A8509 View Datasheet(PDF) - Allegro MicroSystems

Part Name

Description

Manufacturer

A8509

Wide Input Voltage Range, High Efficiency 4-Channel Fault Tolerant LED Driver

Allegro MicroSystems 

A8509

Wide Input Voltage Range, High Efficiency

4-Channel Fault Tolerant LED Driver

from powering-up and putting an uncontrolled amount of current

through the LEDs. After the short is removed the affected LEDx

pin will rise up to the 500 mV level. When the LEDx pin voltage

exceeds the 260 mV threshold, the IC detects connected LEDs

and proceeds with LED detection and soft start. Figure 2 shows

a case when two LED channels are enabled. During the LED

detection period, voltage on both LEDx pins > 260 mV. Figure 5

shows a case with LEDa temporarily shorted to ground and

LEDb in normal operation.

When the converter senses that there is enough voltage on the

LEDx pins, the converter proceeds to increase the LED current

to the preset regulation current and the boost switch current sense

voltage limit is switched to the ISW(LIM) level to allow the A8509

to deliver the necessary output power to the LEDs (figure 8).

Soft start function

During soft start the LED current gain is reduced to (ILEDSS).

As an example, for a 240 mA output current, the soft-start LED

current would be set to about 9 mA (see figure 7). Also dur-

ing soft start the boost switch sense voltage is reduced to the

VSWSS(LIM) level, to limit the initial inrush current generated by

the charging of the output capacitors. The actual current limit

(ILIM) is equal to:

ILIM = VSWSS(LIM) / RSENSE

(1)

where VSWSS(LIM) is found in the Electrical Characteristics table,

and RSENSE is the current sense resistor value.

VEN

C1

IIN

C2

IIN limited by boost switch

to VSWSS(LIM) / RSENSE

IOUT

C3

VOUT

C4

t

Figure 7. Start-up operation, individual LEDx current = 240 mA, boost

sense resistor = 0.010 Ω; shows VEN (ch1, 2 V/div.), IIN (ch2, 2 A/div.),

IOUT (ch3, 1 A/div.), and VOUT (ch4, 20 V/div.), t = 500 μs/div.

VEN IIN limited by boost switch

C1

to VSWSS(lim) / RSENSE

IIN

C2

IOUT

C3

VOUT

C4

t

Figure 6. Start-up operation, individual LEDx current = 120 mA, boost

sense resistor = 0.020 Ω; shows VEN (ch1, 2 V/div.), IIN (ch2, 1 A/div.),

IOUT (ch3, 200 mA/div.), and VOUT (ch4, 20 V/div.), t = 500 μs/div.

VEN

C1

VCOMP

C2

IIN limited by boost switch

to VSWSS(LIM) / RSENSE

C3

IIN

Boost starts operating in

normal current limit VSW(LIM)

VOUT

C4

VOUT sufficient to begin

normal power-up

t

Figure 8. Normal start-up behavior; shows VEN (ch1, 2 V/div.), VCOMP

(ch2, 2 V/div.), IIN (ch3, 1 V/div.), and VOUT (ch4, 10 V/div.), t = 500 μs/div.

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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