Diodes AL8806Q Manuel d'utilisateur télécharger pdf (Page 10)

AL8806Q

Document number: DS36905 Rev. 1 - 2

10 of 13

www.diodes.com

April 2014

L8806Q

Application Information (cont.)

Reducing Output Ripple

Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor C2 across the LED(s) as shown already in the circuit

schematic.

A value of 1μF will reduce the supply ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with higher capacitor

values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up delay, by reducing the rate of rise of

LED voltage. By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp to a more sinusoidal version

without altering the mean current value.

Capacitor Selection

The small size of ceramic capacitors makes them ideal for AL8806Q applications. X5R and X7R types are recommended because they retain their

capacitance over wider voltage and temperature ranges than other types such as Z5U.

A 2.2μF input capacitor is sufficient for most intended applications of AL8806Q; however a 4.7μF input capacitor is suggested for input voltages

approaching 30V.

Diode Selection

For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the

maximum operating voltage and temperature. The Schottky diode also provides better efficiency than silicon PN diodes, due to a combination of

lower forward voltage and reduced recovery time.

It is important to select parts with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output

load current. In particular, it is recommended to have a diode voltage rating at least 15% higher than the operating voltage to ensure safe operation

during the switching and a current rating at least 10% higher than the average diode current. The power rating is verified by calculating the power

loss through the diode.

Schottky diodes, e.g. B240 or B140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8806Q applications.

Thermal and Layout Considerations

For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The maximum power dissipation

depends on several factors: the thermal resistance of the IC package θ

, PCB layout, airflow surrounding the IC, and difference between junction

and ambient temperature.

The maximum power dissipation can be calculated using the following formula:

D(MAX)

= (T

J(MAX)

− T

) / θ

where

J(MAX)

is the maximum operating junction temperature,

is the ambient temperature, and

is the junction to ambient thermal resistance.

The recommended maximum operating junction temperature, T

, is 125°C and so maximum ambient temperature is determined by the AL8806Q’s

junction to ambient thermal resistance, θ

. To support high LED drive at higher ambient temperatures the AL8806Q has been packaged in

thermally enhanced MSOP-8EP package.

, is layout dependent and the AL8806Q’s θ

in MSOP-8EP on a

51 x 51mm double layer PCB with 2oz copper standing in still air is

approximately 69°C/W.

Therefore the maximum power dissipation at T

= 25°C is:

()

W45.1

W/C69

C25C125

)MAX(D

°−°

Figure 5, shows the power derating of the AL8806Q on an FR4

51x51mm PCB with 2oz copper standing in still air.

As the ambient temperature increases and/or the PCB area reduces

the maximum allowable power dissipated by the AL8806Q will

decrease.

Figure 5 Derating Curve

MSOP-8EP

200

400

600

800

1000

1200

1400

1600

-40 -25 -10 5 20 35 50 65 80 95 110 125

Ambient temperature (°C)

Power dissipation (mW)

1 2 ... 5 6 7 8 9 10 11 12 13

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Diodes AL8806Q Manuel d'utilisateur Page 10

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