A8503 View Datasheet(PDF) - Allegro MicroSystems

Part Name

Description

MFG CO.

A8503

High Efficiency 6-Channel, 2 MHz, WLED/RGB Driver for Medium Displays, with Integrated 55 V Power Switch

Allegro MicroSystems 

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A8503

High Efficiency 6-Channel, 2 MHz, WLED/RGB Driver for

Medium Displays, with Integrated 55 V Power Switch

PWM signal is low, the LED sinks turn off. Referring to figure 5,

there is a 4 μs ramp-up delay between when the PWM signal is

applied and when the current reaches the 90% level. 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 0.4 A 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 VOUT reaches the required level, LEDx

current toggles between 0% and 100% with each PWM command

signal.

In case of a heavy overload on output voltage at startup, the

device may stay in soft start mode indefinitely, if the output volt-

age cannot rise to the LED regulation level and the MODE pin is

tied high. To avoid this scenario, A8503 has a soft start timeout

when the MODE pin is tied low. With the MODE pin low, if the

device does not finish soft start during 131,072 switching cycles,

it is shut down.

LED Open and Short Detect

All unused LED pins should be connected to ground to prevent any

undesired faults from triggering. For LED short detect, any enabled

LEDx pins that have a voltage exceeding the short circuit detect

voltage, VSC , causes the device to shut down irrespective of what

mode the A8503 is in. The open LED fault will be triggered as

soon as an enabled LEDx pin does not have sufficient current flow-

ing through it to stay in regulation. This will result in increased

output voltage until the LED is back in regulation or overvoltage

protection (OVP) is tripped. If OVP is tripped, depending on the

mode of operation, the A8503 will either shut down (MODE =

low) or will remove the LED string from operation and continue to

operate normally (MODE = high). Please refer to the Fault Mode

table for latched and non-latched fault conditions.

Overvoltage Protection

The A8503 has two independent overvoltage protection features

to protect the device against output overvoltage. The overvoltage

level can be set, from 44 to 50 V typical, with an external resistor,

ROVP. When the current though the OVP pin exceeds 240 μA,

the OVP comparator goes high and the device shuts down in an

OVP fault state when the MODE pin is low. If the MODE pin is

high, the OVP fault disables all LEDx strings that are below regu-

lation, thus preventing them from controlling the boost output

voltage.

The device also offers open Schottky diode protection. If for any

reason the voltage on the SW pins exceeds more than 57 V, the

IC shuts down and remains latched irrespective of the MODE pin

level. The overvoltage protection circuit is shown in figure 6.

Calculate the value for ROVP as follows:

ROVP = (VOVP – 44) / 240 μA ,

where VOVP is the desired typical OVP level in V, and ROVP is in Ω.

Overcurrent Protection

The IC provides pulse-by-pulse current limiting at 2.7 A for the

boost MOSFET. If the overcurrent fault state persists, the boost

control loop will force the compensating capacitor to rise in volt-

age until it reaches the overcurrent fault level. This fault shuts

down the IC and is latched when MODE pin is low (MODE =

AGND). If MODE pin is high, the overcurrent fault forces the

device into soft start.

VOUT

IOUT

VFPWM

Figure 4. PWM Pin Dimming, fPWM = 200 Hz, duty cycle = 10%. C1, IOUT

50 mA / div; ; C2, VFPWM (signal on PWM pin) 2 V / div; C3, VOUT 5 V / div,

AC coupled.

PWM

4 μs

ILEDx

90% Current

Figure 5. ILEDx versus PWM input

Allegro MicroSystems, Inc.

12

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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