LM4040-N: Output Impedance vs Frequency

Masakazu Adachi

Intellectual 2490 points

Part Number: LM4040-N
Other Parts Discussed in Thread: LM4040

Hello team,

Please tell me about an elementary question.

Regarding Output Impedance vs. Frequency in Fig 2 and 3.

What frequency does "frequency" in this case refer to? Does it refer to the frequency on the load side?

The customer is dividing the voltage of the LM4040-N and using it as a reference voltage for the comparator.

Also, does the LM4040-N ever oscillate? I don't think this has anything to do with the above...

The customer says that the LM4040-N oscillates and that he has added a capacitor but cannot completely suppress the oscillation.

Please let me know your opinion.

Regards,

Masa

over 1 year ago

+1 Trailokya Rai over 1 year ago

TI__Genius 10951 points

Hi Masa

Dynamic impedance at any frequency (let's say F Hz) is shows the amount of ripple in VR at F Hz If IR has a ripple at F frequency. Ripple in IR may come either due to switching in the load or switching in the supply.

LM4040-N is stable across all the capacitor.

Can you please share more details about the schematic?

Regards

Trailokya

+1 Masakazu Adachi over 1 year ago in reply to Trailokya Rai

TI__Intellectual 2490 points

Hi Trailokya,

Thanks to the support.

Explain to the customer and ask for more details.

Regards,

Masa

0 Masakazu Adachi over 1 year ago in reply to Masakazu Adachi

TI__Intellectual 2490 points

Hello Trailokya,

I received additional questions.

Does CL=1uF mean when a 1uF tantalum capacitor is connected in parallel?

Is CL=0 the graph without tantalum capacitor?

What does XCL mean in Figures 2 and 3?

Regards,

Masa

0 Trailokya Rai over 1 year ago in reply to Masakazu Adachi

TI__Genius 10951 points

Hi Masa

I have marked CL in below diagram -

CL= 0 means CL is disconnected.
XCL is the impedance of the load capacitor over the frequency.

Regards

Trailokya

0 Masakazu Adachi over 1 year ago in reply to Trailokya Rai

TI__Intellectual 2490 points

Hi Trailokya,

XCL is the impedance when CL is how many uF?

The impedance at 1uF is the solid line, right?

Regards,

Masa

0 Trailokya Rai over 1 year ago in reply to Masakazu Adachi

TI__Genius 10951 points

Hi Masa

XCL is the impedance for 1uF. You can verify as it is = 1ohm little before 200K (actual value is around 159).

The impedance at 1uF is solid line. The dotted line represents impedance without load capacitor.

Regards

Trailokya

0 Masakazu Adachi over 1 year ago in reply to Trailokya Rai

TI__Intellectual 2490 points

Hi .Trailokya,

What is the difference between the CL = 0 line and XCL line ?

Regards,

Masa

+1 Trailokya Rai over 1 year ago in reply to Masakazu Adachi

TI__Genius 10951 points

HI Masa

CL=0 line is dynamic impedance of LM4040 for no load capacitor.

XCL line is just impedance of 1uF cap. That is just for the reference. This line has been shown for comparison purpose only.

XCL = 1/(2*pi*F* (1uF))

Regards

Trailokya

0 Masakazu Adachi over 1 year ago in reply to Trailokya Rai

TI__Intellectual 2490 points

Hello Trailokya,

I understand. Thanks to the support!

Regards,
Masa

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LM4040-N: Output Impedance vs Frequency