Application information – Diodes ZXLD1356/ ZXLD1356Q User Manual
Page 22
ZXLD1356/ ZXLD1356Q
Document number: DS33470 Rev. 4 - 2
22 of 29
October 2012
© Diodes Incorporated
ZXLD1356/ ZXLD1356Q
A Product Line of
Diodes Incorporated
Application Information
(cont.)
V
IN
Capacitor Selection
A low ESR capacitor should be used for input decoupling, as the ESR of this capacitor appears in series with the supply 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.
To avoid transients into the IC, the size of the input capacitor will depend on the V
IN
voltage:
V
IN
= 6 to 40V
C
IN
= 2.2µF
V
IN
= 40 to 50V
C
IN
= 4.7µF
V
IN
= 50 to 60V
C
IN
= 10µF
When the input voltage is close to the output voltage the input current increases which puts more demand on the input capacitor. The minimum
value of 2.2µF may need to be increased to 4.7µF; higher values will improve performance at lower input voltages, especially when the source
impedance is high. The input capacitor should be placed as close as possible to the IC.
For maximum stability over temperature and voltage, capacitors with X7R, X5R, or better dielectric is recommended. Capacitors with Y5V
dielectric are not suitable for decoupling in this application and should NOT be used.
When higher voltages are with C
IN
= 10µF, an electrolytic capacitor can be used provided that a suitable 1mF ceramic capacitor is also used and
positioned as close to the V
IN
pin as possible.
A suitable capacitor would be NACEW100M1006.3x8TR13F (NIC Components).
The following web sites are useful when finding alternatives:
Inductor Selection
Recommended inductor values for the ZXLD1356 are in the range 68 µH to 220 µH.
Higher values of inductance are recommended at higher supply voltages in order to minimize errors due to switching delays, which result in
increased ripple and lower efficiency. Higher values of inductance also result in a smaller change in output current over the supply voltage range.
(see graphs pages 10-16). The inductor should be mounted as close to the device as possible with low resistance connections to the LX and V
IN
pins.
The chosen coil should have a saturation current higher than the peak output current and a continuous current rating above the required mean
output current.
Suitable coils for use with the ZXLD1356 may be selected from the MSS range manufactured by Coilcraft, or the NPIS range manufactured by
NIC components. The following websites may be useful in finding suitable components.
The inductor value should be chosen to maintain operating duty cycle and switch 'on'/'off' times within the specified limits over the supply voltage
and load current range.
Figures 3 and 4 (following) can be used to select a recommended inductor based on maintaining the ZXLD1356 case temperature below 60°C
for the different package types. For detailed performance characteristics for the inductor values 68, 100, 150 and 220µH see graphs on pages
10-16.