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LMC7660: LMC7660 Schematic Review

Tom Tsao
Expert 1200 points
Part Number: LMC7660
Other Parts Discussed in Thread: TMUX7234, TL431

Tool/software:

Dear TI support team,

Please help me review the schematic of the LMC7660.

Thanks!

  • 0
    Guru 297030 points

    This will not work because the voltage divider does not leave any current for the charge pump or for any other devices connected to the ±10 V supplies.

    What is the actual problem you're trying to solve? Why do you need ±10 V, and how large is the load?

  • 0
    TI__Genius 14500 points

    Hi Tom,

    Pls double check the application with customer at first. Looking forward to your reply. Thanks.

    Aurora

  • 0 in reply to Aurora Tie
    Expert 1200 points

    Hi Aurora and Clemens,

    My application uses the LMC7660 to generate a -10V power supply for the TMUX7234, with a current of approximately 10mA.

    Can I use a 10V Zener diode to clamp Vin at 10V to generate a -10V output? Or do you have any other recommended solutions?

    Thanks!

  • +1 in reply to Tom Tsao
    Guru 297030 points

    You can use a Zener, but then you have to remove R801. R800 must be sized to provide enough current for the Zener, the LMC7660, and the TMUX7234.

  • +1 in reply to Tom Tsao
    TI__Genius 14500 points

    Hi Tom, 

    You can refer to Clemens' suggestion. Thanks.

    Aurora

  • 0 in reply to Clemens Ladisch
    Expert 1200 points

    Hi Clemens,

    The Zener diode I have selected has the following specifications:

    Assuming the LMC requires an output current of 10mA,
    Plus the Zener diode's current consumption of 5mA,
    The resistor value is calculated as:

    R = (14V - 10V) / 5mA = 267Ω

    Is this suitable?

    Thanks!

  • 0 in reply to Tom Tsao
    Guru 297030 points

    The LMC itself might need up to 400 µA. (14 V − 10 V) / (5 mA + 400 µA + 10 mA) ≈ 259 Ω.

    Where do you get that 10 mA value from? The TMUX7234's supply current is much less than 1 mA, and independent from the current that flows through the switches.

    (The Zener voltage will vary with current, so if you care about a stable output voltage or want to save power, better use the TL431.)

  • 0 in reply to Tom Tsao
    TI__Genius 14500 points

    Hi Tom,

    Pls see below update. Thanks.

    Aurora