LM7705: How to select CFLY and CRES

Takashi Yamaguchi

Genius 11080 points

Part Number: LM7705
Other Parts Discussed in Thread: LM27761

Hi team,

My customer is interested in the LM7705 for op-amp negative supply.

According to the d/s, CFLY and CRES influence to the magnitude of the Vout ripple.

Could you tell me the equation to calculate the CFLY and CRES capacitance?

Regards,

Yamaguchi

over 5 years ago

0 Chris Glaser over 5 years ago

TI__Guru*** 142730 points

Hi Yamaguchi-san,

There is no simple equation for ripple. As the D/S explains, the LM7705 is basically an inverting charge pump followed by an LDO. It will give very low ripple.

The LM27761 is another adjustable charge pump + LDO, if they need a different output voltage.

0 Takashi Yamaguchi over 5 years ago in reply to Chris Glaser

TI__Genius 11080 points

Hi Chris-san,

Thank you for the answer. I understand that there is no simple equation.

Was the capacitance of CRES and CFLY described in the d/s determined by the bench test?

Regards,

Yamaguchi

0 Chris Glaser over 5 years ago in reply to Takashi Yamaguchi

TI__Guru*** 142730 points

Hi Yamaguchi-san,

The capacitor values shown in the D/S and used on the EVM give good performance for most applications. The IC was designed to work with these values, and its operation was confirmed in the bench validation of the IC.

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LM7705: How to select CFLY and CRES