UCC28950: SLUSA168, SLUM276 input resistances

SAMRAT PUJARI

Intellectual 730 points

Part Number: UCC28950

hello,

What is the use of C2_2 and three 1 megaohm resistances R4_2, R14_2, R17?

I simulated without these and I did not found any significant change. How is it affecting the input transient?

any references?

thanks

samrat

over 7 years ago

0 Colin Gillmor over 7 years ago

TI__Mastermind 25570 points

Hi Samrat

Can you post the schematic - or tell me which document you are referring to.

Thanks
Colin

0 SAMRAT PUJARI over 7 years ago in reply to Colin Gillmor

Intellectual 730 points

slum276 (2).tschello colin

please see the attached file

thanks

samrat

0 Colin Gillmor over 7 years ago in reply to Colin Gillmor

TI__Mastermind 25570 points

Hi Samrat

In the real world you will need some local input capacitance to provide a low impedance source for high frequency currents at the input to the PSU converter. C2_2 and C3_3 provide this. Also, in the real world one needs to make sure that these capacitances are discharged when the input voltage is removed, R4_2, R14_2 and R17 will discharge the input capacitances. This discharge function is normally applicable to AC powered supplies and in that case it is called X-Cap discharge. The X capacitors being the input capacitances mentioned above.

In a simulation, if the output impedance of VIN is 0 (which IS the case in this simulation) then the input capacitances and the discharge resistors are not needed. In fact your observation that it makes no difference when removed is exactly what I would expect.

Regards
Colin

0 SAMRAT PUJARI over 7 years ago in reply to Colin Gillmor

Intellectual 730 points

hello colin
thanks for the info
samrat

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UCC28950: SLUSA168, SLUM276 input resistances