TLV316: Stability question with buffer circuit

Markus M.

Part Number: TLV316
Other Parts Discussed in Thread: OPA316

Hello,

i have a question to the circuit attached. Its a simple buffer circuit with a big capacity at the output.
In simulation it looks stable with a 1Ohm isolation resistor. I tested it on a breadboard but i havent seen a big difference with or without this resistor.

Do i need this resistor or can the ESR of the load capacity be sufficient enough?

Cliffs: Vout=3,3V
Iout up to 30mA

Second question, is the output current guaranteed over temperatur?

Thank you in advance.

Markus Malsam

over 7 years ago

0 Markus M. over 7 years ago

Prodigy 40 points

3,3V Regler direkt mit tlv316 stability.TSC

0 Collin Wells over 7 years ago in reply to Markus M.

TI__Guru 64345 points

Hello Markus,

20uF is a really large capacitive load. Adding the isolation resistor adds a zero to the control loop to compensate the pole from the capacitive load. By setting Riso to 0Ohms and running the phase-margin analysis we find a crossover frequency of only 18.5kHz. To achieve 45 degrees of phase margin the zero needs to fall at or before this frequency. To achieve the zero at that frequency a pretty small resistance is required due to the large capacitance value. The value is calculated to be 0.43Ohms which to your point may be within the ESR range of this capacitor depending on it's construction and dielectric type.

The TLV316 shares the same output stage as the OPA316 which has additional information in the datasheet on the output current vs. temperature as shown in the image below. As you can see the output voltage swing to rail and output short-circuit current levels decrease with temperature.

OPA316 Vout vs. Iout:

0 Markus M. over 7 years ago in reply to Collin Wells

Prodigy 40 points

Thank you very much Collin!

0 kai klaas69 over 7 years ago

Guru 140200 points

Hi Markus,

there IS a difference if you omit the isolation resistor:

If you omit the isolation resistor, the circuit becomes very unstable!

A 20µ cap with ESR can help:

But the ESR has to be stable over temperature.

Kai

0 Collin Wells over 7 years ago in reply to kai klaas69

TI__Guru 64345 points

Hi Kai,

We confirmed above that the ESR must be at least about 0.43 Ohms for the circuit to be stable. Agreed with your comments over temperature.

Thanks!

0 kai klaas69 over 7 years ago in reply to Collin Wells

Guru 140200 points

Hi Collin,

sorry, I didn't want to be disrespectful. I only wanted to say, that Markus could have found out the threat of instability by a simple frequency sweep measurement. If the phase margin is heavily eroded, this can be seen as narrow peak in the frequency response.

Kai

0 Collin Wells over 7 years ago in reply to kai klaas69

TI__Guru 64345 points

No worries. Thanks for the help!

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TLV316: Stability question with buffer circuit