LM5000 compensation network

Ryuji

Guru 20090 points

Other Parts Discussed in Thread: LM5000

Hello,

I simluated LM5000 on webench with the following requirement.

Vin=26V~30V, Vout = 48C, Iout=0.5A, Ta=30deg.

As a result, phase margin is about 24.7° .
If I change the Rcomp from 20kohm to about 50kohm, phase maringe will be about 47°.
However, the Rcomp upper limit is 20kohm.

Could you please let me know this reason?

And since, I couldn't set the good compensation circuit on Webench, I please let me know the reference compensation circuit value for above requirement.

Best Regards,
Ryuji Asaka

over 10 years ago

0 Timothy Hegarty over 10 years ago

TI__Guru 74002 points

Hello Ryuji,

The RC from COMP to GND sets a zero to provide phase boost at crossover to achieve the required phase margin. You can increase the capacitor to lower the zero frequency and obtain additional phase.

Note that increasing the resistor reduces the zero frequency too - however it increases gain, increasing croosver frequency. For more detail, please review my current-mode control article at http://www.how2power.com/newsletters/1406/articles/H2PToday1406_design_TexasInstruments.pdf.

Regards,

Tim

0 Ryuji over 10 years ago in reply to Timothy Hegarty

Guru 20090 points

Hello Tim san,

Thank you for the reply. I understood that the adjutable the phase margin.

Could you please let me know why the phase margin becomes small with 25° by the first simulation of webench?

Ccomp Upper limit is 4.7nF and Rcomp Upper limit is 20kohms in the webench.
Could you please let me know this reason?

Best Regards,
Ryuji Asaka

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LM5000 compensation network