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TLV1117 with low ESR Ceramic Capacitors

simon chen94
Intellectual 260 points
Other Parts Discussed in Thread: TLV1117

dear,

I use TLV1117 (adjustable) for my application: 3.3V to 1.5V@ 200mA; I use ceramic cap 22uFx1pcs (GRM188R60J226MEA0L) for Cout.

When use 150 ohm and 30 ohm for divider resistor, the Vout is stable, but when use 2.74k and 499 ohm, Vout will oscillation.

My question:

1. Could we use ceramic Cap 22uFx1 for TLV1117 output Cap?

2. If yes, how to verify the system is stable? 

3. Why use higher divider resistor (2.74k+499 ohm) cause the Vout oscillation?

Look forward to your reply!

  • 0
    TI__Mastermind 24485 points

    Hello Simon,

    1. The TLV1117 device is not stable with low ESR capacitors such as ceramic capacitors. The device is stable with tantalum and electrolytic caps. Specifically the ESR of the capacitors needs to be between 0.2 and 1.0 Ohms. Please refer to section 9.1 of the TLV1117 device datasheet.

    2. The easiest way is as you mentioned, to look at the output and see if it oscillates.

    3. If you have no load at the output when you are monitoring the output voltage, then that may be causing the instability. The device requires a minimum load of 1.7 (typ) in order to regulate, so when you increase the resistor divider resistances you are decreasing the output load below the minimum if you have no other load.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    Intellectual 260 points
    Dear Michael:

    Thanks for your quickly reply!
    Regarding to Q2, I mean that I want to use ceramic Cap for Cout, but I concern that how to verify the stability?
    Just check the vout condition when no load? Is need to do the transient load or loop gain testing?

    Since datasheet refer to need use 0.2~1 ohm ESR of output Cap, not prefer to use low ESR ceramic cap, right?

    Please recommend, thanks a lot!
  • 0 in reply to simon chen94
    TI__Mastermind 24485 points

    Simon,

    It is not recommended to use a ceramic capacitor because the ESR is typically too low. I would expect there to be instability if you are using a low ESR ceramic capacitor. The recommendation is to use a tantalum or electrolytic capacitor.

    In order to verify the stability you can look at the phase margin of the device. The phase margin should typically be greater than 20degrees for stability.

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    TI__Mastermind 24485 points

    Simon,

    Additionally, you can use the ceramic capacitors if you add a resistor in series with the capacitor so that the ESR range in the datasheet is achieved.

    Best,
    Michael