ZXLD1360

Issue 1 - March 2007

21

www.zetex.com

© Zetex Semiconductors plc 2007

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:

A value of 1 F will reduce the supply 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.

By adding this capacitor the current waveform through the LED(s) changes from a triangular ramp

to a more sinusoidal version without altering the mean current value .

Operation at low supply voltage

The internal regulator disables the drive to the switch until the supply has risen above the start-

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). The drive to

This prevents the switch working with excessive 'on' resistance under conditions where the duty

cycle is high.

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

with 1oz copper standing in still air.

LX

ZXLD1360

Rs

L1

Cled

LED

D1