ZXLD1366_10 Datasheet, PDF (27/32 Pages) Diodes Incorporated – HIGH ACCURACY 1A, 60V LED DRIVER WITH INTERNAL SWITCH

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Diodes Incorporated
ZXLD1366
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 ZXLD1366 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.
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.
3.3k
100nF
GND
ADJ ZXLD1366
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.
ZXLD1366
Document number: DS31992 Rev. 5 - 2
27 of 32
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June 2010
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