ZXSC4X0 datasheet(6/17 Pages) DIODES

6 / 17 page

ZXSC410/ZXSC420/ZXSC440

Document number: DS33618 Rev. 5 - 2

6 of 17

www.diodes.com

March 2013

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 VCE(SAT)

and high gain is required. The VCEO 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, IRIPPLE and ESR. The

capacitance value is selected to meet the load transient requirements. The capacitors IRIPPLE 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 IRMS

ESR.

Peak Current Definition

In general, the IPK 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 IPK

is decided the value of RSENSE can be determined by:

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, VSENSE.

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