zxld1362 Zetex Semiconductors plc., zxld1362 Datasheet - Page 26

• Manufacturer Part Number: zxld1362
• Description: 1a Led Driver With Internal Switch
• Manufacturer: Zetex Semiconductors plc.

ZXLD1362
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
ZXLD1362
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
Below the under-voltage lockout threshold (V
) the drive to the output transistor is turned off to
SD
prevent device operation with excessive on-resistance of the output transistor. The output
transistor is not full enhanced until the supply voltage exceeds approximately 17V. At supply
voltages between V
and 17V care must be taken to avoid excessive power dissipation due to
SD
the on-resistance.
If the supply voltage is always less than 30V continuous (or less than 40V for less than 0.5s) an
alternative device is available, the ZXLD1360.
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 - December 2007
26
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© Zetex Semiconductors plc 2007