zxld1360 Zetex Semiconductors plc., zxld1360 Datasheet - Page 20

• Manufacturer Part Number: zxld1360
• Description: 1a Led Driver With Internal Switch
• Manufacturer: Zetex Semiconductors plc.

Example:
For V
T
T
This gives an operating frequency of 280kHz and a duty cycle of 0.35.
These and other equations are available as a spreadsheet calculator from the Zetex website at
www.zetex.com/zxld1360
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.
They also provide better efficiency than silicon diodes, due to a combination of lower forward
voltage and reduced recovery time.
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 and if close to the load may create a thermal runaway
condition.
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 - March 2007
© Zetex Semiconductors plc 2007
ON
OFF
= (33e-6 x 0.3)/(12 - 3.6 - 0.693) = 1.28 s
= (33e-6 x 0.3)/(3.6 + 0.49 + 0.193)= 2.31 s
IN
=12V, L=33 H, rL=0.093, R
S
=0.1 , R
LX
=0.15 , V
20
LED
=3.6V, I
avg
=1A and V
ZXLD1360
D
=0.49V
www.zetex.com