Diodes ZXLD1352 - Manual (Page 17)

ZXLD1352
ZXLD1352
Document number: DS33469 Rev. 3 - 2
17 of 21
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December 2010
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Application Information (Continued)
Reducing Output Ripple
Peak to peak ripple current in the LED can be reduced, if required, by shunting a capacitor C
led
across the LED(s) as shown
below:
A value of 1µF 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 (V
SU
). 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.
V
IN
V
IN
I
SENSE
LX
ZXLD1352
Rs
L1
Cled LED
D1