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A8516

LED Backlight Driver for LCD Monitors and Televisions

Allegro MicroSystems 

A8516

LED Backlight Driver for LCD Monitors and Televisions

Functional Description

Overview The A8516 is a multi-output WLED/RGB control-

ler for backlighting medium-size displays. It has an integrated

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

The gate driver voltage is regulated at 7 V, which allows a wide

selection of power MOSFET (in contrast to being limited to

logic-level MOSFETs when using a 5 V gate driver). The boost

controller operates in fixed-frequency current-mode control. The

switching frequency can be set in the range from 300 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.

LED Current Setting The maximum LED current can be set, at

up to 130 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:

ILED = ISET × 640 .

(2)

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. Conversely, we can calculate RISET according to the

LED current required:

RISET = ( 1.235 / ILED ) × 640 .

(3)

In steady-state operation, the maximum average LED current that

can be handled by the IC depends on its thermal budget. That

is, maximum power dissipation and acceptable temperature rise.

The thermal budget is affected by various parameters, such as

PCB size, copper plane around IC, LED Vf mismatch, selection

of power components (MOSFET, inductor and diode), maximum

board temperature, and so on.

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 ,

(4)

where fSW is in MHz and RFSET is in kΩ. The typical range of

RFSET is approximately 51 to 174 kΩ, which corresponds to

1 MHz to 300 kHz.

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

pin, and turns off when this pin is pulled low. The LED current

sinks are turned on when both the EN and the PWM inputs are high.

Channel Selection The A8516 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 twice the

current per LED.

PWM Dimming The A8516 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 to 5 μs to compensate for this delay.

PWM

tD

ILED

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

reaching the 90% level

Allegro MicroSystems, Inc.

11

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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