Application information (continued) – Diodes ZXLD1350 User Manual
Page 19
ZXLD1350
30V 350mA LED DRIVER with AEC-Q100
ZXLD1350
Document number: DS33468 Rev. 8 - 2
19 of 22
March 2011
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Application Information (Continued)
A value of 1
μF will reduce nominal ripple current by a factor three (approx.). Proportionally lower ripple can be achieved with
higher capacitor values. Note that the capacitor will not affect operating frequency or efficiency, but it will increase start-up
delay, by reducing the rate of rise of LED voltage.
Operation at low supply voltage
The internal regulator disables the drive to the switch until the supply has risen above the start-up threshold (VSU). Above
this threshold, the device will start to operate. However, with the supply voltage below the specified minimum value, the
switch duty cycle will be high and the device power dissipation will be at a maximum. Care should be taken to avoid operating
the device under such conditions in the application, in order to minimize the risk of exceeding the maximum allowed die
temperature. (See next section on thermal considerations).
Note that when driving loads of two or more LEDs, the forward drop will normally be sufficient to prevent the device from
switching below approximately 6V. This will minimize the risk of damage to the device.
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.
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 ZXLD1350 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.