ACT334
TYPICAL APPLICATION CONT’D
to a single point (star ground configuration).
output information can be sent back into the IC to
do the appropriate control.
V
FB
waveforms of Figure 3, Figure 4, and Figure 5
violate the sampling design margin and are not
recommended. Figure 3 has very long overshoot
period. Figure 4, and Figure 5 have very long
ringing period. The undesired waveforms cause the
IC to operate in an unstable mode easily due to
wrong feedback information.
Rev 2, 14-Nov-12
NPN Selection Guideline
NPN transistors with HFE of 20 to 30 are highly
recommended in the design due to the start up
time. If the HFE is too low the start up time
becomes longer because of 30M start up resister.
V
FB
Sampling Waveforms
ACT334 senses the output voltage information
through the V
FB
waveforms. Proper V
FB
waveforms
are required for IC to operate in a stable status. To
avoid mis-sampling, 1.8µs blanking time is added to
blank the ringing period due to the leakage
inductance and the circuit parasitic capacitance.
Figure 2 is the recommended V
FB
waveform to
guarantee the correct sampling point so that the
Figure 2
1.8µs
Figure 3
1.8µs
Figure 4
Figure 5
1.8µs
1.8µs
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