Diodes AL5812 Manuel d'utilisateur télécharger pdf (Page 7)

AL5812

Document number: DS35616 Rev. 2 - 2

7 of 11

www.diodes.com

November 2013

NEW PRODUCT

L5812

Application Information (cont.)

PWM Dimming

The AL5812 can be used to provide LED current dimming driving the R

SET

pin via the current setting resistor (R

SET

) and a series MOSFET

switch to ground (Figure 17). The R

SET

pin current is then effectively switched on and off causing the LED current to turn on and off. The linearity

is shown in Figure 18.

Figure 17 PWM Dimming

T = +25°C

2 LEDs

R = 7.5k

f = 500Hz

SET

PWM

10 200901008040 5030 7060

DUTY CYCLE (%)

Figure 18 PWM Dimming Linearity

110

100

(

)

Thermal Considerations

When designing linear LED drivers careful consideration must be given to:

1. the power dissipation within the LED driver

and

2. PCB layout/heat sinking.

A Linear LED driver has to be able to handle the large potential input voltage variations due to the supply voltage tolerance and also the variation

in LED forward voltage due to binning and temperature.

This can result in a large potential difference across the LED driver resulting in a larger than anticipated power dissipation.

For example in a 12V powered system with a 5% output voltage tolerance; the input voltage could typically vary from 12.6V down to 11.4V

driving 3 LEDs with a voltage varying from 3V to 3.5V at 100mA. This means that the LED driver has to cope with a voltage drop across varying

from approximately 3.6V to 0.9V. This means that the power dissipation of the AL5812 could be as much as 366mW.

Figure 19 below shows how the AL5812’s power dissipation capability varies with package; these values will vary with PCB size and area of

metal associated with the ground plane used for heat sinking. By increasing the area on the top layer the thermal impedance of both packages

could be improved.

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

AMBIENT TEMPERATURE (C)

Figure 19 Power Dissipation Derating

1.6

1.4

0.2

0.0

0.6

0.4

1.0

0.8

1.2

(W)

U-DFN3030-6

MSOP-8EP

1 2 3 4 5 6 7 8 9 10 11

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