zxld1360 Zetex Semiconductors plc., zxld1360 Datasheet - Page 21

• Manufacturer Part Number: zxld1360
• Description: 1a Led Driver With Internal Switch
• Manufacturer: Zetex Semiconductors plc.

ZXLD1360
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:
Rs
V
IN
LED
Cled
L1
D1
V
I
LX
IN
SENSE
ZXLD1360
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-
up threshold (V
). Above this threshold, the device will start to operate. However, with the
SU
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
the switch is turned off when the supply voltage falls below the under-voltage threshold (V
).
SD
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
2
below gives details for power derating. This assumes the device to be mounted on a 25mm
PCB
with 1oz copper standing in still air.
Issue 1 - March 2007
21
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© Zetex Semiconductors plc 2007