ZHCS2000 datasheet(11/19 Pages) ZETEX

11 / 19 page

ZXSC100

ISSUE 1 - JANUARY 2001

Inductor selection

The inductor value must be chosen to satisfy

performance, cost and size requirements of the overall

solution. For the reference designs we recommend an

inductor value of 22µH with a core saturation current

rating greater than the converter peak current value.

Inductor selection has a significant impact on the

converter efficiency. For applications where efficiency

is critical, a 5% improvement can be achieved with a

high performance inductor. This should be selected

with a core saturation current rating much higher than

the peak current of the converter, say 3 times greater.

The resultant reduction in core losses brings about the

efficiency improvement.

Peak current definition

The peak current rating is a design parameter whose

value is dependent upon the overall application. For

the reference designs, a peak current of 1.2A was

chosen to ensure that the converter could provide the

required output power.

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 temperature extremes.

Once IPK is decided the value of RSENSE can be

determined by:

SENSE

ISENSE

Output power definition

By making the above assumptions for the inductor and

IPK the output power can be determined by:

Output Power

x I

x T

2x(T

OUT

DIS

OFF

−

)

where

IxL

and

IxL

DIS

OUT

−

()

Note: VOUT = output voltage + rectifier diode VF

Figure 3 shows the discontinuous inductor current and

the relationship between output power, TON,TDIS and

TOFF.

Figure 3

Discontinuous inductor current

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; their capacitance

value, 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 solution, and indicated in the

reference designs, are optimised to provide the best

overall performance.

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