Output voltage adjustment

The ZXSC410/420 are adjustable output converters

allowing the end user the maximum flexibilty. For

adjustable operation a potential divider network is

connected as follows:

The output voltage is determined by the equation:

VV

1

RA

RB

OUT

FB

=+













The resistor values, RA and RB, should be maximised

to improve efficiency and decrease battery drain.

Optimisation can be achieved by providing a minimum

transistor, Q1.

Note: For the reference designs, RA is assigned the

label R2 and RB the label R3.

Layout issues

Layout is critical for the circuit to function in the most

efficient manner in terms of electrical efficiency,

thermal considerations and noise.

For ‘step-up converters’ there are four main current

loops, the input loop, power-switch loop, rectifier loop

and output loop. The supply charging the input

capacitor forms the input loop. The power-switch loop

is defined when Q1 is ‘on’, current flows from the input

Q1 is ‘off’, the energy stored in the inductor is

transferred to the output capacitor and load via D1,

forming the rectifier loop. The output loop is formed by

the output capacitor supplying the load when Q1 is

switched back off.

To optimise for best performance each of these loops

kept separate from each other and interconnected with

short, thick traces thus minimising parasitic

inductance, capacitance and resistance. Also the

trace length, between emitter lead of Q1 and ground,

again minimising stray parasitics.

ZXSC410

ZXSC420

SEMICO NDUC TORS

ISSUE 2 - May 2003

7

GND

STDN

GND

DRIVE

SENSE

EOR

GND

DRIVE

SENSE

ZXSC410

SOT23-6

ZXSC420

SOT23-6

CONNECTION DIAGRAMS