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ST1S14 Vout=22v issue at Vin close to Dutycycle max

Question asked by Alexey Savchenko on Feb 16, 2018
Latest reply on Mar 9, 2018 by T J


I am facing a problem with ST1S14. Conditions:

 - if Vin is 24.5V or less and load current less 30mA, output voltage drops to about 15V. (Problem)

- If Vin becomes about 16V and less and load current still less 30mA, ->Vout=Vin-2V


However if load >40mA output voltage behavior is ok:

 If Vin is 24.5V and more, output voltage is stable 22V

 - If Vin gets <24.5V, the output voltage get less but always Vout=Vin-2V, what is fully ok.  

Seems when Vin comes to a level that Duty Cycles reaches it maximum and there is no load condition DCDC can't maintain proper Vout.


Interesting that it happen when Vout>20V because when I made Vout=18V DCDC worked properly in full load and Vin range with the same schematic(just resistor divider is different)


I tried to change 22uH to 10uH and 100uF. No result

Tried to change Cout from 100uF(electrilytic) to 2x100uF(electrolytic), 10uF electrolytic and 1uF ceramic. No result

Tried to change Cboot from 100nF to 200nF. No result

Tried to change resistor divider to 51k and 3k. No result.

Tried to add capacitor in parallel to R31. No results with 1nF and 100nF.


Vout and ST1S14 pin8  diagrams are attached at different Vin level. Seems fluctuations start in my case. But why they disappear in a case of Load >40mA?


We even tried to change the board with different routing but no results.


Can somebody explain this issue and help to solve it?