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LM4140: ESR requirement of Low ESR Ceramic Capacitors?

BX27
Genius 3880 points
Part Number: LM4140
Other Parts Discussed in Thread: DAC7811

Hi there,

We use LM4140 for DAC REF2.5V. SCH as below.  On Datasheet page9 Figure24-26 shows a Cout's ESR value range for STABLE operation. 

1.We use high quality C1005X5R1A475K050BC 4.7u, which has a very low ESR as pic below. Not sure how to decide the freq because ESR changes with frequency .  Our system has DC/DC all worked at 800KHZ. 

So shou we serial one resistor R1018 with Cout ?  

2. I also need to ask,  there is mechanical stress relief action required on page 12 of Datashseet, is it necessary ? How much Mechanical Stress/Distortion will it affect its performance?  Is there quantitative relationships? 

Thanks!

  • 0
    TI__Mastermind 19570 points
    Hi Yi Xiao,

    A good example of LM4140 design is TIDA00499
    www.ti.com/.../TIDA-00499

    In this TIDA they use a LM4140 in the schematics shown on pg 41 of the user guide.

    I would connect the ESR resistor to only one capacitor. I would recommend to have at least a layout spot for a ESR resistor just in case. Most of the time ceramic does not meet the requirements.

    Stress is the main reason the voltage reference will deviate from the expected norm.
    Temperature drift, long term hysteresis, thermal hysteresis are all Vout deviations from stress. In this case it is stress on the die due to thermal expansion of die and of the package.
    Mechanical stress does affect the device and mechanical stress should be reduced. This can be done by laying out the VREF closer to the center of the PCB or making a notch around the device if the PCB is expected go to under stress.
    Most of the time this is not needed because the environment does not require it. If you expect the PCB to go under mechanical stress such then I would recommend to take it into account and add notches around the device.
    If you are looking for X amount of stress causes Y amount of deviation, that data does not exist.

    Also, some DACs require the voltage reference to sink current when the output voltage is larger than the VREF. The LM4140 cannot sink current, it can only source. This means the LM4140 requires a buffer or you need to switch it to a REF50xx family level of device. This is DAC specific and you should look into it.

    -Marcoo
  • 0 in reply to Marcoo Z
    Genius 3880 points

    Hi Macroo,

    1. The TIDA sch use R90 = 1Kohm in serial with Cout C54/10uF. This resistance seems too large, and too much exceed the Cout ESR value specified in datasheet which is several ohms. pls help confirm.

    2. I think you words "This can be done by laying out the VREF closer to the center of the PCB" is different from datasheet section 10.1 "The simplest ways to reduce the stress related shifts are: 1. Mounting the device near the edges or the corners of the board where mechanical stress is at its minimum. The center of the board generally has the highest mechanical and thermal expansion stress. " 

    3. Thanks for mention "DAC may require the voltage reference to sink current when the output voltage is larger than the VREF" . Now we use LM4140-2.5V to dircetly provide VREF for DAC7811. But I did not found infomation in DAC7811 datasheet about whether it requires voltage reference to sink current when the output voltage is larger than the VREF. Where should I confirm this ? 

    Thank you.

  • 0 in reply to BX27
    TI__Mastermind 35190 points
    1. I agree with you in that the series resistance of the "electrolytic" capacitor is high. These designs have been proven out in the lab, but if you have concerns, I would follow the datasheet to ensure datasheet specifications can be met.

    2. I would consider your specific application to determine how you layout the device. Be mindful of where the device will be located and neighboring components and how the board will be positioned and/or mounted. Additionally, if possible by your board manufacturer and design cost, a u-shaped slot can be placed around the device to allow for thermal isolation and mechanical relief.

    3. I think your best bet on figuring out specific device information on the DAC7811 is to start a new thread for that device information request. You should get a product expert to get back to you quickly. Unfortunately I am not too familiarized with that device.