LTM4668A datasheet vs LTPowerCAD

I've tried to recreate design from 1st page of LTM4668A datasheet in LTPowerCAD and it looks strange.

1) Datasheet Vin range 5-17 V with output as low as 1.5 V - in LTPowerCAD it's visible that Ton@Vin max is way too low and out of range, and it will not work in that Vin range.

2) In datasheet switching frequency is 2.25 MHz, yet at LTPowerCAD it is 2 MHz fixed. Frequency can't be changed, but module could be synchronized to external source.

3) In LTPowerCAD desired BW is about 333 kHz - way high. Using 47uF as suggested output capacitors brings actual BW to like 20-30 kHz. Using lower capacitor values to get high BW yelds to low phase margin. Will low BW not cause instability and/or poor performance?

4) LTPowerCAD suggest Cff capacitor to about 10-15 pF, but datasheet suggest a value of 150 pF. 

  • 0
    •  Analog Employees 
    on Nov 4, 2019 6:38 PM

    Hi RKR,

    1) That is correct - there are some conditions where minimum on-time constraints will be a factor depending upon your VIN, VOUT and FSW parameters. For this part, the minimum on-time constraint is 60ns (PowerCad can guide your design to make sure it does not hit this constraint).

    2) Thank you for the correction. The default switching frequency for the LTM4668A is 2.25MHz. LTPowerCad does not support using a external clock's switching frequency as the switching frequency of the module. 

    3) The typical bandwidth suggested is usually 1/8 of the switching frequency. Having a lower bandwidth will not cause instability as long as there is sufficient phase margin at the BW frequency. However, there are other design considerations such as peak-to-peak output voltage deviation during a load transient event that will be affected by a slow BW (higher peak-to-peak voltage). 

    4) The 150pF feedforward value in LTPowerCad provides sufficient phase margin and gain margin as a starting design. The datasheet's suggestion of 10 - 15pF is also a starting point for any initial design. The stabilty criterion will change based on the operating condtions, which PowerCad can help guide the design in terms of sizing the feedfoward capacitance for a stable system.

    Hope this helps!

  • 0
    •  Analog Employees 
    on Nov 6, 2019 6:09 PM in reply to A_Luan

    Hi RKR,

    Please sync release for LTPowerCad as some changes have been implemented:

    1) Standard default switching frequency is 2.25MHz for LTM4668A

    2) The switching frequency can now be changed to mimic an external switching frequency source (window of default switching frequency +/- 30%). This feature is allowed for both the LTM4668 and the LTM4668A.

    Thanks!