A8507 데이터 시트보기 (PDF) - Allegro MicroSystems
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A8507 Datasheet PDF : 16 Pages
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A8507
LED Backlight Driver for LCD Monitors and Televisions
fault disables all LEDx strings that are below regulation, thus pre-
venting them from controlling the boost output voltage.
Calculate the value for ROVP (Ω) as follows:
ROVP = (VOVP – 36) / 240 μA ,
(3)
where VOVP is the required OVP level in V.
Open LED Protection Unused LEDx pins should be connected
to GND. During normal operation, if any enabled LED string
opens, voltage on the corresponding LEDx pin goes to zero. The
boost loop operates in open loop till the OVP level is reached.
The A8507 identifies the open LED string when overvoltage is
detected. Open strings are then removed from the regulation loop.
Afterwards, the boost controller operates in normal manner, and
the output voltage is regulated to drive the remaining strings. If
the open LED string is reconnected, it will sink current up to the
programmed current level.
Note: Open strings are removed from boost regulation, but not
disabled. This keeps the string in operation if LEDs open for only
a short length of time, or reach OVP level on a transient event.
The disconnected string can be restored to normal mode by reen-
abling the IC. It can also be restored to normal operation if the
fault signal is removed from the corresponding LEDx pin, but an
OVP event occurs on any other LEDx pin.
Overcurrent Protection The IC provides pulse-by-pulse
current limiting for the boost MOSFET. The current limit level,
ISC (A), can be set by selecting the external resistor, RSC (Ω):
RSC = 0.095 / ISC .
(4)
If the boost output voltage is unable to reach the regulation target
even when the switch is operating at maximum current limit, the
boost control loop will force the compensating capacitor, CCOMP,
to rise in voltage until it reaches the overcurrent fault level
(3.4V approximately). The overcurrent fault forces the device
into soft start.
Channel Selection The A8507 can be used to drive 1 to 6 LED
channels. During startup, the IC detects LED sink pins which are
shorted to ground, and disables the corresponding LED channel.
Therefore, any unused LED pins must be connected to ground,
otherwise the IC will go into overvoltage protection fault during
startup. LED pins can be paralleled together for higher cur-
rent. For example for a 3 parallel string configuration, connect
LED1-2, LED3-4, and LED5-6 together to deliver up to 160 mA
per LED.
Thermal Shutdown (TSD) The IC shuts down when junction
temperature exceeds 165°C. It will recover automatically when
the junction temperature falls below 125°C.
VIN Undervoltage Lockout (UVLO) The device is shut down
when input voltage, VIN , falls below VUVLO.
Fault Mode The IC functions in various fault states:
Fault State
Auto-
Restart
Description
Over-
voltage
Protection
Fault occurs when OVP pin exceeds the VOVP
Yes threshold. Used to protect the output voltage
from damaging the part.
Pulse-
by-Pulse
Yes
Current Limit
Fault occurs when the current through the
external MOSFET increases exceeds such that
the voltage across the SENP and SENN pins
exceeds 95 mV typical. The MOSFET switch is
turned off on a cycle-per-cycle basis.
Fault occurs when the COMP pin exceeds the
overcurrent detect threshold. Multiple pulse-by-
Overcurrent
Protection
Yes
pulse current limits will result in the COMP pin
voltage to rise. After a time period determined
by the COMP pin current and the output
capacitor, COUT , the COMP voltage will exceed
the overcurrent detect threshold, forcing a fault.
Over-
temperature
Protection
Yes
Fault occurs when the die temperature exceeds
the over-temperature threshold, 165°C typical.
LED Short
Protection
VIN UVLO
No
Fault occurs when the LED pin voltage exceeds
VSC , 18.7 V typical.
No
Fault occurs when VIN drops below VUVLO,
4.0 V typical. This fault resets all latched faults.
Allegro MicroSystems, Inc.
10
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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