ZXLD1366

ZXLD1366

Document number: DS31992 Rev. 7 - 2

25 of 30

www.diodes.com

May 2012

© Diodes Incorporated

A Product Line of

Diodes Incorporated

Figure. 8 Suggested Layout for SO-8EP Package

Vias ensure an effective path to the ground plane for the heat flow therefore reducing the thermal impedance between junction and ambient

temperature. Diodes came to the conclusion that the compromise is reached by using more than 10 vias with 1mm of diameter and 0.5 hole

size.

Finally the same scheme in Figure 7 (without the exposed paddle) can be usde for the TSOT25 package guaranteeing an effective thermal

path.

Thermal Compensation of Output Current

High luminance LEDs often need to be supplied with a temperature compensated current in order to maintain stable and reliable operation at

all drive levels. The LEDs are usually mounted remotely from the device so, for this reason, the temperature coefficients of the internal

circuits for the ZXLD1366 have been optimized to minimize the change in output current when no compensation is employed. If output

current compensation is required, it is possible to use an external temperature sensing network normally using Negative Temperature

Coefficient (NTC) thermistors and/or diodes, mounted very close to the LED(s). The output of the sensing network can be used to drive the

ADJ pin in order to reduce output current with increasing temperature.

Layout Considerations

LX Pin

The LX pin of the device is a fast switching node, so PCB tracks should be kept as short as possible. To minimize ground 'bounce', the

ground pin of the device should be soldered directly to the ground plane.

Coil and Decoupling Capacitors and Current Sense Resistor

It is particularly important to mount the coil and the input decoupling capacitor as close to the device pins as possible to minimize parasitic

resistance and inductance, which will degrade efficiency. It is also important to minimize any track resistance in series with current sense

apparent higher measure of current than is actual because of track resistance.

ADJ Pin

The ADJ pin is a high impedance input for voltages up to 1.35V so, when left floating, PCB tracks to this pin should be as short as possible

to reduce noise pickup. A 100nF capacitor from the ADJ pin to ground will reduce frequency modulation of the output under these

conditions. An additional series 3.3k

Ω resistor can also be used when driving the ADJ pin from an external circuit (see next page). This

resistor will provide filtering for low frequency noise and provide protection against high voltage transients.