Diodes ZXSC410/ZXSC420/ZXSC440 - Manual (Page 6)

ZXSC410/ZXSC420/ZXSC440
Document number: DS33618 Rev. 5 - 2
6 of 17
www.diodes.com
March 2013
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ZXSC410/ZXSC420/ZXSC440
Application Information
(cont.)
External Component Selection
Switching Transistor Selection
The choice of switching transistor has a major impact on the converter efficiency. For optimum performance, a bipolar transistor with low V
CE(SAT)
and high gain is required. The V
CEO
of the switching transistor is also an important parameter as this sees the full output voltage when the
transistor is switched off. Diodes SOT26 transistors are an ideal choice for this application.
Schottky Diode Selection
As with the switching transistor, the Schottky rectifier diode has a major impact on the converter efficiency. A Schottky diode with a low forward
voltage and fast recovery time should be used for this application.
The diode should be selected so that the maximum forward current rating is greater or equal to the maximum peak current in the inductor, and
the maximum reverse voltage is greater or equal to the output voltage. The Diodes ZHCS Series meet these needs.
Inductor Selection
The inductor value must be chosen to satisfy performance, cost and size requirements of the overall solution.
Inductor selection has a significant impact on the converter performance. For applications where efficiency is critical, an inductor with a series
resistance of 500mΩ or less should be used.
Output Capacitors
Output capacitors are a critical choice in the overall performance of the solution. They are required to filter the output and supply load transient
currents. There are three parameters which are paramount in the selection of the output capacitors, capacitance, I
RIPPLE
and ESR. The
capacitance value is selected to meet the load transient requirements. The capacitors I
RIPPLE
rating must meet or exceed the current ripple of the
solution.
The ESR of the output capacitor can also affect loop stability and transient performance. The capacitors selected for the solutions, and indicated
in the reference designs, are optimised to provide the best overall performance.
Input Capacitors
The input capacitor is chosen for its voltage and RMS current rating. The use of low ESR electrolytic or tantalum capacitors is recommended.
Capacitor values for optimum performance are suggested in the reference design section.
Also note that the ESR of the input capacitor is effectively in series with the input and hence contributes to efficiency losses in the order of I
RMS
2
ESR.
Peak Current Definition
In general, the I
PK
value must be chosen to ensure that the switching transistor, Q1, is in full saturation with maximum output power conditions,
assuming worse case input voltage and transistor gain under all operating temperatureextremes.Once I
PK
is decided the value of R
SENSE
can be determined by:
I
V
R
PK
SENSE
SENSE

Sense Resistor
A low value sense resistor is required to set the peak current. Power in this resistor is negligible due to the low sense voltage threshold, V
SENSE
.