Diodes ZXLD1350 - Manual (Page 18)

ZXLD1350
30V 350mA LED DRIVER with AEC-Q100
ZXLD1350
Document number: DS33468 Rev. 8 - 2
18 of 22
www.diodes.com
March 2011
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Application Information (Continued)
Example:
For V
IN
=12V, L=47µH, rL=0.64V, V
LED
=3.4V, I
avg
=350mA and V
D
=0.36V
t
ON
= (47e-6 x 0.105)/(12 - 3.4 - 0.672) = 0.622µs
t
OFF
= (47e-6 x 0.105)/(3.4 + 0.36 + 0.322)= 1.21µs
This gives an operating frequency of 546kHz and a duty cycle of 0.34.
These and other equations are available as a spreadsheet calculator from the Diodes website.
Note that in practice, the duty cycle and operating frequency will deviate from the calculated values due to dynamic switching
delays, switch rise/fall times and losses in the external components.
Optimum performance will be achieved by setting the duty cycle close to 0.5 at the nominal supply voltage. This helps to
equalize the undershoot and overshoot and improves temperature stability of the output current.
Diode Selection
For maximum efficiency and performance, the rectifier (D1) should be a fast low capacitance Schottky diode with low reverse
leakage at the maximum operating voltage and temperature. The recommended diode for use with this part is the ZLLS1000.
This has approximately ten times lower leakage than standard Schottky diodes, which are unsuitable for use above 85°C. It
also provides better efficiency than silicon diodes, due to a combination of lower forward voltage and reduced recovery time.
The table below gives the typical characteristics for the ZLLS1000:
Diode
Forward Voltage
@ 100mA (mV)
Continuous
Current
(mA)
Reverse Leakage
@ 30V 85°C
(mA)
Package
ZLLS1000 310 1000 300 TSOT23
If alternative diodes are used, it is important to select parts with a peak current rating above the peak coil current and a
continuous current rating higher than the maximum output load current. It is very important to consider the reverse leakage of
the diode when operating above 85°C. Excess leakage will increase the power dissipation in the device.
The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on the
LX output. If a silicon diode is used, care should be taken to ensure that the total voltage appearing on the LX pin including
supply ripple, does not exceed the specified maximum value.
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:
ZXLD1350
LED Cled
L1
LX I
SENSE
D1
R
S V
IN
V
IN