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.

LM3940: stability

Part Number: LM3940

Team,

Customer already used LM3940 but not satisfied the noise performance. By adding more output caps, it seems help but become non-stable. Figure 12 in datasheet only shows Cout = 33uF stable region. Could you please provide more data for their condition below? Or is there any simulation tool they can run by themselves?

4.5Vin to 3.3Vout/500mA, Cout=10uF and 45uF

By the way, is it fine to use ceramic capacitors as long as ESR vs output current located in the stable region?

Thanks.

Allan 

  • Hi Allan,

    It is fine to use ceramic capacitors as long as the ESR is within the stable region of figure 12. Cout = 33 uF minimum. There is no Cout max limit. No simulation model is provided for this part.

    The minimum ESR line for Iout = 200 mA to 1 A is about 3 mohms. For Iout = 0 A to 200 mA, the minimum ESR is about 7 mohms. For multiple capacitors in parallel, calculate the parallel ESR of the output capacitors.
  • Hi Allan,

    the LM3940 is a very old LDO which is intended to be used with good tantal caps at the output. If you want to take ceramics, then you must imitate the behaviour of tantals by adding a series resistor to increase the ESR of ceramic which is much smaller than the ESR of tantal. A 10µ/10V/1206/X7R has a ESR of about 3mR. To get the minimum output capacitance of 33µ you would to have parallel three of them. This will drop the total ESR of this parallel combination to about 1mR which would make the LM3940 become instable. A remedy could be to add a 100mR/0805 resistor in series to the parallel combination. Or you could add a 330mR resistor in series to each 10µ cap.

    As Eric has already noted, when calculating the load capacitance of LM3940 you must also take into calculation the many decoupling caps which are distributed in your circuit at the chips. To keep their impact minimal you could use a trick and isolate these decoupling caps by a small series resistance of a few Ohms. This would also act as a low pass filter cleaning up the supply voltage.

    The LM3940 is a LDO which produces a standard noise level. The output noise isn't very high. If you experience a very high noise level, this could also come from noise at the input of LM3940! The LM3940 does not have a very high ripple rejection, as many other LDOs also. So, in certain instances, a pi-filter at input can heavily decrease the noise seen at output. This is true for instance if your LDO is powered by a DC/DC-switcher or similar.

    Kai
  • Hi Eric, Kai,

    Thank you both. I will discuss with customer and keep you updated. 

    Regards,

    Allan