LM2937: Voltage dip issue

ACGUY

Part Number: LM2937
Other Parts Discussed in Thread: LM317

Hello E2E,

Our customer is using LM2937 for mass product.
An individual whose output voltage generates a voltage dip exists in the devices.
Please see the attached file.

Voltage dip cycle and diping width are unconstant.
ΔV is about 1.8mV.

Please let me know your opinion.

Regards,
ACGUY

0714.LM2937_Voltage dip.pdf

over 6 years ago

0 Eric.Preiss over 6 years ago

TI__Expert 4355 points

Hi ACGUY,

From your attachment pdf, the voltage dip looks to be 3 mV. The scope is set to 10 mV/div. Are you measuring the voltage at the point marked with the arrow in your circuit?

Where are you measuring a voltage dip of 1.8 V?

0 ACGUY over 6 years ago in reply to Eric.Preiss

Guru 11170 points

Hi Eric,

Thank you for reply.
>> Are you measuring the voltage at the point marked with the arrow in your circuit?
Yes.

>> Where are you measuring a voltage dip of 1.8 V?
I was mistake, it is 1.8mV.
When measuring the oscilloscope's cursor at the center of the waveform, ΔV was 1.8mV.

0 Eric.Preiss over 6 years ago in reply to ACGUY

TI__Expert 4355 points

Hi ACGUY,

ΔV = 1.8mV is a very small output change, only 0.036% of the 5 V output voltage. This is within output tolerance with lots of margin. Is the customer concerned about 1.8 mV change in output?

0 Eric.Preiss over 6 years ago in reply to Eric.Preiss

TI__Expert 4355 points

Hi ACGUY,

Please refer to your other post: e2e.ti.com/.../662964
Your questions will be addressed in that post. I will close this thread.

0 ACGUY over 6 years ago in reply to Eric.Preiss

Guru 11170 points

>> Is the customer concerned about 1.8 mV change in output?
Of course.
This output is for sensor power supply. sensor output is depends to supply voltage. Sensor output is amplified by opamp. Voltage dip is appears remarkably to opamp output.

Why LDO output is shake at no load?
Is improvement possible?

0 ACGUY over 6 years ago in reply to Eric.Preiss

Guru 11170 points

Ryan would not reply.
you same to him?

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Acguy,

this is not unusual. I have that seen several times with other voltage regulators (e.g. LM317). This could be popcorn noise or something else.

You shouldn't use the output of a voltage regulator as sensor supply, if noise is such a big problem. Remember, a voltage regulator is no voltage reference.

Kai

0 Eric.Preiss over 6 years ago in reply to kai klaas69

TI__Expert 4355 points

Thanks Kai.

Hi Acguy,

I agree with Kai, that a voltage reference is needed in your application. Voltage regulators are sometimes used as a cheap reference, but linear regulators do not have the same performance as a voltage reference in several parameters.

Voltage references have: lower noise, smaller output voltage tolerance and smaller variations due to temperature and age.

For more information on noise sources, you can read this app note:
www.ti.com/.../slva043b.pdf

Link to the TI e2e voltage reference forum:
e2e.ti.com/.../216

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LM2937: Voltage dip issue