This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS81256 application

Other Parts Discussed in Thread: TPS81256, TPS61251

Dear Sir

My customer is use TPS81256  in 3.3Vin and Vout 5V application in SSD product.

Due to have super cap function, my customer paraell 1.6mF capacitor in output.

then sometime will case 3.3Vin voltage drop. i have check datasheet page 3.

here suggest Cext maximum is 30uF. Is that mean my customer  parallel too much capacitor?

If yes, may you have some suggestion for us?

 

  • it seem to be caused by two factor: large output capacitor and high impedance of input power supply. because of large COUT, the device draw large current when it start switching, and the large current cause the voltage drop in VIN.  you can try to increase the input capacitor.

    thanks

    Jasper Li

  • The TPS61251 is likely a better device, since it is designed to drive this much Cout.

  • Dear Glaser

    Due to my customer have space concern. so now Just can use tps81256.

    input side switch IC 的Rds(on) is160m ohm (max. 320m ohm) and TPS81256  input current limit is1180mA,

    The maximum drop Voltage is 320m ohm * 1,180mA = 0.3776V

    Vcap = 3.3V – 0.3776V = 2.9224V

    We want check Vin drop below 2.7V, or we have loss someting?

                                                               Best regards,

                                                                                   Bogey

  • Dear Sir

     

    May we also check if TPS81256 have optput capacitor limit?

    From DS, Cext  maxium seems 30uf....

                                               Best regards,

                                                                  Bogey

     

  • Yes, we do not recommend operating the TPS81256 with more than 30 uF Cout per the D/S.  It does not seem like you do not have room for an appropriate device since you use so many caps, etc.

    The voltage drop is not due to the TPS81256, but rather the impedance in your input source.  This is the resistance from wherever you are measuring the 3.3V to the TPS81256's input.  Is it possible to reduce this resistance?

    Another option used by some is to add the large caps with a parallel resistor + diode in series with them.  The resistor decouples them from the circuit and reduces their charging current demands, while the diode allows for a fast and low drop discharge.