A8513 ALLEGRO [Allegro MicroSystems], A8513 Datasheet - Page 18

• Manufacturer Part Number: A8513
• Description: Wide Input Voltage Range, High Efficiency Fault Tolerant LED Driver
• Manufacturer: ALLEGRO [Allegro MicroSystems]

A8513
Step 4a: Determine the Duty Cycle:
where V
Step 4b: Determine the maximum and minimum input current to
the system. The minimum input current will dictate the inductor
value. The maximum current rating will dictate the current rating
of the inductor. Given I
where η is efficiency. Next, calculate minimum input current, as
follows:
A good approximation of efficiency η can be taken from the effi-
ciency curves located in the data sheet. A value of 90% is a good
starting approximation.
STEP 4c: Determine the inductor value.
To assure that the inductor operates in continuous conduction
mode the value of inductor should be set such that the ½ inductor
ripple current is not greater than the average minimum input cur-
rent. A good inductor choice for inductor ripple current is 30% of
the maximum input current:
D(max)
I
I
IN
IN
(max) =
(min) =
d
ΔI
is the voltage drop of the boost diode.
L
= I
= 0.595 A × 0.3 = 0.18 A
=
=
=
=
IN
1 –
1 –
V
V
22.32 V
22.32 V
(max) × 0.3
V
OUT(OVP)
OUT(OVP)
V
IN
IN
V
22.32 V + 0.4 V
5 V
16 V
(min)
OUT(OVP)
(max)
OUT
V
5 V
IN
= I
0.9
120 mA
(min)
120 mA
0.9
I
LED
I
OUT
OUT
+
V
= 120 mA:
d
=
=
=
0.595 A
0.19 A
78%
Wide Input Voltage Range, High Efficiency
(14)
(15)
(16)
(17)
then:
Double-check to make sure the ½ current ripple is less than
I
A good inductor value to use would be 10 μH.
Step 4d: This step verifies that there is sufficient slope compensa-
tion for the inductor chosen. The required slope compensation
value for different frequencies is listed below:
Next insert the inductor value used in the design:
Calculate the minimum required slope:
Required Slope (min)
IN
(min):
L =
=
Fault Tolerant LED Driver
ΔI
ΔI
V
0.18 A
Lused
IN
L
0.500
0.250
(min)
(MHz)
f
I
SW
2
1
0.19 A > 0.09 A
=
=
IN
f
SW
=
=
5 V
2.0 MHz
(min) >
f
SW
1
ΔI
V
10 μH
0.20 A
2 MHz
1
Lused
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
IN
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
D(max)
(min) D(max)
L
(1 – D(max))
5 V
used
Slope Compensation
1
/
2
1
(1 – 0.78)
ΔI
1 10
2.0 MHz
10
0.78
L
f
(A/μs)
0.78
SW
3.73
1.85
3.70
1.83
–6
–6
=
10.83 μH
= 1.8 A/μs
=
0.20 A
(18)
(19)
(20)
(21)
18