LMR12010: LMR12010 CFF capacitor application questions

Red Yen

Part Number: LMR12010

Hi Support team

may i have your comments about the side effect of CFF capacitor (C606) on upper feedback resistor?

my customer was encountered FB pin out of SPEC under low temp, but after shipped the suspect IC back to TI, it's passed ATE and bench,

i assumed the issue was caused by CFF capacitor, may i have your deep explain on this cap?

over 5 years ago

0 JohnTucker over 5 years ago

TI__Guru**** 191445 points

Red,

Let me take a look.

0 JohnTucker over 5 years ago in reply to JohnTucker

TI__Guru**** 191445 points

Red,

I am not aware of any effect that a normal Cff cap would have on the FB voltage. That said, 0.068 uF is an unusually large feed forward value. It is possible that it may cause large ac ripple at VFB that may cause some DC offset. Can you provide the VFB voltage waveform with and without the capacitor?

0 JohnTucker over 5 years ago in reply to JohnTucker

TI__Guru**** 191445 points

Red,

Pspice simulation does have a lot more ripple on VFB with Cff. VFB dc voltage is actually slightly lower. Please provide the requested waveforms.

0 Red Yen over 5 years ago in reply to JohnTucker

TI__Expert 4360 points

Hi John

allow me to check w/ customer and get you back soon,

thanks a lot

0 Red Yen over 5 years ago in reply to Red Yen

TI__Expert 4360 points

Hi John,

pls refer to following about FB waveform,

if CFF value is too huge, what is recommend value?

0 JohnTucker over 5 years ago

TI__Guru**** 191445 points

Red,

The wave forms from you last post do not provide any useful information. In any case, there are two methods I can recommend to calculate Cff:

1. Calculate Cff so the zero frequency is slightly above the crossover frequency. Fz = 1 / (2 * pi* Cff *Rtop) where Rtop is the upper FB divider resistor.

2. Calculate Cff so that the zero / pole pair are geometrically symmetric about the crossover frequency. This method gives the most phase boost at crossover. However is is slightly more difficult to calculate than method 1. Placing the feed forward zero below the initial crossover frequency will move the new crossover frequency higher, so this method becomes an iterative process.

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LMR12010: LMR12010 CFF capacitor application questions