TPS54341: Issue with TPS54341 Buck as auxiliary power source

Part Number:TPS54341

I am implementing a full bridge LLC resonant main circuit using a Resonant controller which has soft start capability i.e. during startup the operating frequency starts from 1MHz and rolls off till feedback takes over the control. For providing the required bias voltage of 15V, I am using TPS54341 controller based buck converter (500KHz) as auxiliary circuit for converting 28VDC input to 15VDC output.  

Problem: When the resonant controller is ENABLE (after bias voltage (Vcc) is applied) , output of buck converter is falling from 15V and oscillating between 10V and 8V with low frequency defined by soft start capacitor. i.e. buck converter is found to be undergoing a hiccup, with time interval defined by soft start capacitor.

Steps Performed: 

1. When the resonant controller is biased from external 15V power source, the controller is starting and operating fine. The external 15V power source is showing 0.4A as steady state load current with a current spike on ENABLE of 1.5A peak.
2. When auxiliary circuit output is connected to rheostat (separately, without connecting to resonant controller) and loaded, the circuit maintained 15V output till 3.5A  load current is drawn. The circuit output voltage is satisfactory even for sudden 3.5A loads. On further increasing the load current (above 3.5A) the output voltage started to fall gradually with load current. 
3. Auxiliary circuit output is connected to VCC pin of resonant controller,
   1. Resonant controller is in DISABLE state, Vcc voltage of 15V from auxiliary output is present.
   2. Once ENABLE for resonant controller is given, voltage falls and oscillates between 8V & 10V (ramp up and down). Probing switch node shows IC is in hiccup mode.

Please help me understand why I am facing issue only when I use voltage from Buck converter for powering resonant controller. Thanks.