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Series resistor in output bypass capacitor for LDO

Rohit Baranwal
Expert 1025 points
Other Parts Discussed in Thread: TMS320F28335

Hi, I intend to use the LDO TPS767D301QPWPRQ1 for powering a microcontroller board based about TMS320F28335 microcontroller. I observed in the datasheet that output capacitor ESR of 0.06 to 1 ohm is needed for stability for 22uF output capacitor.

I have used a 22uF ceramic capacitor as output capacitor C1 and a 0.068 ohm resistor R1 to emulate the ESR. Then, I have another capacitor C2 of 10uF connected across the output voltage, with no ESR. I have used a 0.25W resistor for emulating the ESR. Here are my questions:

1. Is the ESR resistor necessary to be a thin film resistor, or can I use a precision thick film resistor, something like the following?

www.digikey.com/.../1705798

The above resistor is in 0402 package, while thin film resistors are in 0603 package for 0.25W. I will be using this for automotive application, for which the said resistor rated.

2. PCB layout question: Can this ESR be on the bottom side of the pcb, while the output capacitor is on the top side? OR does this resistor have to be on the top side too?

3. Is the arrangement of C1+R1 and C2 a good one, or should I insert an ESR in series with C2 as well?

Please advise.

  • 0
    TI__Guru 55940 points
    Hi Rohit,

    As long as the ESR requirements are met, it does not really matter what type of resistor.

    It would be best to have the resistor and capacitor on the top side. Dropping through a via will add inductance;therefore, it would increase the ESL.

    In your indicated arrangement you would be paralleling the 0.068ohm resistor with a 0ohm resistor. This would essentially eliminate the resistance. Please ensure that the parallel combination of ESR for the output capacitors stays within the datasheet range.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Expert 1025 points
    That makes sense. I will just remove the 10uF capacitor and place 0.068 ESR with 22uF.
  • 0 in reply to Rohit Baranwal
    Expert 1025 points
    Actually, when a zero resistance is present in series with C2, it will not result in zero ESR. The real part of the impedance is actually a frequency dependent quantity. But I will have only C1 anyway.