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 reverse voltage rating should be such that, during any opera-
tion condition, the voltage rating of the device is larger than the
maximum output voltage. In this case, the maximum output volt-
age is VOUT(OVP) .
The peak current through the diode is:
Id(peak) = IIN(max) + ∆ILused
(23)
= 3.72 A + 0.56 A
= 4.28 A
The other major component in determining the switching diode is
the reverse current characteristic of the diode. This characteristic
is especially important when PWM dimming is implemented.
During PWM off-time the boost converter is not switching. This
results in a slow bleeding-off of the output voltage due to leakage
currents (IR). IR , or reverse current, can be a huge contributor
especially at high temperatures. On the diode that was selected in
this design, the current varies between 1 and 100 μA.
STEP 7: Choosing the output capacitors. The output capacitors
must be chosen such that they can provide filtering for both the
boost converter and for the PWM dimming function. The biggest
factor that contributes to the size of the output capacitor is PWM
dimming frequency and the PWM duty cycle. Another major
contributor is leakage current (ILK ). This current is the combina-
tion of the OVP current sense as well as the reverse current of the
switching diode.
In this design the PWM dimming frequency is 200 Hz and the
minimum duty cycle is 1%. Typically the voltage variation on the
output during PWM dimming must be less than 250 mV (VCOUT)
so that no audible hum can be heard:
COUT = ILK×
1 – DPWM(min)
fPWM × VCOUT
(24)
1 – 0.01
= 300 μA × 200 Hz × 0.250 V
= 5.94 μF
A capacitor larger than 5.94 μF should be selected due to deg-
radation of capacitance at high voltages on the capacitor. Two
ceramic 4.7 μF 50 V capacitors are a good choice to fulfill this
requirement.
The rms current through the capacitor is given by:
ICOUTrms = IOUT
D(max)
+
∆ILused
IIN(max)
12
(25)
1 – D(max)
= 0.960 A
= 1.67 A
0.745
+
1.24
3.72 A
A
12
1 – 0.745
The output capacitor must have a current rating of at least 1.67 A.
The output capacitors selected in this design have a combined
rms current rating of 2 A.
STEP 8: Selection of input capacitor. The input capacitor must be
selected such that it provides a good filtering of the input voltage
waveform. A good rule of thumb is to set the input voltage ripple
(ΔVIN) to be 1% of the minimum input voltage. The minimum
input capacitor requirements are as follows:
CIN = 8
∆ILused
fSW ∆VIN
(26)
=
1.24 A
8 600 kHz 0.1 V
= 2.65 μF
The rms current through the capacitor is given by:
IINrms =
IOUT
×
∆ILused
IIN(max)
(27)
(1 – D(max)) 12
0.960 A ×
1.24 A
3.72 A
=
(1 – 0.765) 12
= 0.363 A
A good ceramic input capacitor with ratings of 50 V, 4.7 μF will
suffice for this application.
Corresponding capacitors include:
Vendor
Murata
Murata
Value
4.7 μF 50 V
2.2 μF 50 V
Part number
GRM32ER71H475KA88L
GRM31CR71H225KA88L
Allegro MicroSystems, Inc.
23
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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