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A8507

LED Backlight Driver for LCD Monitors and Televisions

Allegro MicroSystems 

A8507

LED Backlight Driver for LCD Monitors and Televisions

Functional Description

Overview The A8507 is a multi-output WLED/RGB controller

for backlighting medium-size displays. It has a integrated gate

driver for driving an external N-channel boost MOSFET. The

boost controller is fixed frequency current mode converter. The

switching frequency can be set in the range from 250 kHz to

1 MHz, by an external resistor, RFSET , connected between FSET

and ground.

The external MOSFET switch is protected by pulse-by-pulse cur-

rent limiting. The current limit is independent of duty cycle, and

is set using an external sense resistor, RSC.

The A8507 has six well-matched current sinks that provide regu-

lated current through the LEDs for uniform display brightness.

The boost converter is controlled by monitoring all LEDx pins

simultaneously and continuously.

LED Current Setting The maximum LED current can be set,

at up to 80 mA/channel, through the ISET pin. Connect a resistor,

RISET , between this pin and ground to set the reference current

level, ISET . The value of ISET (mA) is determined by:

ISET = 1.235 / RISET (kΩ) .

(1)

The resulting current is multiplied internally with a gain of 640

and mirrored on all enabled LEDx pins. This sets the maximum

current through each LEDx, referred as the 100% Current. The

LEDx current can be reduced from the 100% Current value by

applying an external PWM signal on the PWM pin.

Boost Switching Frequency Setting Connect an external

resistor between the FSET pin and GND, to set boost switching

frequency, fSW . The value of fSW (MHz) is determined by:

fSW = 52 / RFSET ,

(2)

where fSW is in MHz and RFSET is in kΩ.

Enable The IC turns on when a high signal is applied on the EN

pin, and turns off when this pin is pulled low.

PWM Dimming The A8507 has a very wide range for PWM

signal input. It can accept a PWM signal from 100 Hz to 5 kHz.

When a PWM high signal is applied, the LEDx pins sink

100% Current. When the PWM signal is low, the LED sinks

turn off.

Referring to figure 13, there is a ramp-up delay between when

the PWM signal is applied and when the current reaches the

90% level. To improve current dimming linearity for PWM pulse

widths less than 100 μs, increase the applied PWM pulse-width

by 3 μs to compensate for this delay.

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.

After the first PWM positive trigger, the boost current is limited

to 35% 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, VOUT , reaches

the required level, LEDx current toggles between 0% and 100%

with each PWM command signal.

In case of a heavy overload on VOUT 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, VSC , 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 A8507 has an adjustable over-

voltage protection feature to protect the external MOSFET

against output overvoltage. The overvoltage level can be set,

from 36 V to a higher voltage, with an external resistor, ROVP.

When the current though the OVP pin exceeds 240 μA, internal

OVP comparator goes high and the device shuts down. The OVP

PWM

tD

ILED

Figure 13. Propagation delay from the PWM signal rising edge to ILEDx

reaching the 90% level

Allegro MicroSystems, Inc.

9

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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