TPS7B6950 Input Capacitor

Hello, the appropriate engineer has been assigned. Scott

Hello,

Thanks very much for reaching us!
For this application, does input voltage come from the trucks battery directly? How much and how quick would the voltage transient be during the normal operations?

If the voltage changes with very short time (e.g. <40us), big input capacitor is required. Or the line transient would be worse and there would be voltage glitch at the output, usually several hundred mV.
Electrolytic capacitors are also been widely used by many automotive OEMs. Is there any special condition in the trucks that would shorten the life time?

BTW, what is the maximum current value would the load consume? As there is already 19V voltage drop in 24V systems, high load current would result in thermal issue. If with SOT223 package and 85degC ambient operating temperature, load current must be smaller than 50mA. If with SOT23 package and same 85degC ambient operating temperature, load current must be smaller than 15mA.

Best Regards,

Jason Liu

Hello Jason,

    sorry for the delay in my answer, I didn't see your reply. 

    Yes, the voltage comes directly from the vehicle battery (generally after a fuse), honestly I have no info about the transient time, I haven't measured that parameter.

     An output transient of some mV in our application is not so important, becuase the output of that regulator is the input of a low noise ldo that will stabilize the voltage.

     The reason why I don't like electrolytic capacitors is because they are rated to 10 000 hours of lifetime that is not much time, and the system will be always plugged, maybe you can recommend me one with a good performance and lifetime.

      The consumption will be about 80 mA in 3.3V with peaks at startup. The device will be outside the truck so the ambient temperature will be no more than 45 degrees.

       Thanks for your support,

       Regards,

 

Hello,

Thanks for the additional information! If line transient is not so important, ceramic capacitor could be used, I would suggest, 4.7uF or higher. Please also take care of the voltage ratings.

To be honest, we haven't seen operating conditions outside of the vehicle before. Beside the ambient temperature, is there any special requirement for the device to work under the outside conditions?

Best regards,
Jason Liu

Hello,

    for avoiding capacitance problems due to DC bias / capacitance, I think that I will choose a 10uF/50V reference, I have bought this reference for prototyping GRM31CR61H106MA12L , do you think is a good option ?

     A part from the ambient temperature we haven't any other "special requirement" (apart from the IP and vibration requirements), do you think that 100mA is too much current for this choice of regulators ?

      Thanks for your support!,

       Regards,

Hello again,


we have assembled our firsts prototypes and as you adviced we are having temperature issues.

Now we are changing our power supply module. What we are thinking is to place 2 TPS7B6933 (one for the main uController and one for the rf part, because the rf module specification, indicates that the ripple voltage must be under 50mVpp and we have measured less than that for the TPS7B6933 without the P5900SD-3.3) for dividing the current consumption and avoid the thermal problems. Our calculations are 126degC for 40 degC ambient temperature, 50 mA max consumption and 40 VIN. And 106.75 degC for the same situation and 24 VIN that is the correct working voltage.

Taking the temperature and ripple information in count, I go back to the first question the inputs capacitors. Do you think that placing 2 GRM31CR61H106MA12L in parallel and 1 Low-ESR 100 nF will be enough for input capacitance ?

Thanks you very much for your incredible support !!!,

Regards,