LTspice for LTC3642

chenpenghao · Jan 3, 2012

i try to use LTspice to simulate LTC3642,
it should be can create the efficiency report for the circuit,
but spice told me : don't know how to detect the circuit's steady state.
it's really confused me.
could some pro can figure this out for me?
how can i get the efficiency report ?

simonbramble · Jan 4, 2012

You need to do: Simulate -> edit simulation command -> check the box saying stop simulating if steady state is detected.

Alternatively, look at my LTSpice tutorial: http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm

.. and use a behavioural voltage source to model the efficiency at any point in time

simonbramble · Jan 4, 2012

To improve the ripple performance of the output voltage, put a 47pF cap across the top feedback resistor. This will insert some phase lead to offset the phase lag caused by the high value feedback resistor

chenpenghao · Jan 4, 2012

simonbramble said:
You need to do: Simulate -> edit simulation command -> check the box saying stop simulating if steady state is detected.

Alternatively, look at my LTSpice tutorial: http://www.simonbramble.co.uk/lt_spice/ltspice_lt_spice.htm

.. and use a behavioural voltage source to model the efficiency at any point in time

i did it, but the software can not find steady state,
and i already changed the load resisitor with a load current source.
thanks your tutorial.

simonbramble · Jan 4, 2012

Ah Ha! I see the problem... I guess I should have tried simulating it myself without giving you the 'standard answer'. OK, LTSpice looks at the current flowing into the compensation pin to detect steady state. This chip does not have a compensation pin, so I guess it gets upset when you try to measure steady state. Attached is what you are looking for using behavioural voltage sources. This actually gives you the efficiency over the whole operating range of the converter. Let me know if you have problems.

Simon

chenpenghao · Jan 5, 2012

simonbramble said:
Ah Ha! I see the problem... I guess I should have tried simulating it myself without giving you the 'standard answer'. OK, LTSpice looks at the current flowing into the compensation pin to detect steady state. This chip does not have a compensation pin, so I guess it gets upset when you try to measure steady state. Attached is what you are looking for using behavioural voltage sources. This actually gives you the efficiency over the whole operating range of the converter. Let me know if you have problems.

Simon

Thanks a lot simon.
you help me a lot

chenpenghao · Jan 15, 2012

simonbramble said:
Ah Ha! I see the problem... I guess I should have tried simulating it myself without giving you the 'standard answer'. OK, LTSpice looks at the current flowing into the compensation pin to detect steady state. This chip does not have a compensation pin, so I guess it gets upset when you try to measure steady state. Attached is what you are looking for using behavioural voltage sources. This actually gives you the efficiency over the whole operating range of the converter. Let me know if you have problems.

Simon

I wanna make a plot of efficiency vs output current of lTC 3642,
when i increase the load resisitor value, the circuit efficiency will increase beyond 100%.
it really confused me.
thanks very lot if you can help me.

simonbramble · Jan 16, 2012

I see what is happening. A dc/dc converter is a switched system - you have a pulsed current (hence pulsed power) on the input with a constant power on the output. Therefore it is possible to have no power flowing into the input yet still have power delivered on the output. This is what the low pass filters are for on B1 and B2. YOu need to play with these values to get Pin and Pout responding quickly to the circuit changes, yet no too quickly such that they do not filter out the pulsed input power. The attached circuit should work better

chenpenghao · Jan 16, 2012

simonbramble said:
I see what is happening. A dc/dc converter is a switched system - you have a pulsed current (hence pulsed power) on the input with a constant power on the output. Therefore it is possible to have no power flowing into the input yet still have power delivered on the output. This is what the low pass filters are for on B1 and B2. YOu need to play with these values to get Pin and Pout responding quickly to the circuit changes, yet no too quickly such that they do not filter out the pulsed input power. The attached circuit should work better

Thanks the circuit works better,
but the curve of efficiency vs load current seems constant.
the efficiency dosen't change with the different level currents.
it doesn't match the curve i found in the datasheet.
Thanks a lot

simonbramble · Jan 16, 2012

SPICE uses ideal components. The curves in the datasheet use an evaluation kit (real components), so SPICE shoudl give higher efficiency

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