ZXLD1350

ZXLD1350

Document number: DS33468 Rev. 7 - 2

20 of 22

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November 2010

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Application Information (Continued)

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 ZXLD1350 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

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

resistance and inductance, which will degrade efficiency. It is also important to take account of any track resistance in series

with current sense resistor RS.

ADJ pin

The ADJ pin is a high impedance input, 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 10k

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

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

ZXLD1350

GND

ADJ

100nF

GND

10k

High voltage tracks

Avoid running any high voltage tracks close to the ADJ pin, to reduce the risk of leakage due to board contamination. Any such

leakage may raise the ADJ pin voltage and cause excessive output current. A ground ring placed around the ADJ pin will

minimize changes in output current under these conditions.