ZHCS2000 datasheet(12/19 Pages) ZETEX

12 / 19 page

ISSUE 1 - JANUARY 2001

ZXSC100

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

2 x

ESR.

Output voltage adjustment

The ZXSC100 is an adjustable converter allowing the

end user the maximum flexibility in output voltage

selection. For adjustable operation a potential divider

network is connected as indicated in the diagram.

The output voltage is determined by the equation:

VOUT= VFB (1 + RA / RB),

where VFB=730mV

The resistor values, RA and RB, should be maximised

to improve efficiency and decrease battery drain.

Optimisation can be achieved by providing a

minimum current of IFB(MAX)=200nA to the VBATT pin.

The output is adjustable from VFB to the (BR)VCEO of

the switching transistor, Q1.

Note: For the reference designs, RA is assigned the

label R3 and RB the label R4.

External Transistor base drive selection

Optimisation of the external switching transistor base

drive may be necessary for improved efficiency in low

power applications. This can be achieved by

introducing an external resistor between the supply

and the RE pin of the ZXSC100. The resistor value can

be determined by:

DREF

VOUT

VFB

ZHCS2000 Datasheet(PDF) 12 Page - Zetex Semiconductors