ZXLD1350

ZXLD1350

Document number: DS33468 Rev. 7 - 2

19 of 22

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November 2010

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Application Information (Continued)

Reducing Output Ripple

Peak to peak ripple current in the LED(s) can be reduced, if required, by shunting a capacitor Cled across the LED(s) as

shown below:

ZXLD1350

LED

Cled

L1

LX

I

SENSE

D1

R

S

V

IN

V

IN

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)

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.