Application information – Diodes ZXLD1356/ ZXLD1356Q User Manual
Page 25
ZXLD1356/ ZXLD1356Q
Document number: DS33470 Rev. 4 - 2
25 of 29
October 2012
© Diodes Incorporated
ZXLD1356/ ZXLD1356Q
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Thermal Considerations
(cont.)
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 ZXLD1356 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 Isense 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.
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 3.3k
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.
GND
ZXLD1356
ADJ
3.3k
100nF
GND
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.