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ZXLD1356Q

AUTOMOTIVE GRADE 60V 550mA LED DRIVER

Diodes Incorporated. 

ZXLD1356Q

Q

Application Information (Cont.)

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. For this reason, the temperature coefficients of the internal circuits for the

ZXLD1356Q 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 resistor

RS. Its best to connect VIN directly to one end of RS and Isense directly to the opposite end of RS with no other currents flowing in these tracks. It

is important that the cathode current of the Schottky diode does not flow in a track between RS and VIN as this may give an 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 3.3kΩ 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.

High Voltage Tracks

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

soft-clamped for voltages above 1.35V to desensitize it to leakage that might raise the ADJ pin voltage and cause excessive output current.

However, a ground ring placed around the ADJ pin is recommended to minimize changes in output current under these conditions.

Evaluation Boards

ZXLD1356 evaluation boards are available on request, which have connection terminals that allow customers to connect their own LED products

to the board.

ZXLD1356Q

Document number: DS37077 Rev. 2 - 2

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www.diodes.com

October 2015

© Diodes Incorporated

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