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A Product Line of
Diodes Incorporated
ZXLD1352
Application Information (Continued)
Example
For V IN =12V, L=47 μ H, rL=0.64 Ω , V LED =3.4V, I avg =350mA and VD =0.36V
T ON = (47e-6 x 0.105)/(12 - 3.4 - 0.672) = 0.622 μ s
T OFF = (47e-6 x 0.105)/(3.4 + 0.36 + 0.322)= 1.21 μ s
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
Zetex
website.
Go
to
www.diodes.com/ZXLD1352
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:
Forward Voltage at
Continuous
Reverse Leakage
Diode
ZLLS1000
100mA
(mV)
310
Current
(mA)
1000
At 30V 85oC
( μ A)
300
Package
TSOT23-5
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.
ZXLD1352
Document number: DS33469 Rev. 3 - 2
16 of 21
www.diodes.com
December 2010
? Diodes Incorporated
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