ACT337 datasheet(8/11 Pages) ACTIVE-SEMI

8 / 11 page

ACT337

Rev 2, 14-Nov-12

Innovative Power

- 8 -

www.active-semi.com

TYPICAL APPLICATION CONT’D

to a single point (star ground configuration).

NPN Selection Guideline

NPN transistors with HFE of 20 to 25 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.

VFB Sampling Waveforms

ACT337 senses the output voltage information

through the VFB waveforms. Proper VFB waveforms

are required for IC to operate in a stable status. To

avoid mis-sampling, 1.38µs blanking time is added

to blank the ringing period due to the leakage

inductance and the circuit parasitic capacitance.

Figure 2 is the recommended VFB waveform to

guarantee the correct sampling point so that the

output information can be sent back into the IC to

do the appropriate control.

VFB 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.

Figure 2

Figure 3

Figure 4

Figure 5

1.8µs

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