Hi,
I have a TPS 54540 to generate +15 VDC from 28VDC and in the cascaded structure, I have used LM 25575 to generate -15 VDC from +15 VDC. I do like to synchronize both power module to an external CLK. After running this circuit in practice, in the Sync pin of the LM2557, there is -15 V and according to the datasheet, I need to pull -15 V up to zero to generate a pulse train by using the external CLK. Would you please suggest me what type of circuit I need to use to sync LM 25575 with negative -15 V output to the external clk? Would you please clarify what is the minimum frequency that LM25575 can be synchronized with using the external CLK?
The other problem, when I connected RT/CLK pin of TPS 54540 to the Sync pin of LM 25575, since the voltage level of the sync pin for LM25575 is -15 VDC, TPS 54540 doesn't run as it was before, and the switching frequency of that has dropped to 1/4 of the SW which is designed. I wonder of TPS 54540 has got a damage?
Thanks!
