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LM27761: Please review the schematic to make ±2.5V

Part Number: LM27761
Other Parts Discussed in Thread: ADS1299, , LP5907

Hi E2E Team,

we are designing the power block to make ±2.5V to operate ADS1299, using 3.7V battery power(VSYS).

the schematic is as below.

(we modified VFB net on U7 between R1 and R2 already.)

also we want to know what is the difference if we remove ferrite bead? (L1,L4)

Please review this schematic, and let me know the changes for improvement. Thanks.

Best regards,

Chase

  • Hi Chase,

    As you mentioned, LM27761 requires the FB modification apart from that it looks reasonable. I would expect input is 3.7V as it is hard to read on the schematic.

    Ferrite bead on output normally uses for high-frequency noise reduction. You can remove it if no certain noise-damping requires or you can use the low-value capacitance to reduce noise on a high frequency.

    Please reach out to us for more questions regarding LM27761 device. I am loop in an expert for LP5907 device for schematic review.

    Best regards,

    Tanvee

  • Hi Tanvee,

    the input is the battery with 3.7V nominal voltage, as you said.

    okay, i expect that there is no problem on LM27761 schematic.

    could you tell someone for reviewing LP5907 too?

    Best regards,

    Chase

  • Hi,

    I have looped in the expert and you will get feedback soon from them.

    Best regards,

    Tanvee

  • Hi Chase,

    There is too much capacitance on the output of the LP5907 LDO. To remain stable the total amount of output capacitor should be between 1uF and 10uF but I see 20.2uF shown in the schematic. If you need to add the LC filter, I'd recommend using two 4.7uF caps to stay within the requirement. Below is the relevant section from the datasheet which discusses the output capacitor requirements. 

    Other than that the only other recommendation I have is that the NC pin be connected to GND to improve thermal dissipation. This isn't a requirement just a  standard recommendation for our LDOs. 

  • Hi Kyle,

    Thank you for your review. i will ask you if i have more questions.

    Best regards,

    Chase