A8509 View Datasheet(PDF) - Allegro MicroSystems

Part Name

Description

MFG CO.

A8509

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

Allegro MicroSystems 

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A8509

Wide Input Voltage Range, High Efficiency

4-Channel Fault Tolerant LED Driver

Design Example

This section provides a method for selecting component values

when designing an application using the A8509.

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

given as the application requirements:

• VIN: 10 to 16 V

• Quantity of LED channels, #CHANNELS: 4

• Quantity of series LEDs per channel, #SERIESLEDS : 10

• LED current per channel, ILED : 240 mA

• Vf(240) at 240 mA: 3.2 V (max)

• fSW : 600 kHz

• TA(max): 65°C

• PWM dimming frequency: 200 Hz, 1% duty cycle

Electrical Characteristics table. Choose a value of resistor that is

the closest value higher than the calculated ROVP . In this design

example, a value of 750 kΩ is selected.

Below is the actual value of the minimum OVP trip level with the

selected resistor, applying equation 8:

VOVP(master) = ROVP × IOVPH + 1.25 V

= 750 kΩ × 49 μA + 1.25 V

= 38.75 V

STEP 4: Determine the inductor. The inductor must be chosen

such that it can handle the necessary input current. In most appli-

cations, due to stringent EMI requirements, the inductor must

operate in continuous conduction mode throughout the whole

input voltage range.

Step 1: Connect LEDs to pins LED1 through LED4.

Step 2: Determine the LED current by setting resistor RISET . To

do so, apply equation 2:

RISET = (1.000 / ILED ) × 2320

= (1.000 V / 0.240 A ) × 2320

= 9.67 kΩ

Choose an 9.53 kΩ resistor.

STEP 3: Determine 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. Then the regulation voltage of

650 mV should be added, along with 2 V for noise and regulation.

Given the regulation voltage (VLED) of the A8509 is 650 mV, the

minimum required voltage can be determined as follows:

VOUT(OVP) = #SERIESLEDS ×Vf(240) + VLED + 2 V (10)

= 10 × 3.2 V + 0.650 V + 2 V

VOUT(OVP)(min) = 34.65 V

The OVP resistor (ROVP) value can be calculated as:

ROVP =

VOUT(OVP)(min) – VOVP(th)(min)

IOVPH(min)

(11)

= 34.65 V – 1.11 V

45 μA

= 745 kΩ

where both IOVP(th)(min) and VOVP(th)(min) are found in the

STEP4a: Determine the maximum duty cycle of the system:

D(max) =1– VIN(min) × η

(12)

VOUT(OVP) + Vf(boost)

=1–

10

34.65

V×

V+

0.9

0.4

V

= 74.5%

A good approximation of efficiency (η) is 90%. The voltage drop

of the boost diode can be approximated to be about 0.4 V.

STEP 4b: Determine the maximum and minimum input current

to the system. The minimum input current dictates the inductor

value. The maximum current rating dictates the current rating of

the inductor.

To calculate the maximum input current, first determine the

required output current:

IOUT = #CHANNELS × ILED

(13)

= 4 × 240 mA

= 0.960 A

Then substitute into the formula for maximum input current:

IIN(max)

=

VOUT × IOUT

VIN(min) × η

(14)

=

34.65 V × 0.960 A

10 V × 0.90

= 3.7 A

Allegro MicroSystems, Inc.

21

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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