ZXLD1350

30V 350mA LED DRIVER with AEC-Q100

ZXLD1350

Document number: DS33468 Rev. 8 - 2

19 of 22

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March 2011

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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.