LM7322: No phase reversal ?

user5163865

Genius 3980 points

Part Number: LM7322
Other Parts Discussed in Thread: LM7332, OPA2192, OPA2202

Hi,

Our Customer has a question about LM7322 & LM7332.

Are these two OPAMPs guaranteed to be no phase reversal with a large C Load?

Customer requirement is that the input voltage is within the power supply rail.

I think phase reversal occurs with overvoltage inputs.
I think C Load is irrelevant.
Is this correct?

Best regards,

Hiroshi

over 5 years ago

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Hiroshi,

can you show a schematic?

Kai

0 Ron Michallick over 5 years ago

TI__Guru**** 164976 points

Hello Hiroshi-san,

RRIO devices will never phase reverse for input voltages that are within the supply rails. That's the main feature of rail to rail input. High output capacitance can lead to other problems. The schematic will be useful to see what other problems may exist.

0 user5163865 over 5 years ago in reply to Ron Michallick

Genius 3980 points

Ron-san,

Thanks a lot.

There is no detailed schematic, but the circuit information is below.
24V、Non-inv Gain=4.5　Load : MOSFET Ciss = 5600pF Output swing : 0.1V～12V.

Too large CLoad,,,
OPA2192 can drive 1nF.

In customer simulation, there was no problem with the 1k ohm series resistance, but the evaluation caused oscillation.

The customer will reselect the device.

I also considered OPA2202, but the SR is too low.
Since it is used for the FET drive circuit of the high voltage generation circuit and it is difficult to measure the OPA output, there is no measurement data.

So they are testing the device in cut and try.

Best regards,
Hiroshi

0 kai klaas69 over 5 years ago in reply to user5163865

Guru 140200 points

Hi Hiroshi,

I'm sorry, but I cannot help without schematic.

Kai

0 Ron Michallick over 5 years ago in reply to user5163865

TI__Guru**** 164976 points

Hiroshi-san,

Is it like this? What are the values for RF, RG, CF?

0 user5163865 over 5 years ago in reply to Ron Michallick

Genius 3980 points

Hi Ron-san,

I asked the customer for circuit information. It is a similar circuit.

RG 3kΩ
　RF 16kΩ
　CF 1000pF
　ROUT 3.6kΩ　
　Ciss = 5600pF

OPA2192 is unstable. It seems to have improved by changing OPA to LM7332 and then changing to a larger ROUT.

This constant is the value of their cut and try.

I think that it is difficult because the actual circuit and the simulation are different, but TI's advice would be helpful.

Best regards,

Hiroshi

0 Ron Michallick over 5 years ago in reply to user5163865

TI__Guru**** 164976 points

Hiroshi-san,

Those values for OPA2192 would produce a very stable 76 degrees of phase margin.

However changing the feedback tap would reduce phase margin to just 4 degrees (potentially unstable)

Is the red line the real feedback location?

0 user5163865 over 5 years ago in reply to Ron Michallick

Genius 3980 points

Hi, Ron-san

Thank you for your support.

Feedback is the original circuit(not red line).
The OPA2192 simulation results were good.
However, it sometimes becomes unstable in a real circuit.
It may be an external factor because it is the drive circuit of the high frequency power supply unit.

Due to the time of design FIX, they will adopt LM7332 which has good real circuit evaluation result.
Customers would like to know if LM7332 has no problem with this constant value.
Please let me know if you have the best value.

When I do an AC analysis with TINA it looks like it's fine.

Best regards,

Hiroshi

0 Ron Michallick over 5 years ago in reply to user5163865

TI__Guru**** 164976 points

Hiroshi-san,

Those values are good for LM7732

0 user5163865 over 5 years ago in reply to Ron Michallick

Genius 3980 points

Ronald-san

Thanks a lot.

Best regards,

HIroshi

Amplifiers

Amplifiers forum

LM7322: No phase reversal ?