A8508 View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
A8508
Allegro MicroSystems
'A8508' PDF : 28 Pages View PDF
A8508
Wide Input Voltage Range, High Efficiency
8-Channel Fault Tolerant LED Driver
The minimum input current can be calculated as:
IIN(min)
= VOUT × IOUT
VIN(max) × η
(15)
=
34.65 V × 0.960 A
16 V × 0.90
= 2.31 A
STEP 4c: Determining the inductor value. To assure that the
inductor operates in continuous conduction mode, the value of
inductor must be set such that 1/2 of the inductor ripple current is
not greater than the average minimum input current.
As a first pass, take Iripple to be 30% of the maximum inductor
current:
∆IL = IIN(max) × (Iripple / IIN(max))
(16)
= 3.72 A × 0.30
= 1.1 A
Check to make sure that 1/2 of the inductor ripple current is less
than IIN(min):
IIN(min)
>
1
2
× ∆IL
2.31 A > 0.56 A
The inductor value can be calculated as:
L
=
VIN(min)
∆IL × fSW
×
D(max)
(17)
=
10 V
1.1 A × 600 kHz
× 0.745
= 10.62 μH
A good inductor value to use would be Lused = 10 μH.
STEP 4d: Determining the inductor current rating. The inductor
current rating must be greater than the IIN(max) value plus the
ripple current ΔIL , calculated as:
IL(min)
=
IIN(max)
+
1
2
× ∆ILused
(18)
= 3.72 A + 0.56 A
= 4.28 A
STEP 4e: Choosing the RSENSE resistor. The sense resistor
value can be calculated as follows:
RSENSE =
VSENSEP
IL(min)
(19)
=
0.086 V
4.28 A
= 0.02 Ω
0.018 Ω is a good value to use for the resistor.
STEP 4f: This step is used to verify that there is sufficient slope
compensation for the inductor chosen. The internal slope com-
pensation value is determined by the following formula:
Slope Compensation = 2.81×10–7 × fSW
(20)
where fSW is in Hz. Substituting:
= 0.168 V/μs
With RSENSE = 0.02 Ω this translates to:
0.168 / 0.02 = 8.4 A/μs
Next invert equation 17 and insert the inductor value used in the
design:
∆ ILused
=
VIN(min)
Lused × fSW
×
D(max)
(21)
where fSW is in MHz. Substituting:
=
10 V
10 μH × 600 kHz
× 0.745
=
Inductor Current Slope =
1.24 A
∆ ILused× 1 × 10 –6
(22)
1 × (1 – D(max))
fSW
1.24 A × 1 × 10–6
=
1 × (1 – 0.745)
600 kHz
= 2.91 A/ μs
Note: that the 1×10 –6 is a constant multiplier. This slope should
be smaller than the internal slope compensation.
STEP 5: To determine the resistor values for a switching fre-
quency use figure 9.
STEP 6: Choosing the proper switching diode. The switching
diode must be chosen for three characteristics when it is used in
LED lighting circuitry: reverse voltage rating, current rating, and
reverse current characteristic of the diode.
Allegro MicroSystems, Inc.
22
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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