A8510
Wide Input Voltage Range, High Efficiency
Fault Tolerant LED Driver
LED short detect
All of the LEDx pins are capable of handling the maximum VOUT
that the converter can deliver, thus providing protection from the
LED pin to VOUT in the event of a connector short.
Any 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 the LEDx pin.
While the IC is being PWM-dimmed, the IC rechecks the dis-
abled LEDx pin every time the PWM signal goes high, to prevent
false tripping of an LEDx short event. This also allows some self-
correction if an intermittent LEDx pin short-to-VOUT is present.
Overvoltage protection
The A8510 has overvoltage protection (OVP) and open Schottky
diode (D1) protection. The OVP protection has a default level of
8 V and can be increased up to 55 V by connecting 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
(4)
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: an
open LED string, and a disconnected output. Examples of these
are provided in figures 21 and 22.
Figure 21 illustrates when the output of the A8510 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.
VLED
C1
EN/PWM
C2
C3
ILED
t
Figure 20. Example of the disabling of an LED string when the LED pin
voltage is increased above 4.6 V; shows VLED (ch1, 5 V/div.), EN/PWM
(ch2, 5 V/div.), and ILED (ch3, 50 mA/div.), t = 20 μs/div.
Allegro MicroSystems, Inc.
17
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com