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TPS40200 shuts down in ESD test

Andrew Sykes
Prodigy 20 points
Other Parts Discussed in Thread: TPS40200

We have experienced a problem when performing an ESD test on one of our products which uses TPS40200 as the controller for a 24V - 15V non-isolated power supply. The output switches off and then resets - we assume this is the same response as we would see if the overcurrent protection operates when the pin 7 signal exceeds the reference level. The attached Word document gives some details of the effects observed, schematic and layout.

We have found an interim solution which raises the ESD voltage required to trigger this effect, but we would like to raise the threshold even further by improving the PCB layout. In the attached docment I have listed the changes that I plan to make to the layout. I would value any comments  that you have on the possible causes of the effect and the solutions both already applied and planned.

ESD problem 24 - 15V DC DC converter.docx
  • 0
    TI__Mastermind 35165 points

    Hi Andrew,

    Thanks for the detailed document. You're plans are the same as I would have suggested so I don't have much to add. It is possible there is some type of glitch triggering the comparator. Well placement of the decoupling capacitor from VDD to GND and filter capacitor from ISNS to VDD will help filter any glitch. Any inductance between the pin and the decoupling capacitor will act as an antenna and increase the amount of noise picked up by the ESD pulse.

    I wonder if it is possible to measure any ripple on the VDD or ISNS pins during the ESD pulse which can help confirm this.

    Best Regards,
    Anthony 

  • 0 in reply to Anthony F
    Prodigy 20 points

    Dear Anthony,

    Thanks for your response.

    Regarding the placement of the decoupling capacitor and the filter capacitor from ISNS to VDD: The decoupling capacitor is already as close to the VDD and GND pins as is physically possible. The capacitor from ISNS to VDD could be moved closer and we plan to do that in our next PCB up-issue. However, please note that we experienced the power supply shut down even when we shorted the VDD pin to the ISNS pin.

    We never attempted to measure the signals on any of the TPS40200 pins. I think that if a 'scope probe is connected it will change the parasitic impedance of the pin and possibly affect the test result (adversely or improve it)? In addition, detecting small differential signals whilst a large common mode disturbance is applied could prove difficult.

    I would value your opinion on the change that I am proposing to add the 10R resistor in series with the +24V supply to the VDD pin, thus forming an RC filter. Do you think this change will be effective in reducing the sensitivy of the IC to ESD disturbances?

    I have noted that in one of the  forum discussions on TPS40200 a comment from TI as follows:

    "Without this filter ripple on VIN could result in false triggers of the current limit circuit due to the internal time-constant tracking of the VDD - 150mV reference."

    This was posted by Peter Miller of TI on Jan 10 2011. Could you explain what Peter meant by "internal time-constant tracking of the VDD - 150mV reference"?

    Regards,

    Andrew

  • 0 in reply to Andrew Sykes
    TI__Mastermind 35165 points

    I believe he is referring to the internal nodes tracking the ripple on VDD at both inputs to the current limit comparator. I also think he meant the VDD - 100 mV reference. The ISNS pin goes directly to the comparator while the VDD - 100 mV reference might have internal delays to adjust to changes in VDD.

    Adding the 10R resistor may help but it depends on where the noise is coming from causing the ESD event. If there is a big VIN copper are it is possible there is a glitch on which also goes to the VDD pin of the TPS40200. Adding the 10R resistor will help filter any glitch in this area. The RC time constants of the added resistor with C25 should match the RC time constant of the filter added to the ISNS pin so they are in phase.

    I was thinking it might not be so to measure the glitches. But if you can and find a frequency of the glitch, the bypass capacitors could potentially be tuned to be more effective at filtering the glitch. Lower capacitance can be more effective at filtering higher frequency noise because they become inductive at higher frequencies than larger capacitance.