ZXSC100

SEMICO NDUC TORS

ISSUE 3 - JANUARY 2004

13

Layout issues

Layout is critical for the circuit to function optimally 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

should be kept separate from each other and

interconnections made with short, thick traces thus

minimising parasitic inductance, capacitance and

resistance. Also the sense resistor R2 should be

connected, with minimum trace length, between

emitter lead of Q1 and ground, again minimising stray

parasitics.

The layout for the 0.33W solution is shown below.

Top silk

Drill holes

Actual Size

Bottom Copper

Top Copper

0.33W solution demo board layout