Layout considerations

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.

ZXSC400

ISSUE 1 - JANUARY 2003

9