ZXSC400
Layout considerations
Layout is critical for the circuit to function in the m ost To optim ise for best perform ance each of these loops
efficient m anner in term s of electrical efficiency, kept separate from each other and interconnected with
therm al considerations and noise.
s h o rt, th ick tra ce s th u s m in im is in g p a ra s itic
inductance, capacitance and resistance. Also the
RSENSE resistor should be connected, with m inim um
trace length, between em itter lead of Q1 and ground,
again m inim ising stray parasitics.
For ‘step-up converters’ there are four m ain current
loops, the input loop, power-switch loop, rectifier loop
and output loop. The supply charging the input
capacitor form s the input loop. The power-switch loop
is defined when Q1 is ‘on’, current flows from the input
through the inductor, Q1, RSENSE and to ground. When
Q1 is ‘off’, the energy stored in the inductor is
transferred to the output capacitor and load via D1,
form ing the rectifier loop. The output loop is form ed by
the output capacitor supplying the load when Q1 is
switched back off.
ISSUE 1 - J ANUARY 2003
9
DIODES [ DIODES INCORPORATED ]
