Diodes ZXLD1352 - Manual (Page 18)

ZXLD1352
ZXLD1352
Document number: DS33469 Rev. 3 - 2
18 of 21
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December 2010
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Application Information
(Continued)
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 ZXLD1352 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 R
S
.
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.
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.
GND
ZXLD1352 ADJ
10k
100nF
GND