Diodes ZXLD1356/ ZXLD1356Q - Manual (Page 24)

ZXLD1356/ ZXLD1356Q
Document number: DS33470 Rev. 4 - 2
24 of 29
www.diodes.com
October 2012
© Diodes Incorporated
ZXLD1356/ ZXLD1356Q
A Product Line of
Diodes Incorporated
Application Information
(cont.)
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:
V
IN
V
IN
I
SENSE
LX
ZXLD1356
Rs
L1
Cled LED
D1
A value of 1µF 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
SD
) the drive to the output transistor is turned off to 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 SD and 17V care must be taken to avoid excessive power dissipation due to the on-resistance.
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.
TSOT23-5
DFN3030-6
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
-40 -25 -10 5 20 35 50 65 80 95 110 125
Ambient Temperature (°C)
140
Maximum Power Dissipation
P
o
we
r Dis
s
ip
a
tio
n
(W
)