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
Analog dimming
The A8502 can also be dimmed by using an external DAC or
another voltage source applied either directly to the ground side
of the RISET resistor or through an external resistor to the ISET
pin (see figure 19). The limit of this type of dimming depends on
the range of the ISET pin. In the case of the A8502 the limit is
40 to 125 μA.
The advantage of this circuit is that the DAC voltage can be
higher or lower, thus adjusting the LED current to a higher or
lower value of the preset LED current set by the RISET resistor:
▫ VDAC = 1.003 V; the output is strictly controlled by RISET
▫ VDAC > 1.003 V; the LED current is reduced
▫ VDAC < 1.003 V; the LED current is increased
• For a single resistor (panel A of figure 18), the ISET current is
controlled by the following formula:
ISET
=
VISET – VDAC
RISET
(3)
where VISET is the ISET pin voltage and VDAC is the DAC output
voltage.
When the DAC voltage is 0 V the LED current will be at its
maximum. To keep the internal gain amplifier stable, the user
should not decrease the current through the RISET resistor to less
than 40 μA
• For a dual-resistor configuration (panel B of figure 19), the ISET
current is controlled by the following formula:
ISET
=
VISET
RISET
–
VDAC – VISET
R1
(4)
LED short detect
Both LEDx pins are capable of handling the maximum VOUT
that the converter can deliver, thus providing protection from the
LEDx pin to VOUT in the event of a connector short.
An LEDx pin that has a voltage exceeding VLEDSC will be
removed from operation (see figure 20). This is to prevent the IC
from dissipating too much power by having a large voltage pres-
ent on an LEDx pin.
While the IC is being PWM-dimmed, the IC rechecks the dis-
abled LED every time the PWM signal goes high, to prevent false
tripping of an LED short event. This also allows some self-cor-
rection if an intermittent LED pin short to VOUT is present.
Overvoltage protection
The A8502 has overvoltage protection (OVP) and open Schottky
diode (D1 in figure 1) protection. The OVP protection has a
DAC
RISET
A8502
VDAC
ISET
GND
C1
GND
(A)
IOUT
LED1
DAC
VDAC
GND
R1
RISET
A8502
ISET
GND
(B)
Figure 19. Simplified diagrams of voltage control of ILED: typical
applications using a DAC to control ILED using a single resistor (upper),
and dual resistors (lower).
C2
PWM/EN
C3
t
Figure 20. Example of the disabling of an LED string when the LED pin
voltage is increased above 4.6 V; shows IOUT (ch1, 200 mA/div.), LED1
(ch2, 5 V/div.), and PWM/EN (ch3, 5 V/div.), time = 10 μs/div.
Allegro MicroSystems, Inc.
18
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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