A8514KLPTR-T 查看數據表（PDF） - Allegro MicroSystems

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A8514KLPTR-T

Wide Input Voltage Range, High Efficiency Fault Tolerant LED Driver

Allegro MicroSystems 

A8514

Wide Input Voltage Range, High Efficiency

Fault Tolerant LED Driver

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) + Vd

VIN(min) + VOUT(OVP) + Vd

(35)

= 15.9 (V) + 0.4 (V)

= 76.5%

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

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

(36)

= 4 0.060 (A) = 0.240 A

then:

IIN(max) =

VOUT(OVP)

VIN(min)

IOUT

H

(37)

= 15.9 (V) 0.24 (A) = 0.848 A

5 (V) 0.90

then:

L = VIN(min) D(max)

(40)

ΔIL fSW

=

5 (V)

0.765 = 7.53 μH

0.254 (A) 2 (MHz)

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:

ΔILused =

VIN(min)

Lused

D(max)

fSW

(42)

= 5 (V) 0.765 = 0.191 A

10 (μH) 2.0 (MHz)

where η is efficiency.

Next, calculate minimum input current, as follows:

IIN(min) =

VOUT(OVP)

VIN(max)

IOUT

H

(38)

= 15.9 (V) 0.24 (A) = 0.265 A

16 (V) 0.90

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

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.

Allegro MicroSystems, Inc.

31

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

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