A8521 datasheet(28/35 Pages) ALLEGRO

28 / 35 page

Wide Input Voltage Range, High Efficiency

Fault Tolerant LED Driver

A8521

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

The reverse voltage rating should be such that during operation

condition, the voltage rating of the device is larger than the maxi-

mum output voltage. In this case it is VOUT(OVP).

The peak current through the diode is calculated as:

Idp = IIN(max) + 1/2 ΔILused

(23)

= 0.94 (A) + 0.36 (A) / 2 = 1.12 A

The third major component in deciding the switching diode is the

reverse current, IR, 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 can be a large contributor, especially at high tempera-

tures. 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

factors that contribute to the size of the output capacitor are:

PWM dimming frequency and PWM duty cycle. Another major

contributor is leakage current, ILK. This current is the combina-

tion of the OVP leakage current 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 volt-

age variation on the output, VCOUT, during PWM dimming must

be less than 250 mV, so that no audible hum can be heard. The

capacitance can be calculated as follows:

COUT =

fPWM(dimming)

1 – D(min)

1 – 0.01

200 Hz

ILK

200 μA

3.96 μF

0.250 V

VCOUT

(24)

A capacitor larger than 3.96 μF should be selected due to degra-

dation of capacitance at high voltages on the capacitor. A ceramic

4.7 μF 50 V capacitor is a good choice to fulfill this requirement.

Corresponding capacitors include:

Vendor

Value

Part number

Murata

4.7 μF 50 V

GRM32ER71H475KA88L

Murata

2.2 μF 50 V

GRM31CR71H225KA88L

The rms current through the capacitor is given by:

ICOUTrms =

1 – D(max)

D(max) +

∆ILused

IOUT

0.240 (A)

0.39 A

× 12

IIN(max)

1 – 0.72

0.72 + 0.36 (A)

0.94 (A)× 12

(25)

The output capacitor must have a current rating of at least

390 mA. The capacitor selected in this design was a 4.7 μF 50 V

capacitor with a 3 A current rating.

Step 8

Selecting 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 =

fSW

0.36 (A)

∆ILused

0.23 μF

∆VIN

2 (MHz) 0.1 (V)

(26)

The rms current through the capacitor is given by:

CINrms

(1 – D)×

IOUT ×

∆ILused

0.095 A

IIN(max)

(1 – 0.72)×

0.240 (A) × 0.36 (A)

0.94 (A)

(27)

A good ceramic input capacitor with ratings of 2.2 μF 50 V or

4.7 μF 50 V will suffice for this application. Corresponding

capacitors include:

Vendor

Value

Part number

Murata

4.7 μF 50 V

GRM32ER71H475KA88L

Murata

2.2 μF 50 V

GRM31CR71H225KA88L

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