Diodes ZXLD1360 - Manual (Page 18)

ZXLD1360
ZXLD1360
Document number: DS33471 Rev. 4 - 2
18 of 25
www.diodes.com
March 2011
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Application Information (cont.)
Example:
For V
IN
=12V, L=33µH, rL=0.093, R
S
=0.1
Ω , R
LX
=0.15
Ω, V
LED
=3.6V, I
avg
=1A and V
D
=0.49V
t
ON
= (33e-6 x 0.3)/(12 - 3.6 - 0.693) = 1.28µs
t
OFF
= (33e-6 x 0.3)/(3.6 + 0.49 + 0.193) = 2.31µs
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 Diodes website at www.diodes.com
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.