Diodes ZXLD1615 - Manual (Page 8)

Capacitor selection
A ceramic capacitor grounded close to the GND pin
of the package is recommended at the output of the
device. Surface mount types offer the best
performance due to their lower inductance. A
minimum value of 0.22
␮F is advised, although higher
values will lower switching frequency and improve
efficiency especially at lower load currents. A higher
value will also minimize ripple when using the device
to provide an adjustable dc output current.
A good quality, low ESR capacitor should also be
used for input decoupling, as the ESR of this
capacitor is effectively in series with the source
impedance and lowers overall efficiency. This
capacitor has to supply the relatively high peak
current to the coil and smooth the current ripple on
the input supply. A minimum value of 4.7
␮F is
acceptable if the input source is close to the device,
but higher values will improve performance at lower
input voltages, when the source impedance is high.
The input capacitor should be mounted as close as
possible to the IC.
Inductor selection
The choice of inductor will depend on available
board space as well as required performance. Small
value inductors have the advantage of smaller
physical size and may offer lower series resistance
and higher saturation current compared to larger
values. A disadvantage of lower inductor values is
that they result in higher frequency switching, which
in turn causes reduced efficiency due to switch
losses. Higher inductor values can provide better
performance at lower supply voltages. However, if
the inductance is too high, the output power will be
limited by the internal oscillator, which will prevent
the coil current from reaching its peak value. This
condition will arise whenever the ramp time (I
LX(peak)
x L/V IN ) exceeds the nominal 5 ␮s maximum 'on' time
limit for the LX output.
Part No. L
( H)
DCR
( )
I
SAT
(A)
Manufacturer
CMD4D11-100MC 10 0.457 0.5 Sumida
www.sumida.com
DO1608-103 10 0.16 1.1 Coilcraft
www.coilcraft.com
LQH31CN100 10 1.3 0.23 Murata
www.murata.com
LB2012Y100MR 10 0.5 0.1 Taiyo Yuden
www.t-yuden.com
For maximum stability over temperature, capacitors
with X7R dielectric are recommended, as these have
a much smaller temperature coefficient than other
types.
A table of recommended manufacturers is provided
below:
Manufacturer Website
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Kemet www.kement.com
AVX www.avxcorp.com
Recommended inductor values for the ZXLD1615 are
in the range 6.8
␮H to 22␮H. The inductor should be
mounted as close to the device as possible with low
resistance connections to the LX and V
IN
pins.
Suitable coils for use with the ZXLD1615 are shown
in the table below:
ZXLD1615
S E M I C O N D U C T O R S
ISSUE 3 - AUGUST 2004
8