LM21305: LM21305 FREQ CFRQ

Wenhao Wu

Part Number: LM21305

Hi Team,

Looking at Figure 19 in LM21305 datasheet, do you know the reason why we should add that CFRQ? Do you know how LM21305 sets Fsw by FREQ pin?

By my understanding, there should be a triangle waveform whose frequency is same as Fsw. Adding an additional CFRQ will filter the triangle signal. Another way is to source a current into RFRQ and read the voltage acorss it, namely pinstrapping.

Thanks!

-Wenhao

over 5 years ago

0 Jenson Lam over 5 years ago

TI__Prodigy 450 points

Hi WenHao,

Please refer to section 8.3.3, 100pF of C_FRQ is added as a noise filter for the oscillator.

Fsw freq can be controlled by the size of R_FRQ, and section 9.2.2.5 shows you how.

Fsw [kHz] = 31000 x (R_FRQ ^ -0.9) [kOhm]

For example, if your R_FRQ=60kohm, then Fsw will become 778kHz

regards,

jenson

0 Wenhao Wu over 5 years ago in reply to Jenson Lam

TI__Genius 9046 points

Hi Jenson,

Understood. Do you know why we should add a CFRQ here, even though for noise filter, it could filter the Fsw signal which converter needs too. Refer to all other converters, we do not need this CFRQ. Thanks!

-Wenhao

0 Wenhao Wu over 5 years ago in reply to Wenhao Wu

TI__Genius 9046 points

Hi Jenson,

Any updates to it?

-Wenhao

0 Jenson Lam over 5 years ago in reply to Wenhao Wu

TI__Prodigy 450 points

Hi Wenhao,

I have replied to your email.

regards,

jenson

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LM21305: LM21305 FREQ CFRQ