ZXLD1362
Document number: DS33472 Rev. 6 - 2
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March 2014
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ZXLD1362
Application Information (cont.)
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
2
PCB
with 1oz copper standing in still air.
-10 10-50 -30 30 90 110
AMBIENT TEMPERATURE (°C)
Maximum Power Dissipation
130 15050 70
1100
1000
900
800
500
200
100
0
700
600
WE
(mW)
400
300
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 ZXLD1362 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
R
S
. Its best to connect V
IN
directly to one end of R
S
and I
SENSE
directly to the opposite end of R
S
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 R
S
and V
IN
as this may give an apparent higher
measure of current than is actual because of track resistance.
