A8516 Datasheet, PDF (12/17 Pages) Allegro MicroSystems – The A8516 is a multi-output WLED/RGB driver for backlighting LCD monitors and televisions.

A8516
LED Backlight Driver for LCD Monitors and Televisions
Startup Sequence When EN is pulled high, the IC enters
soft start. The IC first tries to determine which LEDx pins are
being used, by raising the LEDx pin voltage with a small current.
After a duration of 512 switching cycles, the LEDx pin voltage
is checked. Any LEDx channel with a drain voltage smaller then
100 mV is removed from the control loop. This is the reason why
unused LEDx pins should be connected to GND,
After the first PWM positive trigger, the boost current is limited
to 30% of normal value and all active LEDx pins sink ¹/₁₂ of
the set current until output voltage reaches sufficient regulation
level. When the device comes out of soft start, boost current and
the LEDx pin currents are set to normal operating level. Within
a few cycles, the output capacitor charges to the voltage required
to supply full LEDx current. After output voltage, V_OUT , reaches
the required level, LEDx current toggles between 0% and 100%
with each PWM command signal.
In case of a heavy overload on V_OUT at startup, the device will
stay in soft start mode indefinitely, as the output voltage cannot
rise to the LED regulation level.
LED Short Detect Any LEDx pins that have a voltage exceed-
ing the Short Circuit Detect Voltage, V_SC , cause the device to
shut down and this condition is latched. This faults occurs when
multiple LEDs short. In case only a few LEDs short, the IC will
continue to work as long as power dissipation in the IC is limited.
Overvoltage Protection The A8516 has an adjustable over-
voltage protection feature to protect the power components
(external MOSFET, output diode and capacitors) against output
overvoltage. The overvoltage level can be set, from 19.5 V to a
higher voltage, with an external resistor, R_OVP. When the current
though the OVP pin exceeds 200 μA, internal OVP comparator
goes high and the device shuts down. The OVP fault disables all
LEDx strings that are below regulation, thus preventing them
from controlling the boost output voltage.
Calculate the value for R_OVP (Ω) as follows:
R_OVP = (V_OVP – 19.5) / 200 μA ,
(5)
where V_OVP is the required OVP level in V.
For single-IC operation, select R_OVP such that its OVP setpoint is
approximately 10 V above the LED operating voltage at cold. For
example, given the pin regulation voltage, V_LEDx of 1.4 V (typ.), if
LED V_F = 3.4 V (max.) and there are 15 LEDs in series, then the
operating voltage is approximately:
V_OUT = 3.4 V × 15 + 1.4 V = 52.4 V .
In this case, select OVP at about 60 V, which gives R_OVP = 200 kΩ.
Open LED Protection 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 A8516 identifies the open LED string when over-
voltage is detected. Open strings are then removed from the regu-
lation 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 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,
I_SC (A), can be set by selecting the external resistor, R_SC (Ω):
R_SC = 0.095 / I_SC .
(6)
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, C_COMP,
to rise in voltage until it reaches the overcurrent fault level
(3.4 V approximately). The overcurrent fault forces the device
into soft start.
Allegro MicroSystems, Inc.
12
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com