ZXLD1350Q

Document number: DS37076 Rev. 2 - 2

17 of 21

www.diodes.com

September 2017

© Diodes Incorporated

ZXLD1350Q

Thermal Considerations

When operating the device at high ambient temperatures, or when driving maximum load current, care must be taken to avoid exceeding the

package power dissipation limits. The graph below gives details for power derating. This assumes the device to be mounted on a (25mm)

with 1oz copper standing in still air.

Note that the device power dissipation will most often be a maximum at minimum supply voltage. It will also increase if the efficiency of the circuit

is low. This may result from the use of unsuitable coils, or excessive parasitic output capacitance on the switch output.

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

ZXLD1350Q 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,

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 10kΩ

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.

ZXLD1350Q