A8519 and
A8519-1
Wide Input Voltage Range, High-Efficiency,
Fault-Tolerant LED Driver
Step 4a: Determine the Duty Cycle.
IL = IIN(max) × Kripple
VIN(min)
(VOUT(ovp) + V )
DMAX = 1 –
IL = 1.06 A × 0.3 = 0.318 A
D
(VIN(min) × DMAX
IL × fSW)
)
L =
10
DMAX = 1 –
= 0.75
(39.9 + 0.4)
10 V
0.318 A × 2 MHz
L =
× 0.75 = 11.79 µH
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.
Double-check to make sure that ½ current ripple is less than
IIN(min)
.
IIN(min) > ½ DIL
V
OUT(ovp)× IOUT
IIN(max)
=
V
×
0.625 A > 0.159 A
IN(min)
A good inductor value to use would be 10 µH.
IOUT = #Channels × ILED
Step 4d: This step is used to verify that there is sufficient slope
compensation for the inductor chosen. 6 A/µs slope compensation
value is applied inside the IC at 2 MHz switching frequency. The
slope compensation at any switching frequency can be deter-
mined by the following formula:
IOUT = 4 × 0.060 A = 0.240 A
A good approximation of efficiency η can be taken from the
efficiency curves located on page 10. A value of 90% is a good
starting approximation.
6 A/µs × fSW
Slope Comp =
2 × 10 6
39.9 V × 240 mA
10 V × 0.90
IIN(max)
=
= 1.06 A
Next, insert the inductor value used in the design:
VOUT × IOUT
IIN(max)
=
VIN(max)
×
VIN(min) × DMAX
ΔIL(used)
=
L(used) × fSW
VOUT = 10 × 3.2 V + 0.85 V = 32.85 V
10 V × 0.75
10 µH × 2 MHz
32.85 V × 240 mA
ΔIL(used)
=
= 0.375 A
IIN(min)
=
= 0.625 A
14 V × 0.90
ΔIL(used) × ΔS × 10-6
1
Required Min Slope =
Step 4c: Determining the inductor value. To ensure that the
inductor operates in continuous conduction mode, the value of
the inductor needs to be set such that the ½ inductor ripple cur-
rent is not greater than the average minimum input current. A
first pass calculation for Kripple should be 30% of the maximum
inductor current.
× (1 – DMAX
)
fSW
where ΔS is taken from the following formula:
0.18
ΔS = 1 –
DMAX
ΔS = 0.76
0.375 × 0.76 × 10-6
Required Min Slope =
= 2.28 A/µs
1
× (1 – 0.75)
2 MHz
Allegro MicroSystems, LLC
115 Northeast Cutoff
29
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com