A8519KETTR-R datasheet(28/35 Pages) ALLEGRO

28 / 35 page

Wide Input Voltage Range, High-Efficiency,

Fault-Tolerant LED Driver

A8519 and

A8519-1

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Design Example

This section provides a method for selecting component values

when designing an application using the A8519.

Assumptions: For the purposes of this example, the following are

given as the application requirements:

•

VIN: 10 to 14 V

•

Quantity of LED channels, #CHANNELS: 4

•

Quantity of series LEDs per channel, #SERIESLEDS: 10

•

LED current per channel, ILED: 60 mA

•

LED Vf at 60 mA: 3.2 V

•

fSW: 2 MHz

•

PWM dimming frequency 200 Hz, 1% duty cycle

Step 1: Connect LED strings to pins LED1, LED2, LED3, and

LED4.

Step 2: Determine the LED current set resistor RISET

R =

ISET

R =

ISET

R = 11.8 k

ISET

= 12 k

(V ×A )

ISET

(1.017 × 710)

LED

0.06 A

An 11.8 kΩ resistor was chosen.

Step 3a: Determining the OVP resistor.

The OVP resistor is connected between the OVP pin and the

output voltage of the converter. The first step is to determine the

maximum voltage based on the LED requirements. The regula-

tion voltage for an LED pin (VLEDx) of the A8519 is 850 mV. A

5 V headroom is added to give margin to the design due to noise

and output voltage ripple.

VOUT(ovp) = #SERIESLEDs × Vf + VLED + 5 V

VOUT(ovp) = 10 × 3.2 V + 0.850 V + 5 V

VOUT(ovp) = 37.85 V

The OVP resistor is:

OVP

– V

)

OUT(ovp)

OVP(th)

Where both IOVP(th) and VOVP(th) values are from the datasheet’s

Electrical Characteristics table.

OVP

= 147.75 k

OVP

37.85 – 8.3

0.2

Choose a value of resistor that is higher value than the calculated

ROVP. In this case, a value of 158 kΩ was selected. Below is the

actual value of the minimum OVP trip level with the selected

resistor.

VOUT(ovp) = 158 kΩ × 0.2 mA + 8.3 V

VOUT(ovp) = 39.9 V

Step 3b: At this point, a quick check needs to be done to see

if the conversion ratio is adequate for the selected frequency.

Where VD is the boost diode forward voltage, minimum off-time

(tSW(off)) is found in the datasheet:

Theoretical Max V

OUT =

1 – D

MAX(boost)

= 1– (85 ns × 2.2 MHz) = 0.813

MAX(boost)

= 1– t

× f

MAX(boost)

SW(off)

SW(max)

– V

IN(min)

VD is the voltage drop of the boost diode.

10 V

Theoretical Max V

OUT =

1 – 0.813 – 0.4 = 53.1 V

Theoretical Max VOUT value needs to greater than the value

VOUT(ovp). If this is not the case, the switching frequency of the

boost converter is going to have to be reduced to meet the maxi-

mum duty cycle requirements.

Step 4: Inductor selection.

The inductor needs to be chosen such that it can handle the neces-

sary input current. In most applications, due to stringent EMI

requirements, the system needs to operate in continues conduc-

tion mode throughout the whole input voltage range.

APPLICATION INFORMATION

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