zxld1352 Zetex Semiconductors plc., zxld1352 Datasheet - Page 19

• Manufacturer Part Number: zxld1352
• Description: 350ma Led Driver With Internal Switch And Enhanced Pwm Dimming Range
• Manufacturer: Zetex Semiconductors plc.

ZXLD1352
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
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.
10k
ZXLD1352
ADJ
GND
100nF
GND
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.
Issue 1 - August 2008
19
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