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A8509GLBTR-T

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

indicated by the specifications for tPWSYNCON and tPWSYNCOFF .

Thus any pulse with a duty cycle of 19% to 85% at 800 kHz will

synchronize the IC.

It is recommended to also use the RFSET resistor with the

external clock signal. If a synchronization clock is lost during

operation, the IC will revert to the preset switching frequency

that is set by the RFSET resistor. In this configuration the preset

frequency does not have any restrictions other than the normal

operating range of 300 to 800 kHz. During the changeover period

the IC stops switching for an approximately 5 μs period to allow

for the synchronization detection circuitry to switch over to exter-

nal preset switching frequency.

Although examples shown in figures 13 and 14 are extreme

cases of clock-to-resistor frequency changes, it is recommended

that actual applications not have such large switching frequency

changes. In most applications the RFSET resistor and clock fre-

quency should be very close to each other in terms of frequency.

Setting the frequencies close together will prevent the system

from experiencing large changes on frequency-dependent signals

and components, such as the inductor ripple current and the com-

pensation resistor and capacitor.

fSW = 800 kHz from Changeover

RFSET value

period

fSW = 600 kHz

from SYNC pulse

C1

VGATE

VCOMP

C2

IOUT

C3

VFSET/SYNC

C4

t

Figure 13. Synchronization feature with 200 kHz difference between RFSET

and external clock signal. The synchronization frequency is 600 kHz,

and the resistor preset frequency is 800 kHz. Note that there is very little

disturbance in the LED current at the time of changeover; shows

VGATE (ch1, 5 V/div.), VCOMP (ch2, 1 V/div.), IOUT (ch3, 1 A/div.), and

VFSET/SYNC (ch4, 5 V/div.), t = 5 μs/div.

LED current setting and LED dimming

fSW = 800 Hz from

RFSET value

Changeover

fSW = 300 Hz

period

from SYNC pulse

The maximum LED current can be up to 300 mA per channel.

The LED current is set through the RISET resistor connected

between the ISET pin and ground. The ILED current is set accord-

ing to the following formula:

RISET = (1.000 / ILED ) × 2320

(2)

where RISET is in Ω, and ILED is in A. This sets the maximum cur-

rent through the LEDs, referred to as the 100% current. Standard

RISET values are as follows:

Standard Resistor Value

Closest to RISET

(kΩ)

7.87

9.53

11.5

14.3

19.1

LED current per LED, ILED

(mA)

300

240

200

160

120

C1

VGATE

VCOMP

C2

IOUT

C3

VFSET/SYNC

C4

t

Figure 14. Synchronization feature with 500 kHz difference between RFSET

and external clock signal, illustrating the flexibility of the RFSET/SYNC pin;

synchronization frequency is 300 kHz, and the resistor preset frequency

is 800 kHz; shows VGATE (ch1, 5 V/div.), VCOMP (ch2, 1 V/div.),

IOUT (ch3, 1 A/div.), and VFSET/SYNC (ch4, 5 V/div.), t = 10 μs/div.

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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