A8504 データシートの表示（PDF） - Allegro MicroSystems

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A8504

WLED/RGB Backlight Driver for Medium Size LCDs

Allegro MicroSystems 

A8504

WLED/RGB Backlight Driver for Medium Size LCDs

Open LED Protection. The A8504 has protection against open

LEDs. If any enabled LED string opens, voltage on the corre-

VIN

D1

VOUT

sponding LEDx pin goes to zero. The boost loop operates in open

loop till the OVP level is reached. The A8504 identifies the open

LED string when overvoltage on the OVP pin is detected. This

SW SW

ROVP DZ

OVP

COUT

string is then removed from the boost controlling loop. The boost

circuit is then controlled in the normal manner, and the output

OVP

Disable

28.8 V

A

voltage is regulated, to provide the output required to drive the

22 kΩ

remaining strings. If the open LED string is reconnected, it will

1.23 V

sink current up to the programmed current level.

4.4 kΩ

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 re-

enabling the IC. It can also be restored to normal operation if the

Figure 5. Overvoltage protection circuit. Three alternative

configurations at (A) are available, as follows:

fault signal is removed from the corresponding LEDx pin, but an

OVP event occurs on any other LEDx pin.

External Component

ROVP only

OVP Rating

up to 45 V

DZ only

up to 47 V

both ROVP and DZ

redundancy

C1

VPWM

C1

VPWM

Startup

C2

Overvoltage condition

detected

VPWM

VOUT

Normal operation

established

IIN

VOUT

IOUT

IOUT

C4

C3

t

Symbol Parameter Units/Division

C1

VPWM

5.00 V

C2

IIN

500 mA

C3

vout

5.00 V

C4

Iout

200 mA

t

time

200 μs

LED opens

C2

Normal

operation

Overvoltage condition

detected

IIN

Normal operation restored

with open LED string

removed from control loop

VOUT

IOUT

C4

C3

t

Symbol Parameter Units/Division

C1

VPWM

5.00 V

C2

IIN

500 mA

C3

vout

5.00 V

C4

Iout

100 mA

t

time

200 μs

Figure 4. Timing of overvoltage protection (OVP) function when open

LEDs are detected at startup (left ) and during normal operation (right).

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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