hello,
I am using LM3481 to build an ultra-wide flyback converter.
My question is about how to shutdown PWM switching immediately.
Initially I would like to use FA/SYNC/SD pin 6 of the LM3481 to shut down PWM IC if my fault detection circuit finds something wrong. Can I do this by using the schematic shown in Fig. 27 in the datasheet?
There is no external freq sync at Pin 6 of my design. I just simply use a 215 kohm resistor to get PWM at 100kHZ. As in frequency adjust mode, not external freq sync mode.
I found a scope waveform from thread, https://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/t/51436
Question 1: Is this a good waveform example of immediate shut down of LM3481 at freq adjust mode? SW node voltage bounces back to high because of the LC resonance after no PWM. Is this correct?
This post says: Here is a capture of the LM3481 with the SD pin pulled high (GREEN waveform). Notice that the switch node stops pulsing immediately (BLUE waveform) because the internal circuit setting the clock frequency sees a high voltage beyond its range and so the clock stops running. This is with a part running with internal clock generation, not synchronized to a signal but the result should be similar.
if there is no load on the output, the output voltage might not drop much during 10us but if there is any load, that could create an under-voltage condition on the output. In this case we need to ensure that the part can keep switching during the interval when the MCU is reconfiguring the sync signal.
Question 2: Is this another method to shut down PWM IC by pulling the COMP pin to low?
Ref: http://www.ti.com/lit/ds/symlink/uc3845.pdf
Question 3: Is this a good way to shut down PWM IC by pulling down UVLO pin? It seems more direct than COMP pin and shutdown pin.
according to its functional block diagram.
Thanks
Please let me know.
Ye
