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LP2951-50DRGR Output capacitance/ESR requirement (ESR curves)

Ramasamy Venkatachalapathy33
Prodigy 100 points

Hi,

We are planning to use LP2951-50DRGR in our design. In the device datasheet, it is mentioned that there is minimum output capacitance requirement with ESR range for stability of the LDO. I have provided LDO requirement for our product, Input/ Output capacitors chosen for our design below. Could you please confirm whether these capacitors satisfy LDO Input/Output capacitance requirement?

Design Requirement, Capacitor values data:

1. I/P voltage: 24V

2. Output Voltage: 5V

3. Load current: 6mA to max 60 mA.

4. Input Capacitor: P/N: GRM31CR61H106KA12L. 10uF ceramic capacitor (placed approx 2 inch away from LDO IC)

5. Output capacitor: P/N: TPSB106K016R0500 10uF tantalum capacitor (ESR value: 0.5 Ohms @ 100 kHzResonant freq: 1 MHz), further there are 6 x 0.1uF bypass capacitors placed near different Digital ICs (Loads).

6. I am unable to find ESR curves showing LDO stable region in Typical characteristics section of datasheet.

  • 0
    TI__Mastermind 24975 points
    Hello,

    Yes, while there are no ESR curves, we recommend that the output capacitor be between 100mOhms and 2Ohms. Our typical recommendation is 1Ohm, but 500mOhms will work perfectly fine as well.

    The input capacitor doesn't have a specified ESR, and the input capacitor you have chose is appropriate for this design.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 100 points

    Hi Michael,

    Thank you very much for your kind support. I would like to know whether 6x0.1uF decaps used in PCB board will affect stability of LDO considering  10uF tantalum capacitor (ESR value: 0.5 Ohms @ 100 kHzResonant freq: 1 MHz) is used as Cout for LDO.

  • 0 in reply to Ramasamy Venkatachalapathy33
    TI__Mastermind 24975 points

    Hello,

    I believe you are saying the 0.1uF caps are in parallel with the 10uF.

    If so, the key is the equivalent output ESR. The 0.1uF capacitors will effectively reduce the output ESR, but the largest capacitor typically dominates the ESR equation, especially when the capacitance is two orders of magnitude larger.

    In general, I wouldn't expect that this would affect it that much because the cap with the higher ESR is the largest cap, but it may be a good idea to calculate the effective output ESR.

    Best,
    Michael