A8519KETTR-R datasheet(31/35 Pages) ALLEGRO

31 / 35 page

Wide Input Voltage Range, High-Efficiency,

Fault-Tolerant LED Driver

A8519 and

A8519-1

Allegro MicroSystems, LLC

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Step 8: Selection of input capacitor. The input capacitor needs to

be selected such that it provides good filtering of the input volt-

age waveform. A good rule of thumb is to set the input voltage

ripple ΔVIN to be 1% of the minimum input voltage. The mini-

mum input capacitor requirements are as follows:

C =

ΔIL(used)

8 × f × ΔV

C =

= 0.234 µF

0.375 A

8 × 2 MHz × 0.1 V

IN(rms)

= 0.1 A

I ×

OUT

ΔI

L(used)

(1 – D )× 12

MAX

IN(max)

= 0.1 A

0.240 A × 0.375 A

(1 – 0.75) × 12

1.06 A

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

50 V / 4.7 µF will suffice for this application.

Vendor

Value

Part Number

Murata

4.7 µF / 50 V

GRM32ER71H475KA88L

Murata

2.2 µF / 50 V

GRM31CR71H225KA88L

If long wires are used for the input, it is necessary to use a much

larger input capacitor. A larger input capacitor is also required to

have stable input voltage during line transients. Combinations of

aluminum electrolytic and ceramic capacitors can be used.

Step 9: Choosing the input disconnect switch components.

Set the input disconnect current limit to 4.25 A.

= 0.0259

0.11 V

4.25 A

The RSC chosen is 0.024 ohms. Therefore, the voltage drop across

RSC is:

adj

V= 4.25 A×0.024 = 0.102 V

V– V

VSENSE(trip)

adj

= 372

adj

0.11 V – 0.102 V

21.5 µA

A value of 383 W was chosen for this design. The disconnect

switch Q1 works as on or off. Therefore, the Radj value is not

really critical.

For the input disconnect switch, an AO4421 6.2 A / 60 V P-chan-

nel MOSFET is selected.

To achieve proper operation at low dimming ratios, connect an

RC filter to the VOUT pin. Use R = 10 kW and C = 47 pF.

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