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

ground. When Q1 is 'off', the energy stored in the

transformer is transferred from the secondary 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 optimize for best performance each of these loops

kept separate from each other and interconnected with

short, thick traces thus minimizing parasitic

inductance, capacitance and resistance. Also the

trace length, between emitter lead of Q1 and ground,

again minimizing stray parasitics.

ZXSC440

SEMICO NDUC TORS

ISSUE 1 - JANUARY 2005

9