Zxld1370, Application information – Diodes ZXLD1370 User Manual
Page 25
ZXLD1370
Document number: DS32165 Rev. 5 - 2
25 of 39
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application 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 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:
PP
LED
LED
SW
PP
L
OUTPUT
I
x
r
x
f
x
8
I
C
−
−
Δ
Δ
=
Boost and Buck-Boost
PP
LED
LED
SW
PP
LED
OUTPUT
I
x
r
x
f
I
x
D
C
−
−
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
12
I
I
PP
LED
RMS
COUTPUT
−
−
=
Boost and Buck-Boost
MAX
MAX
LED
RMS
COUTPUT
D
1
D
I
I
−
=
−
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.