• Resolved

LM7705: How to select CFLY and CRES

Part Number: LM7705

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

 

  • 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.

    Chris Glaser

    Texas Instruments

    ________________________________

    TPS62147 – New 17Vin, 2A buck with forced-PWM option for low output ripple.
    1% accuracy over temperature, 0.8V to 12Vout, pin-compatible to 4A (TPS62136), for industrial applications.

  • In reply to Chris Glaser:

    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

  • In reply to Takashi Yamaguchi:

    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.

    Chris Glaser

    Texas Instruments

    ________________________________

    TPS62147 – New 17Vin, 2A buck with forced-PWM option for low output ripple.
    1% accuracy over temperature, 0.8V to 12Vout, pin-compatible to 4A (TPS62136), for industrial applications.