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LM2733 soft start

Genius 3870 points
Other Parts Discussed in Thread: LM2733

Hi there,

We are using LM2733X to boost VCC12V to Vout(24V), output current load is small, like 10~20mA.

When LM2733 is enabled, VCC12V has momentary drop down, which is not we want to see. As VCC12V is generated from upstream LDO TPS7A4901, whose I_out capacity is max. 150mA. So I think this drop down problem may be caused by inrush current when Vout is ramping. 

We do not want to change TPS7A4901. Is there any way to add soft-start to LM2733 circuit, so VCC12V will hold on? SCH and test wave below.

Thanks!

Channel 2 and Channel 3 is VCC12V and Vout(24V) respectively.

  • Hello,

    You may be able to increase the 39pF feed-forward cap to substantially improve the matter. Start with a value like 0.1uF as a staring point. use only enough C to get the desired amount of improvement. The problem with this approach is  it may work on the first enable but if you disable and re-enable quickly it may not properly perform. This capacitor can impact transient response but probably is not a factor in your loading condition.

    A second method is to add a PNP and RC network as follows: this allows large freedom in adjustment and a good candidate for a simulator. I just reused the feedback divider resistor values as a starting point. The values will need to be optimized. The diode resets the cap during shutdown.

     2733-Soft-Start.docx

  • Hi Alan,

    At first, I tried increase C68 to 10nF, the drop down of VCC12V indeed decreased(as picture 1 below). Unfortunately the voltage ripple of 24V output became abnormally larger(as picture 2 below), which is not wanted.  

    I changed this feed forward cap value for several times, here is the dilemma:

    (1)If the cap value is too small, VCC12V will have large drop down; Vout ripple is normal, very small, like less than 20mVpp.

    (2)If the cap value is too large(about > 5.6nF), Vout will be "dirty" with much larger ripple, looks like circuit become unstable. I am not sure how this cap will influence  loop response of boost circuit, but from the test phenomenons, I think lager cap value leads to bad dynamic performance, which means bad ripples on Vout.

    So, the soft-start is not easy to add for LM2733. Could you please give some insight? Thanks, and happy weekend!

    Pic 1: 10nF C68 reduced Vin drop down.

    Pic 2: Chanel2 ripple, measured by AC coupling .

  • Sorry for the late reply. Next try adding some resistance in series with the feed forward cap. maybe a value that is 10 or 20% of RFBT.
    Did you try the other circuit I sent?
  • Hi Alan,
    I did not try the other method because the PCB layout is very "crowded" ,not much room left for flying wires.
    Could you please help explain why should adding some res in series with Cff? Or how will the Cff influence the loop response of this boost circuit?
    Thanks.
  • Hi Alan,
    Today I tried use adding 56K resistor in series with the feed forward cap 10nf. The result is unchanged with single 10nF condition.