Zxld1374, Application information – Diodes ZXLD1374 User Manual
Page 23
ZXLD1374
Document number: DS35032 Rev. 3 - 2
23 of 39
September 2012
© Diodes Incorporated
ZXLD1374
A Product Line of
Diodes Incorporated
Application Information
(cont.)
The chosen coil should saturate at a current greater than the peak sensed current. This saturation current is the DC current for which the
inductance has decreased by 10% compared to the low current value.
Assuming ±10% ripple current, we can find this peak current from Equation 8, adjusted for ripple current:
I
COILPEAK
= 1.1 I
LED
for Buck
I
COILPEAK
= 1.1 I
INMAX
for Boost
Equation 21
I
COILPEAK
= 1.1 I
INMAX
+ I
LED
for
Buck-boost
where I
INMAX
is the value of I
IN
at minimum V
IN
.
The mean current rating is also a factor, but normally the saturation current is the limiting factor.
The following websites may be useful in finding suitable components
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 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 LX 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 LX pin. 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.
*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:
I
x
r
x
xf
8
I
C
PP
LED
LED
SW
PP
L
OUTPUT
Δ
Δ
=
−
−
Boost and Buck-boost
I
x
r
x
f
DxI
C
PP
LED
LED
SW
LED
OUTPUT
Δ
=
−
where:
•
ΔI
L
is the ripple of the inductor current, usually ± 20% of the average sensed current
•
ΔI
LED
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).