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LM2941 Selecting Caps for Input and Output Pins

Gary Cederquist
Prodigy 70 points
Other Parts Discussed in Thread: LM2941, LMZ21700

I am designing a Power Conversion small circuit card using the LM2941LD part. The estimated maximum output current is 0.225 amps with an input voltage of 15-19 V DC. I have selected Resistor values for R1 = 1 K and R2 = 10 K for an output of approximately 14 V.  The maximum operating temperature is 55 Deg C.

1)  For Cin, I have tentatively selected a 0805, X5R, 35 V 0.47 uF, 10%.

     a. Is this style(X5R), value and DC rating appropriate for my circuit application?  Yes, I will place Cin as close to the LM2941 as possible - same side as LDO.

2)  For Cout, I have tentatively selected a 1206, X5R, 25 V, 22 uF, 10%.

     a. Is this style (X5R), value and DC rating appropriate for my circuit application ?  I will be operating Cout  at approx 14 V (56% of 25V rating)

     b. Should I also include a series resistor ?  How do I calculate value and wattage ?

For this design, the physical space is limited so I do not have room for a large size tantalum output cap.

Thanks for any design assistance.

Best Regards, Gary

  • 0
    TI__Mastermind 22610 points

    Hello Gary,

    I see that this question has also been posted in the High Reliability Forum as well.

    The concern with using ceramic capacitors, especially for Cout, is that the capacitance value decreases as the applied DC voltage increases, as well as the lack of useful ESR.

    For the input cap with 19V applied, I would recommend that you increase the value to 1uF ceramic, as the capacitance derating for 0805 case size is noticable.

    Your choice for Cout would probably be closer to (22uF x 0.9 x 0.86)= 17uF in the circuit.

     For Vout= 14V, the 17uF should be OK.

    Yes, you absolutely will need a resistor in series with your Ceramic Cout in order to simulate the required ESR. The ESR Value comes from the chart in the datasheet (Figure 13. Output Capacitor ESR). I recommend 500 milli-ohms, the middle of the valid ESR range. Power dissipation is minimal, there is dissipation only when Vout is changing. Case size 0603 is OK.

    As for " ... large size tantalum output cap ...", there are a number of suitable 22uF 20V Tantalum capacitors available in small case sizes.


    For a 1210 case see : AVX TPSB226K020R0400

  • 0 in reply to Donald Jones
    Prodigy 70 points
    Donald:
    Thanks for the responses. Here are my follow-up questions:
    1) For Cin shall I use - note DC rating. My worst case Input V is 19 V:
    0.47 uF, 35 V, 0805 Type X5R or 1 uF, 25V, 0805 Type X5R
    2) For the Cout we are trading off physical size limitations of:
    TBSB226K020R0400, 1210, 22 uF, 10%, 20V DC, 0.4 Ohm ESR (Tantalum Low ESR) vs.
    12063D226KAT2A , 1206, 22 uF, 10%, 25 V, X5R + series Resistor of 0.5 Ohm R0603

    3) For Figure 13 on the data sheet My output voltage will be approx. 14 V and estimated current about 225mA max.
    Does this mean for the LM2941 to be stable I will need and ESR of 5 Ohms or less on Cout ?

    System usage note: I will be connecting the LM2941 to an LMZ21700 Simple Switcher on a physically small circuit board. This means that the Cout of the LM2941 will also be the Cin for the LM21700 part.
    Regards, Gary
  • 0 in reply to Gary Cederquist
    TI__Mastermind 22610 points

    Q1) "... For Cin shall I use ..."

    A1) Use the 1uF 25V 0805.

    Q2) " ... Does this mean ... I will need and ESR of 5 Ohms or less on Cout ? ..."

    A2) Yes

    Q3) "... the Cout of the LM2941 will also be the Cin for the LMZ21700 part..."

    A3) Then capacitor must meet the requirements of both devices, and the Tantalum is no longer an option. The LMZ21700, like most SMPS, should not use Tantalums on their inputs.