ZXSC100 Zetex Semiconductors plc., ZXSC100 Datasheet - Page 12

• Manufacturer Part Number: ZXSC100
• Description: Single Cell Dc-dc Converter Solution
• Manufacturer: Zetex Semiconductors plc.

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
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:
V
where V
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 I
The output is adjustable from V
the switching transistor, Q1.
Note: For the reference designs, RA is assigned the
label R3 and RB the label R4.
ZXSC100
OUT
S E M I C O N D U C T O R S
= V
FB
FB
=730mV
(1 + RA / RB),
FB(MAX)
=200nA to the V
FB
to the (BR)V
BATT
RMS
CEO
pin.
2
of
x
12
R
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 ZXSC300. The resistor value can
be determined by:
1
=
V
DREF
I
B
V
V
OUT
FB
0V
ISSUE 3 - JANUARY 2004
RA
RB