zxld1352 Zetex Semiconductors plc., zxld1352 Datasheet - Page 17

• Manufacturer Part Number: zxld1352
• Description: 350ma Led Driver With Internal Switch And Enhanced Pwm Dimming Range
• Manufacturer: Zetex Semiconductors plc.

Example:
For V
T
T
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.zetex.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:
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.
Issue 1 - August 2008
© Diodes Incorporated 2008
Diode
ZLLS1000
ON
OFF
= (47e-6 x 0.105)/(12 - 3.4 - 0.672) = 0.622 s
= (47e-6 x 0.105)/(3.4 + 0.36 + 0.322)= 1.21 s
IN
=12V, L=47 H, rL=0.64 , V
Forward voltage
at 100mA
(mV)
310
Continuous
current
1000
(mA)
LED
=3.4V, I
avg
17
=350mA and VD =0.36V
Reverse Leakage
At 30V 85°C
( A)
300
Package
TSOT23
ZXLD1352
www.diodes.com
www.zetex.com
Manufacturer
Zetex