LT8302+LT8640S simualtion


Our customer will use LT8302 fly back and LT8640S.

(24V-> 5.5V by LT8302 and 5.5V-Vf-> 3.3V about 4A by LT8640S.

(Refer to attached slide 1)

He simulated this design by LTspice.

(Result is attached slide 2)


When LT8302 SW current reaches to current limit or input of LT8640 is below 3.0V, LT8640S output is decreased.

After its sequence, LT8640S does not output. (Refer to attached slide 2 waveform)

When we add the LT8640S to 100uF of input capacitance

LT8640S can output .

But, when LT8302 output is decraesed, LT8302 suddenly stop to output current.

(refer to attached slide 4 blue circle)


Question 1

Regarding to slide 2 and 4,  why does LT8640S stop to output ?

I think that LT8302 stops to output but  I does not understand why LT8302 stops.



Ragarding to slide2, why does LT8640S remains  to output 0V ?

I think   that   if VIN returns  above 3V, LT8640S output the voltage.


For your referenec, I attach circuit deisng and used diode spice model.