DC-DC Converter LTSpice confusion

[FotatoPotato]

Hey guys,

I’m working on a DC-DC Buck pre-regulator using the LT3864 and I was testing it in LTSpice and I noticed that the time it took to ramp up to the set 32v was over 7ms!  which seems awfully long. It also has some nasty voltage spikes for the first 0.6ms before ramping up the voltage. I have tried to play around with the ITH resistor and capacitor values, the SS cap values and just about everything else I could think of but nothing seems to work. Is this just a bug in LTSpice or am I doing something wrong? Also, once the voltage gets to the set 32v it is stable so there are no problems there. It’s just the ramp up time and the voltage spikes during ramp up.

The ramp-up time is directly related to the current draw on the LT3081 and On top of that, I'm only pulling 1/4 of the current I will be in the final design. I plan on putting 4 of the LT3081's in parallel to get a 6A output which I imagine would make the ramp-up time crazy long  . For reference the Green line is the output of the LT3081, the Red line is the output of the DC-DC pre-regulator and the Blue line is the current draw.

Thanks for the help! 

[uer166]

Did you try making the SS capacitor 0 (open circuit)?

According to datasheet: "The  internal  soft-start  voltage  and  the  external  SS  pin 
operate independently. The output will track the lower of
the two voltages. The slew rate of the internal soft-start
voltage  is  roughly  1.2V/ms,  which  translates  to  a  total 
soft-start  time  of  650μs.  "

So you should expect <1ms startup, although it may also be a function of output capacitance and inductor sizing.

[FotatoPotato]

Hey uer166,

I have tried leaving the SS pin floating and it doesn't really change much. The ramp time is a bit better but it is still way to long. I'll link a pic with the resultant graph. As I said before, I have tried just about everything, from changing the frequency, the inductor values, the ss cap values, the ITH pin values (this did affect the ramp time a lot but not enough to get it sub 2ms) and all of the other things that I thought would make a difference. All of them had little to no positive effect and most made the ramp time take longer or made the voltage ripple crazy high.

[T3sl4co1l]

You can always set initial conditions to get further along in startup.  That helps with simulation time, if not with the underlying problem (if it is a problem).

Is compensation near critical damping, or is it heavily overdamped?  A slow loop time constant gives a slow output time constant, of course.

Is the current limit set correctly?  Load capacitance?  Consider the two together: you may simply be doing what you meant to do in the first place, and didn't anticipate that startup will actually take this long.

Tim

[eetech00]

Hey guys,

I’m working on a DC-DC Buck pre-regulator using the LT3864 and I was testing it in LTSpice and I noticed that the time it took to ramp up to the set 32v was over 7ms!  which seems awfully long. It also has some nasty voltage spikes for the first 0.6ms before ramping up the voltage. I have tried to play around with the ITH resistor and capacitor values, the SS cap values and just about everything else I could think of but nothing seems to work. Is this just a bug in LTSpice or am I doing something wrong? Also, once the voltage gets to the set 32v it is stable so there are no problems there. It’s just the ramp up time and the voltage spikes during ramp up.

The ramp-up time is directly related to the current draw on the LT3081 and On top of that, I'm only pulling 1/4 of the current I will be in the final design. I plan on putting 4 of the LT3081's in parallel to get a 6A output which I imagine would make the ramp-up time crazy long  . For reference the Green line is the output of the LT3081, the Red line is the output of the DC-DC pre-regulator and the Blue line is the current draw.

Thanks for the help! 

Try:


Add some series resistance to the voltage source V1: rser=100m

set intial value L1: .IC I(L1)=0

set initial value C10, C8: (Properties) IC=0


eT