A8521 datasheet(31/35 Pages) ALLEGRO

31 / 35 page

Wide Input Voltage Range, High Efficiency

Fault Tolerant LED Driver

A8521

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

tions, due to stringent EMI requirements, the system must operate

in continuous conduction mode throughout the whole input volt-

age range.

Step 4a

Determining the duty cycle, calculated as follows:

D(max)

VOUT(OVP)

VIN(min)

VOUT(OVP)

76.5%

5 (V) + 15.9 (V) + 0.4 (V)

+ 0.4 (V)

15.9 (V)

(35)

Step 4b

Determining the maximum and minimum input current

to the system. The minimum input current will dictate the induc-

tor value. The maximum current rating will dictate the current

rating of the inductor. First, the maximum input current, given:

IOUT = #CHANNELS ILED

0.240 A

0.060 (A)

(36)

then:

IIN(max) =

VIN(min)

VOUT(OVP) IOUT

0.848 A

15.9 (V)

5 (V)

0.90

0.24 (A)

(37)

where η is efficiency.

Next, calculate minimum input current, as follows:

IIN(min) =

VIN(max)

VOUT(OVP) IOUT

0.265 A

15.9 (V)

16 (V)

0.90

0.24 (A)

(38)

Step 4c

Determining the inductor value. To ensure that the induc-

tor operates in continuous conduction mode, the value of the

inductor must be set such that the ½ inductor ripple current is not

greater than the average minimum input current. As a first pass

assume Iripple to be 30% of the maximum inductor current:

ΔIL = IIN(max) × Iripple

(39)

= 0.848 × 0.30 = 0.254 A

then:

VIN(min)

D(max)

fSW

ΔIL

7.53 μH

0.254 (A)

0.765

5 (V)

2 (MHz)

(40)

Step 4d

Double-check to make sure the ½ current ripple is less

than IIN(min):

IIN(min) > 1/2 ΔIL

(41)

0.265 A > 0.127 A

A good inductor value to use would be 10 μH.

Step 4e

Next insert the inductor value used in the design to deter-

mine the actual inductor ripple current:

VIN(min) D(max)

fSW

Lused

ΔILused

10 (μH)

0.191 A

0.765

5 (V)

2.0 (MHz)

(42)

Step 4f

Determining the inductor current rating. The inductor

current rating must be greater than the IIN(max) value plus half of

the ripple current ΔIL, calculated as follows:

L(min) = IIN(max) + 1/2 ΔILused

(43)

= 0.848 (A) + 0.096 (A) = 0.944 A

Step 5

Determining the resistor value for a particular switching

frequency. Use the RFSET values shown in figure 7. For example,

a 10 kΩ resistor will result in a 2 MHz switching frequency.

Step 6

Choosing the proper switching diode. The switching diode

must be chosen for three characteristics when it is used in LED

lighting circuitry. The most obvious two are: current rating of the

diode and reverse voltage rating.

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