AL8806

HIGH EFFICIENCY 30V 1.5A BUCK LED DRIVER

AL8806

Document number: DS35144 Rev. 2 - 2

10 of 15

www.diodes.com

June 2011

© Diodes Incorporated

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 1

μF 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 AL8806 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 2.2

μF input capacitor is sufficient for most intended applications of AL8806; 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

AL8806 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

surrounding the IC, and difference between junction and ambient temperature.

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

()

W

45

.

1

W

/

C

69

C

25

C

125

P

)

MAX

(

D

=

°

°

−

°

=

where

θ

determined by the AL8806’s junction to ambient thermal resistance,

θ

temperatures the AL8806 has been packaged in thermally enhanced MSOP-8EP package.

θ

on a 51 x 51mm single layer PCB with 2oz copper standing

in still air is approximately 69°C/W.

()

W

45

.

1

W

/

C

69

C

25

C

125

P

)

MAX

(

D

=

°

°

−

°

=

Figure 5, shows the power derating of the AL8806 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 power dissipated by the AL8806 will

decrease.

MSOP-8EP

0

200

400

600

800

1000

1200

1400

1600

-40 -25 -10

5

20

35

50

65

80

95

110 125

Ambient temperature (°C)

Figure 5. Derating Curve