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LTM4646 transient response

Hello,

I want to power the VCCINT (0.85V) of a Xilinx FPGA. I intend to use the LTM4646 regulator in dual mode. However, I see the 1V Single Phase Single Output  Load Transient Response in the datasheet and it's 33mV undershoot on Vout. Which in this case is a problem as Vout cannot go below the minimum 0.825V...

Can we expect the same undershoot level in dual mode?

I do not expect the FPGA, when powering up, to draw a such amount of current. Do you have any experience on this matter?

Thanks.

Sébastien

  • The load transient response in LTM4646 datasheet is for that one test condition. The overshoot and undershoot of the output voltage depends on lots of operational parameters of the circuit, like Vin, Vout voltage, output caps design, control loop design, and magnitude and slew rate of transient load current. For smaller transient load current, the overshoot and undershoot will be much smaller.

  • I used LTpowerCAD in order to estimate the load transient response. And I'm not sure if this will be relevant in my case. First, it seems I can't configure the device in dual mode.

    If I just use recommended output capacitance, the overshoot is way beyond the value in datasheet. I use a 5A step @10A/µs. The undershoot is above 40mV. Should I look for another device or there is a better suited configuration of the LTM4646.

    Thanks.

  • By dual mode, do you mean two phase single output mode? In LTpowerCAD, set number of output rails to one and set the number of parallel phases to two, LTpowerCAD will open the parallel configuration for LTM4646. It is hard for me to give specific suggestions since I don't know your design spec. But you can try adding more output caps to see if that can help reducing the undershoot.