Do the transient thermal curves of part# LTC1693-5 allow its output to quickly charge a capacitor as depicted here? Less than several microsec charge time is desired. The datasheet lists Rth of 200 C/W, which suggests this is not possible, but I had hoped that the transient thermal curves might indicate this is a possibility at low duty cycles. Such thermal curves are not included in the datasheet. I need to quickly charge and invert a voltage, but all of the charge pumps I've seen so far are much too slow and/or have soft starts that are not suitable for fast use.

If the answer is no, is it also no for parts LTC4449 and LTC4449A-5?

Thank You.

Top Replies

  • 0
    •  Analog Employees 
    on Apr 13, 2021 5:02 PM


    What's the switching frequency?  With the value of capacitor that you suggested, you will have to use low switching frequency around 100 to a few KHz to keep the thermal rise manageable.  Low duty cycle may not be possible with R1 and internal resistance of the gate driver.  I would suggest keeping the capacitor value around 1nF and increase the switching frequency.

  • 0
    •  Analog Employees 
    on Apr 13, 2021 5:04 PM in reply to Gene323

    See page 4 of the datasheet, switching supply current vs. Cout plot to estimate the power consumption.

  • Hi Gene323,

    Thanks for the answers. The switching current vs Cout graph does not appear to take into account the output pmos and nmos power dissipation. These mosfets appear to achieve peak currents above 1 Amp. It is the impact of these high currents on the chip's thermal behavior that is most concerning at the moment. You can see that fast charging through these mosfets generates high peak power levels, even for a 1-10nF capacitor, but apparently this is acceptable. This implies that the transient thermal behavior is at play here, and is the genesis of my hope that possibly larger capacitances may be quickly charged but at low duty cycle.

    A graph of transient thermal resistance vs square wave duration, and parameterized by duty cycle is how mosfets typically display this information, and would also be somewhat informative here. The caveat here, is that since this is an IC, interpretation of that graph may be more complicated than for a mosfet.