AL8805

HIGH EFFICIENCY 30V 1A BUCK LED DRIVER

AL8805

Document number: DS35030 Rev. 1 - 2

12 of 16

www.diodes.com

September 2010

© 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

AL8805 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 AL8805; however a 4.7

μF 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 AL8805 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:

where

temperature,

θ

The

recommended

maximum

operating

junction

temperature is determined by the AL8805’s junction to

ambient thermal resistance,

θ

θ

is layout dependent and the AL8805’s

θ

JA

on a

25x25mm single layer PCB with 1oz copper standing in still

air is approximately 250°C/W (160°C/W on a four-layer

PCB).

calculated by the following formulas:

layer PCB

four-layer PCB

Figure 34, shows the power derating of the AL8805 on two

(one single-layer and four-layer) different 25x25mm PCB

with 1oz copper standing in still air.

Figure 34. Derating Curve for different PCB

Figure 35 gives details about the PCB layout suggestions:

1.

the capacitor C1 has to be placed as close as

2.

The sense resistor R1 has to be placed as close as

3.

The D1 anode, the SW pin and the inductor have

to be placed as close as possible to avoid ringing.