problem with TL497A circuit in voltage boost mode,

Discussion in 'General Electronics Chat' started by navidnavid, Aug 10, 2005.

  1. navidnavid

    Thread Starter New Member

    Aug 10, 2005

    I used TL497A to design a voltage booster (12 volts to 18 volts / 350 mA).
    Based on the datasheet, I used L=470uH, Ct=470pf, TIP122 darlington NPN transistor for power switching, 0.38 ohm/1W limiter resistor, 300uF filter capacitor and 3A Fast Diode.

    Sometimes it show Short Circuit when start-up (turn on board) and destroy Resistor or IC.
    Output is about 200mA @ 18volts.
    Also, I hear some noise from 470uH Inductor when circuit is on.

    I will be appreciated with your comment & suggestion for troubleshooting.

    Best Regards,
    Navid Navid
  2. Dave

    Retired Moderator

    Nov 17, 2003
    Just for maintenance of the forum I have removed the other similar posts.


  3. freetek

    New Member

    Aug 10, 2005
    Hi Navid,

    The only pitfalls I see are R103 and C34. These should be respectively, low inductance and low ESR parts.
    There are fast transients being developed on Vcc and a combination of too much inductance in R103 and high impedance at frequency (actually rise times) at C34 could cause problems. C107 helps to offset the poor high frequency characteristics of C34 but can't compensate for a really bad cap.
    If you have the advantage of an impedance meter with high and low bridge frequency output (typically 60/120 Hz and 1 kHz or more), take a look at the change in capacitance of C34 at these two settings and if ESR is also a function, check it at both too.
    If you are further blessed with inductance functions, look at R103 at the different frequencies too.

    Good luck-
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Vbe of the TIP122 is about 1.5V. Therefore R100 will draw about 1.5ma. R97 will only pass about ((12-1.5)/12k) 0.87ma. You are probably getting enough extra drive to the TIP122 from the base-emitter current of the internal transistor to marginally turn on the TIP122. Reduce the value of R97, or increase the value of R100. Keeping in mind that I have no experience with this part - I would make R97=3.9k.