【LT1080】Internal charge pump specifications (capacitor type)

Hi,

I want to use LT1080.
I have a question, please let me know.

① What is the frequency of the charge pump?

② The type of capacitor for the charge pump is not listed, but is it possible to use a tantalum capacitor?


The capacitor to be used in the charge pump has been discontinued.
I would like to ask about the required specifications for the type of capacitor.

Should I have a capacity of 1uF or more?

Regarding ②,
If a tantalum capacitor is fine, please give us a reason.

Best Regards,

Naohisa

  • 0
    •  Analog Employees 
    on Dec 22, 2020 2:07 PM 2 months ago

    Hi Naohisa,

    The frequency of the LT1080 charge pump is approx. 100kHz.

    1uF value for the capacitor is fine, you can view the recommended values in the pin functions section, given as >= 1uF. 

    We normally recommend use of ceramic types but the use of either electrolytic, chosen for low ESR or nonpolarized types can be used.

    the following design note may be of use to you - shows LT1080 common configuration.

    https://www.analog.com/media/en/reference-design-documentation/design-notes/dn004f.pdf

    Best Regards,

    Shane

  • Hi,

    Shane

    Thank you for your answer.
    Let me ask you another question.

    LT1080 is a product released in 1987.
    At that time, high-capacity ceramic capacitors and low ESR capacitors were not widely used.(they do not exist in the first place).

    Therefore, I expect that the data sheet does not have many restrictions on capacitors.

    When this product was released (1987), how was the effect of the ESR of the capacitor on the circuit considered?

    I would like to know your views at that time(1987).

    Best Regards,

    Naohisa

  • 0
    •  Analog Employees 
    on Jan 8, 2021 11:09 AM 1 month ago in reply to naohisa

    Hi Naohisa, 

    Apologies for the delay in response.

    You are correct, there would not be many restrictions on capacitors for this part, given this device was designed for use with >=1uF caps. If the capacitor size was smaller, (i.e 0.1uF and below) then use of low ESR types would be of greater importance to incorporate into a design.

    Smaller values of capacitance yield larger ripple, and  ripple also increases with ESR. However the impact the value of capacitance has is more of a dominant factor. (Higher capacitance = lower ripple, albeit higher Capacitance = higher ESR)

    Restrictions on capacitor type, regarding ESR, would more likely be introduced If the capacitors used were in the 0.1uF range. You'll see in many of our other products, LT1180 for example, where these smaller capacitors are designed in, there's constraints introduced that low ESR capacitors must be used for bypassing/charge storage stated in the datasheet. This is to minimize the effect on ripple, as excessive values could arise if higher ESR types were used in conjunction with lower capacitance.

    With regards to LT1080, low/high ESR 1uF cap should be fine, you will observe lower ripple if you opt for the low ESR type.

    Hope this answers your question!

    Best Regards,

    Shane