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
default level of 8.1 V and can be increased up to 53 V by con-
necting resistor ROVP between the OVP pin and VOUT . When
the current into the OVP pin exceeds 199 μA (typical), the OVP
comparator goes low and the boost stops switching.
The following equation can be used to determine the resistance
for setting the OVP level:
ROVP = ( VOUTovp – VOVP(th) ) / IOVPH
(5)
where:
VOUTovp is the target overvoltage level,
ROVP is the value of the external resistor, in Ω,
VOVP(th) is the pin OVP trip point found in the Electrical Charac-
teristics table, and
IOVPH is the current into the OVP pin.
There are several possibilities for why an OVP condition would
be encountered during operation, the two most common being: a
disconnected output, and an open LED string. Examples of these
are provided in figures 21 and 22.
Figure 21 illustrates when the output of the A8502 is discon-
nected from load during normal operation. The output voltage
instantly increases up to OVP voltage level and then the boost
stops switching to prevent damage to the IC. If the output is
drained off, eventually the boost might start switching for a short
duration until the OVP threshold is hit again.
Figure 22 displays a typical OVP event caused by an open LED
string. After the OVP condition is detected, the boost stops
switching, and the open LED string is removed from operation.
Afterwards VOUT is allowed to fall, and eventually the boost will
resume switching and the A8502 will resume normal operation.
A8502 also has built-in secondary overvoltage protection to
protect the internal switch in the event of an open diode condi-
tion. Open Schottky diode detection is implemented by detecting
overvoltage on the SW pin of the device. If voltage on the SW
Output disconnect
event detected
VOUT
SW node
C2
C3
PWM
C1
ILED
C4
t
Figure 21. OVP protection in an output disconnect event; shows VOUT
(ch1, 10 V/div.), SW node (ch2, 50 V/div.), PWM (ch3, 5 V/div.), and
ILED (ch4, 200 mA/div.), time = 1 ms/div.
VOUT
SW node
C2
C3
PWM
C1
ILED
C4
LED string open
condition detected
t
Figure 22. OVP protection in an open LED string event; shows VOUT
(ch1, 10 V/div.), SW node (ch2, 50 V/div.), PWM (ch3, 5 V/div.), and
ILED (ch4, 200 mA/div.), time = 500 μs/div.
Allegro MicroSystems, Inc.
19
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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