
Operation during
ISWSS(lim)
C2
IIN
C3
VOUT
C4
PWM/EN
Normal operation
ISW(lim)
t
Figure 6. Startup diagram showing the input current, output voltage, and
output current; shows IOUT (ch1, 200 mA/div.), IIN (ch2, 1 A/div.), VOUT
(ch3, 20 V/div.), and PWM/EN (ch4, 5 V/div.), time = 1 ms/div.
A8502
A8502
GND
LED1
LED2
1.54 kΩ
GND
LED1
LED2
Figure 5. Channel select setup: (left) using only channel LED1,
(right) using both channels.
2.1
1.9
1.7
1.5
1.3
1.1
0.9
0.7
0.5
10.0 12.5 15.0 17.5 20.0 22.5 25.0 30.0 32.5 35.0
Resistance for RSET (kΩ)
Figure 7. Typical Switching Frequency versus value of RFSET resistor.
Allegro MicroSystems, Inc.
13
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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