A8519KETTR-R datasheet(30/35 Pages) ALLEGRO

30 / 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

If the required minimum slope is larger than the calculated slope

compensation, the inductor value needs to be increased. Note that

the slope compensation value is in A/μs the 1 × 10-6 is constant

multiplier.

Step 4e: Determining the inductor current rating.

= I

+ (1/2) × ΔI

L(min)

IN(max)

= 1.25 A

= 1.06A +

L(min)

0.375 A

Step 5: To determine the resistor value for a switching frequency

refer to the graph in Figure 8. A 10 kW resistor will result in a

2 MHz switching frequency.

Step 6: Choosing the proper output Schottky diode. The diode

needs to be chosen for three characteristics when it is used in

LED lighting circuitry. The most obvious two are the current

rating of the diode and the reverse voltage rating. The reverse

voltage rating should be larger than the maximum output VOVP .

The peak current through the diode is:

= 1.25 A

= 1.06 +

D(pk)

0.375 A

= I

D(pk)

IN(max)

L(used)

The other major factor in deciding 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 or reverse current can be a large contributor espe-

cially at high temperatures. The reverse current of the selected

diode varies between 1 and 100 µA. For higher efficiency, use

a small forward voltage drop diode. For lower high-frequency

noise, choose a small junction capacitor diode.

Step 7: Choosing the output capacitors. The output capacitors

need to 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 boost diode. In this design, the PWM dimming frequency is

200 Hz; the minimum duty cycle is 0.02%. Typically, the voltage

variation on the output during PWM dimming needs to be less

than 250 mV (VCOUT) so there is no audible hum.

C = I ×

OUT

(1– minimum dimming duty cycles)

PWM dimming frequency × V

COUT

The selected diode leakage current at a 150°C junction tem-

perature and 30 V output is 100 μA, and the maximum leakage

current through OVP pin is 1 μA. The total leakage current can be

calculated as follows:

ILK = ILKG(diode) + ILKG(ovp)

= 100 μA + 1 μA

= 101 μA

C = 101 µA ×

OUT

= 2 µF

(1 – 0.02)

200 Hz × 0.250 V

A capacitor larger than 2 µF should be selected. Due to degrada-

tion of capacitance at dc voltages, a 4.7 µF / 50 V capacitor is a

good choice.

Vendor

Value

Part Number

Murata

4.7 µF / 50 V

GRM21BC18H475KE11K

It is also necessary to note that if a high dimming ratio of 5000:1

must be maintained at lower input voltages, then larger out-

put capacitors will be needed. 4 × 4.7 µF / 50 V / X6S / 0805

capacitors are chosen; 0805 size is selected to minimize possible

audible noise.

The RMS current through the capacitor is given by:

= I ×

OUT(rms)

OUT

D +

MAX

L(used)

1 – D

MAX

× 12

IN(max)

= 0.240 ×

OUT(rms)

= 0.424 A

0.75 +

0.375

1 – 0.75

1.06 × 12

The output capacitor needs to have a current rating of at least

0.424 A. The capacitors selected in this design, 4 × 4.7 µF / 50 V

/ X6S / 0805, have a combined current rating of more than 3 A

current rating.

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