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TPS7A4700/3301 capacitor selection and few other things.

Other Parts Discussed in Thread: TPS7A47, TPS3847

Hi all, this my first post, english is not my native language so sorry for all mistakes ;)


I want to build a PSU with those regulators mentioned in thread name.

But datasheet don't exacly mentioned how low ESR should be (or ESL).

There is only few words about X5R/X7R as good choice for typical application.
Almost everyone know that those capacitors have many disadvantages like piezo effect etc.

There is so many low ESR capacitors but I want to ask about one of them, I mean polymer tantalum, they are very low ESR, so can I use it as direct replacement for MLCC's?

Few other things that I need to know (for curiosity) how about FF capacitor? Those dialectric of those capacitors matter? Or maybe I can use any good MLCC or even PPS/PEN film capacitor (in SMD case)? TI recommend 10nF but how about going higher? It's worth and gave me any advantages over typical 10nF?

Last question is linked with sbva042 document, I'm thinking about include shottky diode in my desing and connecting it to FB pin.
But there is a question which diode can safety, any idea/recommendations?

Thank you for any help and time spend for reading this ;)

  • Hi,

    Your question on the capacitor ESR has already been answered in another forum post, which can be found at this link https://e2e.ti.com/support/power_management/linear_regulators/f/321/p/227080/799474#799474.

    Regarding the feedforward capacitor this capacitor should be a ceramic cap as well. A 10nF ceramic cap is recommended using too big of a FF cap can causes glitches in the power good signal during load transients. To avoid problems associated with the FF cap, stick with the recommended 10nF.

    If you choose to put the diode in the schottky diodes are recommended due to thier low forward voltage threshold. Putting the schottky diode in will help to protect the part due to exceeding the abs max negative voltage on the FB pin. If you use the schottky diode be sure to test the performance and make sure it satisfies your use case, also make sure the leakage current is not large enough to affect the voltage at the FB node.

    Regards,
    Mark

  • Thank you Mark.
    Could you sugest me any good (in your opinion) schottky diode, that can I connect to FB pin?
    There is so many diodes and I don't know which one will be good enough.

    Maybe this is stupid question but I also want to connect shunt protection diode with both outputs to avoid reverse biased operation.
    Should I choose something special for that application? I usually saw 1N400x in that place, but only in conjunction with some older type LDO's like LM3x7 or LM7x0x. 1N400x is very generic and cheap diode but I don't want to degrade performance.

    If I want to connect capacitor between both outputs (rail-to-rail decoupling) should I use something special? Like tantalum, ceramic, maybe general type electrolitic? Should I bypass that capacitor with some low value ceramic to achive low impedantion path? How close should I connect/place that capacitor?

    Thank you for help and your precious time :)
  • Hi Mateusz,

    A small, standard schottky should suffice to connect to the FB pin i.e.  BAT54.

    For a shunt protection diode to connect to the output (I will assume you mean anode to GND, cathode to VOUT), any diode will be fine, there should not be any performance degradation.

    I would be hesitant to connect a very large capacitance between the two VOUT voltages.  I fear that if the capacitance between the two rails is too large, it may interfere with the control loop of the opposing LDO.  If you need a common-mode capacitor between the rails, I would suggest a ceramic that is roughly 1/10 the size of the output capacitor to ground to ensure it does not create problems with the loop.  Placement should be near the output capacitors.

    Best Regards,

    Mike

  • Hello Again Mike.

    Could you tell me can I use precission enable like in Fig.1?
    I mean this article:

    e2e.ti.com/.../how-to-control-turn-on-off-thresholds-in-voltage-regulators-using-a-logic-level-uvlo

    If yes how to choose optimal values of those resistors? Theoreticaly I want to enable IC when input voltage will be 12V or higher but if it fall below that value (no matter why) IC should be disabled.
    I don't understand well enable function in linear regulators, I know what it should do, but don't know hot to work with.
  • Hi Mateusz,

    Yes, you can use a resistor divider for enabling the TPS7A47, however, since it is not a "precision" enable, the accuracy will be poor.  Most standard enable circuits are intended to be driven directly from a digital output to enable or disable.  On precision enable devices, the enable can be used by an analog voltage to control the enable/disable of the device.  For precision enable devices, the enable threshold is tightly controlled to a narrow voltage range (for example, 1.2V +- 10% maximum variation).  However, on the TPS7A47, the enable threshold is only determined by a digital logic device internally, so the min/max specified threshold is 0.4 V -> 2.0 V.

    If you used the resistor divider on a precision enable device, then the variation of where the device would turn on (meaning enable would go above the threshold) would be the same as the variation of the enable threshold i.e. 10% enable threshold variation means that the input voltage that the device will turn on above is also 10%, ignoring resistor variation.  So if you set it to turn on at 10 V, then that could vary between 9 V and 11 V. In the TPS7A47, the variation is wide and may not be useful for your application.

    If you need precision enable with the TPS7A47, I would recommend a supervisor circuit such as the  TPS3847.

    Regards,

    Mike

  • TPS3847 looks good.
    But unfortunately I will use those regulators with higher input voltage (aprox. 25VDC).
    And sometimes I'll adjust output voltage to lower values eg. 5V or 3V3 and when it will be done I want to shut down chip when input voltage falls below 7V or something like that.

    If needed I can use 0.1% resistors for that function.
    Any advice how low or how high those resistors should be?

    I saw that LT3042 uses tens and hundreds of kOhms.