Diodes ZXLD1371 - Manual (Page 31)

ZXLD1371
ZXLD1371
Document number: DS35436 Rev. 1 - 2
31 of 42
www.diodes.com
February 2012
© Diodes Incorporated
A Product Line of
Diodes Incorporated
Applications Information (cont.)
Diodes 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 Schottky diode also provides better efficiency
than silicon PN 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. In particular, it is recommended to have a voltage rating at least 15% higher than
the maximum transistor voltage to ensure safe operation during the ringing of the switch node and a current rating at least
10% higher than the average diode current. The power rating is verified by calculating the power loss through the diode.
The higher forward voltage and overshoot due to reverse recovery time in silicon diodes will increase the peak voltage on
the Drain of the external MOSFET. If a silicon diode is used, care should be taken to ensure that the total voltage appearing
on the Drain of the external MOSFET, including supply ripple, does not exceed the specified maximum value.
*A suitable Schottky diode for a switching current of up to about 1.5A would be PDS3100 (Diodes Inc).
Output Capacitor
An output capacitor may be required to limit interference or for specific EMC purposes. For boost and buck-boost
regulators, the output capacitor provides energy to the load when the freewheeling diode is reverse biased during the first
switching subinterval. An output capacitor in a buck topology will simply reduce the LED current ripple below the inductor
current ripple. In other words, this capacitor changes the current waveform through the LED(s) from a triangular ramp to a
more sinusoidal version without altering the mean current value.
In all cases, the output capacitor is chosen to provide a desired current ripple of the LED current (usually recommended to
be less than 40% of the average LED current).
Buck:
Boost and Buck-boost
PP LED LED SW
PP L
OUTPUT
I x r x f
I x D
C
−
−
Δ
Δ
=
where:
• ΔI
L-PP
is the ripple of the inductor current, usually ± 20% of the average sensed current
• ΔI
LED-PP
is the ripple of the LED current, it should be <40% of the LEDs average current
• f
sw
is the switching frequency (From graphs and calculator)
• r
LED
is the dynamic resistance of the LEDs string (n times the dynamic resistance of the single LED from the
datasheet of the LED manufacturer).
The output capacitor should be chosen to account for derating due to temperature and operating voltage. It must also have
the necessary RMS current rating. The minimum RMS current for the output capacitor is calculated as follows:
Buck
Boost and Buck-boost
Ceramic capacitors with X7R dielectric are the best choice due to their high ripple current rating, long lifetime, and
performance over the voltage and temperature ranges.
PP LED LED SW
PP L
OUTPUT
I x r x f x 8
I
C
−
−
Δ
Δ
=
12
I
I PP LED
RMS COUTPUT
− =
MAX
MAX
LED COUTPUTRMS
D 1
D
I I
−
=