LT8750 Shutdown problems

In battery powered (Li-ion 3.7 V) ultrasonic acquisition system, LT8750 IC used as high voltage power supply for ultrasonic transducers. Schematic part related to HV (high voltage) given below.

LT8750 IC used in flyback topology, transformer CJ5143-ALB ratio 1:15, primary inductance 15 µH, I peak 1.2 A. DAC (MCP4725) used to control HV in range 10 V – 400 V.

Red arrows in schematic indicates places where signals are measured.

HV part is switched on and off to save battery (SHDN pin). But at voltages >300 V LT8750 is not always switches off, despite SHDN pin has been driven low. Below given cases then IC is switches off and then not.

Despite SHDN pin has been driven low LT8750 is running in 340 kHz.

We would be grateful if Analog Device could give some advises how to fix this problem.

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